Genetics 2
I may have to do this again... tables did not come
out.
100 Placenta
E
a~
Middle
ii, wall
E
a
a
~
U
Epidermis
10
1 10 100
Ovary diameter (mm)
Fig. 9.7. The relationship of ovary diameter to
diameter of individual cells in the placenta, middle
wall and epidermis of the growing fruit of
`Connecticut Field' pumpkin (Cucurbita pepo). Both
ovary and cell diameters are plotted on a logarithmic
scale (Sinnott, 1939).
period of cell division and cell enlargement to purely
cell enlargement takes place at about anthesis, and
occurred later in outer than in the inner layers of
the fruit. At fruit maturity, cells are largest in the
innermost layers of the fruit, and may be loosely
arranged or even torn apart. Epidermal cells are
small, closely packed, and in some cultivars, have
thickened cell walls to form a hard shell.
Similar patterns of cell division and enlargement were
found for other cultivated cucurbits (Kano et al.,
1957; Sinnott, 1939). In watermelon (Citrullus
lanatus), cell enlargement of the innermost fruit
tissues continued until the cells had attained an
astonishing 350,000-fold increase from their size at
the end of the cell division stage.
Fruit growth in Cucurbita pepo is characterized by an
initial log-linear (exponential) phase, followed by a
gradual growth rate decrease (Sinnott, 1945). A
comparison of cultivars ranging in fruit size from 40
to 7000 cm3 indicated that fruit growth rates varied
little, but the larger-fruited types had longer growth
durations (Fig. 9.8).
The expansion of cucumber fruits has also been found
to consist of initial exponential growth, followed by
a gradual decline (Tazuke and Sakiyama, 1984;
Marcelis,1992b). Increase in fresh weight was closely
correlated to volume growth, which in turn could be
accurately predicted from length and circumference
measurements.
Fruit growth rates can be profoundly affected by the
influence of the rest of the plant, and by
environmental factors. Marcelis and Baan HofmanEijer
(1993) showed that parthenocarpic greenhouse-grown
cucumbers had maximum fruit growth rates three times
higher when one rather than five fruits were
developing on the plant at the same time. Fruit growth
rate
366 H.C. Wien
8
7
E E
6
0
5 •
• R2 = 0.836"
c m 0 4
14 16 18 20 22 24 26 28 30 32 34
Fruit growth duration (days)
Fig. 9.8. The influence of fruit growth duration from
10 mm ovary diameter to maturity on final fruit volume
(logarithmic scale) in Cucurbita pepo (Sinnott, 1945).
increased with higher temperatures most markedly with
single-fruited plants, but reached maximum growth
rates at 25°C in plants with five fruits. Increasing
assimilate supply with higher irradiance also enhanced
fruit growth rates (Marcelis, 1993). The higher
assimilate levels resulted in increased number and
size of fruit cells, if higher light was given early
in fruit development. Later applications of high light
increased cell size only. These studies point out the
need to conduct fruit growth studies under uni
form environmental conditions, and with plants of
similar fruit load..
Considerable research has been devoted to the study of
the biochemistry and enzymology of fruit growth,
tracing the changes in fruit carbohydrates during
development. The principal translocated carbohydrate
of the cucurbit vegetables is the raffinose
polysaccharide stachyose (Webb and Gorham, 1964;
Hughes and Yamaguchi, 1983; Pharr et al., 1985). Once
it reaches the fruit peduncle, this transport sugar is
thought to be transformed into sucrose and hexose
sugars in muskmelon and cucumber (Handley et al.,
1983; Hubbard et al., 1989). Gross and Pharr (1982)
found that cucumber fruit peduncles contain the
necessary enzymes to convert stachyose to sucrose.
Peduncle extracts of Cucurbita moschata, watermelon
and muskmelon also had similar capabilities.
In the early stages of growth, muskmelon fruit sucrose
levels tend to be low, with soluble sugars made up
almost exclusively of glucose and fructose (McCollum
et al., 1988; Hubbard et al., 1989). It is thought
that the high levels of fruit acid invertase prevent
sucrose accumulation. During later stages of fruit
growth, acid invertase activity drops, and sucrose
phosphate syn
The Cucurbits: Cucumber, Melon, Squash and Pumpkin
367
thase (SPS) enzyme activity increases (Hubbard et al.,
1989). At the same time, sucrose levels also rise,
until they make up nearly 50% of the fruit's soluble
sugars. Hubbard et al. (1989) found that SPS activity
correlated well with sucrose concentration at fruit
harvest, when they compared melon cultivars with
contrasting fruit sucrose content. Although reducing
sugars make up between 2 and 3% of fruit fresh weight
of cucumber and melon during development, starch
levels of these fruits are less than 1% in both
species (Schaffer et al., 1987). It is therefore not
possible to increase fruit
sugar content of melons once the fruit has been
detached from the plant'
(Bianco and Pratt, 1977). For optimum fruit quality,
harvest should take
place as close as possible to the time of maturity of
the fruit.,
The changes in carbohydrate content of the fruits of
Cucurbita and of watermelon have not been intensively
investigated. As in muskmelon, watermelon fruit show
earlier increases in reducing sugars than in sucrose
during development (Porter et al., 1940). At maturity,
the fruit typically has
10% total sugars, of which about 35% is sucrose. If
the fruit is allowed to
become overmature on the vine, or stored at room
temperature, the propor-
tion of sucrose increases to around 65% (Porter et
al.,1940). Total sugars, and
soluble solids increase in the fruit until maturity
(Mizuno and Pratt, 1973).
Little is known about the enzyme systems responsible
for these changes in
the fruit.
FACTORS AFFECTING PRODUCTIVITY
Yield production in the annual herbaceous vegetable
crops of the Cucurbitaceae is affected both by factors
that influence overall plant productivity, and those
that determine the partitioning of assimilates to
reproductive tissue. As with other vegetable crops
such as tomato and pepper, crop responses have been
worked out in detail for glasshouse production
systems, in which temperatures, light and C02 levels
can be regulated. Accordingly, information is most
complete for the climatic controls needed for optimum
growth and yield of the gynoecious parthenocarpic
cucumber grown in the glasshouses of Northern Europe
and North America.
Productivity also involves issues of the timing and
concentration of harvests. In pickling cucumber, where
the fruits are harvested at a young stage, much effort
has been expended to devise production systems and
develop genetic types that give high yields in a short
harvest period. Some of these trials will be described
below.
Fruit quality is an important criterion in the
production of muskmelon, watermelon and winter squash.
Production systems must provide conditions which allow
fruit to develop acceptable sweetness and taste, and
the size characteristic of the cultivar.
GLASSHOUSE CUCUMBER
In the European production system, cucumbers typically
are sown in January and February, and bear fruit
during spring and summer. Considerable experimentation
has determined that mean air temperatures of 18-24°C
are optimum for greatest yield accumulation (brews ct
al., 1980; Liebig, 1980b; Slack and Hand, 1980). As
temperature increased, stem extension rate
accelerated, and time to first harvest declined (Krug
and Liebig, 1980). Plants started bearing earliest
under high temperatures, but had a shorter harvest
duration, and a reduced total yield (Liebig, 1980b).
Variation of day and night temperature about the mean
had no effect on earliness and early yield (Slack and
Hand, 1980; Grimstad and Frimanslund, 1993), but
profoundly influenced stem length (Krug and Liebig,
1980; Grimstad and Frimanslund, 1993) (see section on
germination and seedling growth above). At night
temperatures of 18°C or lower, earliness and
productivity was boosted by increasing soil
temperature (brews et al., 1980).
The response of the cucumber crop to temperature can
be modified by the light conditions under which the
crop is grown. Under the limiting light energy levels
of midwinter, stem extension and earliness of cropping
was maximal at 21°C, with no further increase as
temperature was raised beyond this mark (Heij, 1980).
Earliness of yield production and marketable yield at
a given air temperature was further boosted by
increases in light levels (Liebig, 1980a,b) (Fig.
9.9).
Increasing ambient C02 level for glasshouse cucumber
has become the standard practice particularly when
glasshouse vents are closed.
50
45
RADIATION
(1500 WWh m2 day-1)
40-
00__
35 ' O- - _ 2000 Wh
o--0..__._...0
30
3000 Wh
25
20
18 20 22 24 26 28
Temperature (°C)
Fig. 9.9. Influence of growing temperature on time to
harvest of the first fruits of glasshouse cucumbers
grown under three levels of supplementary radiation
(Liebig, 1980b).
Concentrations of 700-1000 frl I-1 COZ are commonly
used, and have been found to increase yields from
20-43% (Hand, 1984; Kimball, 1986). In situations
where temperature control requires ventilation, CO Z
supplementation becomes more difficult. Under warm
spring and fall conditions of North Carolina, Peet and
Willits (1987) found that enrichment periods as short
as 4.5 h still increased cucumber yields by 27%. A
larger benefit could be obtained, however, by use of a
passive rock storage cooling system that allowed the
vents to be kept closed for more than 8 h per day.
An alternative means of maintaining elevated COZ
levels in the glasshouse is to produce cucumbers on
bales of decomposing straw. Growers in Britain found
that this reduces the need for carbon dioxide
supplementation because the gas is being released by
the medium, and keeps COZ levels at nearly 1000 ul 1-1
(Hand, 1984).
FIELD-GROWN CUCUMBER
The climatic optima for growing cucumbers in the field
have not been intensively studied, but broadly confirm
the findings of the glasshouse research. Lorenz and
Maynard (1980) state the optimum temperature of the
crop to be 18-24°C. In establishing a heat unit system
to predict the timing of harvests, Perry et al. (1986)
found that a base temperature of 15.5 and a maximum
temperature of 32°C worked best. Perry and Wehner
(1990) found that these limits were better predictors
of harvest dates for pickling cucumbers than for
slicers in trials conducted in three locations over 3
years. Their results imply that fruit growth rates
during the later stages of development are influenced
by factors other than temperature. This was confirmed
by the research of Marcelis (1993), who showed the
dependence of fruit growth on assimilate supply,
particularly the presence of previously set fruits on
the plant.
The influence of growing fruit on a plant on the
development of younger fruit and further vegetative
growth has been thought to limit cucumber yields.
McCollum (1934) showed the inhibiting influence was
strongest in seeded fruits until the time the
seedcoats hardened. Parthenocarpic fruits had a much
weaker inhibitory effect. Denna (1973) extended these
observations with comparisons of parthenocarpic
glasshouse cultivars that were pollinated or allowed
to set seedless fruits. Among 12 cultivars, the
seedless plants produced 17% more fruit than their
seeded counterparts. Denna also pointed out that seeds
comprised a significant amount of the total plant dry
weight, namely 10% for the glasshouse cultivars, and
20% for two pickling cucumber lines (Table 9.5). From
that, one would expect the yield depression from the
first-formed fruits to be even stronger in seeded
cultivars.
The causes for the inhibition have not been clarified,
but two major factors are thought to be involved. The
first is competition for limited assimilates, and the
priority of reproductive structures for these (Pharr
et al., 1985; Marcelis, 1992a). Pharr et al. (1985)
calculated, for instance, that one actively
Table 9.5. Total plant dry weight and the percentage
of that dry matter in the principal plant components
for two parthenocarpic and two pickling cucumber
cultivars (Denna, 1973).
Total dry
weight Per cent dry weight
Cultivar Fruit condition g plant-' Fruit Seed Vine
Uniflora A Seedless 166 62 - 38
Seeded 108 52 11 37
Toska Seedless 177 57 - 43
Seeded 88 49 9 42
MSMR15 Fruitless 122 - - 100
Seeded 105 34 20 46
GSMR15 Fruitless 11_1 - - 100
Seeded 89 31 20 49
growing cucumber fruit required the photosynthetic
output of 40% of the
plant canopy. During the growth of parthenocarpic
cucumbers, the inhibi-
tion of vegetative growth is most marked during
periods of rapid fruit
growth (Marcelis, 1992a). The greater inhibitory
effect of seeded fruits may
be due to the fact that seeds contain 32% fat and
comprise 20% of fruit dry
weight (Winton and Winton, 1935; Denna, 1973).
Hormonal factors are also likely to be involved in
the competition
between old and younger fruits, and the fruits and
vegetative growth.
Schapendonk and Brouwer (1984) found for instance,
that just starving a
cucumber plant of carbohydrates by defoliation will
reduce fruit growth
rate, but is not sufficient to cause the reproductive
tissue to become necrotic.
They used this as indirect evidence that hormonal
influences are involved in
the fruit-induced growth inhibition. It is also
likely that hormonal activity in
the developing fruit allows that organ to become a
stronger sink for assimi-
lates than other tissues. Although concrete data on
these points have so far
been lacking amongst the cucurbit vegetables,
considerably more is known
a with regard to other vegetable crops (see Chapter
5).
Several strategies have been suggested to overcome
the intraplant com-
petitive effects in order to increase the yield
potential of pickling cucum-
bers. Increasing the number of female flowers per
plant by use of the
gynoecious trait (Tasdighi and Baker, 1981) was
positively correlated with
- increased yields. A genetic trait which increased
the number of female flow-
ers per node from one to three or four was, however,
not successful in
changing fruit number per plant (Uzcategui and Baker,
1979). Others have
crossed pickling cucumbers with C. sativus var.
hardwickii, which tends to
produce many small, seeded fruits per plant (Lower et
al., 1982).
Unfortunately, this species has several undesirable
traits such as photope-
riod sensitivity, small fruit size and chilling
temperature sensitivity.
Morphological traits such as determinate growth habit
and the 'little leaf'
trait, which confers increased branch numbers have
also been advocated as
25
Fruit
20 O Leaf blades
Petioles
o Stem
Q 15
a~
a
F 10
3
5
0
20 40 60 80 100 120 140 160 180 200220
Plant density (1000s ha-1)
Fig. 9.10. Effect of plant density on dry weight of
fruits and vegetative parts of pickling cucumbers at
48 days after planting (Wielders and Price, 1989).
means of increasing pickling cucumber yields,
particularly for once-over, mechanical harvesting
(Staub et al., 1992).
It is doubtful, however, if morphological
modifications of the plant are going to overcome the
inverse relationship between yield per plant and plant
density. As cucumber plants are crowded closer
together, the leaf area, branching and fruit number
per plant are reduced (Cantliffe and Phatak, 1975;
Widders and Price, 1989). Widders and Price (1989)
pointed out the remarkably close relation between dry
matter partitioned to fruits and to leaves as plant
populations are varied from 5 to 20 m-2 (Fig. 9.10).
This supports the findings of Pharr et al. (1985) of
the heavy demand that a rapidly growing cucumber fruit
makes on current photosynthesis. So unless
morphological changes increase photosynthetic rate, or
increase the proportion of the leaf canopy
intercepting light, alterations in branching habit and
leaf size are unlikely to increase fruit yield.
In the development of pickling cucumber production
systems using once-over mechanical harvesting, many
studies have focused on the population density needed
for high yield. Some of these have shown increased
yields as populations were raised to 50 or even 64
plants m-2 (Cantliffe and Phatak, 1975; O'Sullivan,
1980). At such high densities, only half the plants
produced fruit, however (O'Sullivan, 1980), indicating
that plant populations were in excess of what was
needed. In addition, weed control and harvesting
problems with these dense stands have limited their
use (Lower and Edwards, 1986). As a result,
populations of around 15 plants m-2 have generally
been adopted for once-over harvests in recent years
(Staub et al., 1992).
Another approach to increasing cucumber yields at
lower densities has been to delay pollination and thus
fruit set, and allowing the plants to develop more
nodes with female flowers. Connor and Martin (1970)
excluded bees from cucumber plots for the first 11
days of bloom, and achieved a 100% yield increase.
Fruit quality was also improved by the delayed fruit
set, a characteristic in common with late-fruiting
monoecious cultivars (Wehner and Miller, 1985). From a
practical standpoint, cucumber fields would need to be
entirely free of male flowers to achieve such delayed
fruiting, a situation which has been difficult to
accomplish. Many gynoecious cultivars are derived from
crosses between gynoecious by monoecious parents, and
produce significant numbers of male flowers,
especially under long daylengths and high temperatures
(Connor and Martin, 1971; Lower and Edwards, 1986)
(see section on flower differentiation above). The
percentage of female flowers can be increased
significantly in crosses of gynoecious by
hermaphrodite lines, and lines homozygous for the
gynoecious trait have also been developed by use of
ethylene inhibiting chemicals such as silver nitrate
(Lower and Edwards, 1986). Nevertheless, delayed
planting of pollinator lines (Connor and Martin,
1970), or use of the fruit setting chemical
chlorflurenol (see section on hormonal regulation of
fruit set, above), have not been adopted into
commercial practice, likely because of practical and
regulatory constraints.
COMBINING YIELD AND FRUIT QUALITY IN CUCURBIT
VEGETABLES
To be classed as 'marketable', muskmelon, watermelon
and some types of squash must attain a minimum level
of soluble solids, as well as a size and external
appearance characteristic of the cultivar. Sugars make
up about 85% of the soluble solids in muskmelon
(Porter et al., 1940). For muskmelon, soluble solids
must exceed 8% to be eligible for shipping out of
California (Peirce, 1987). Similarly, watermelon
fruits should have sugar content of 10% or greater to
be considered acceptable (Peirce, 1987).
Maintaining quality of fruit harvested while at the
same time increasing yield has generally not been
successful using higher plant densities (Zahara, 1972;
Mendlinger, 1994). As yields of fruit increase,
soluble solids percentage and fruit size declines.
Welles and Buitelaar (1988) found that any factor
which shortened the period from flowering to fruit
maturity reduced soluble solids content of the fruit
(Fig. 9.11). Increasing the night temperature,
reducing leaf area, and increasing the number of fruit
per plant all reduced the maturation period of the
fruit, and simultaneously lowered fruit quality. Davis
and Schweers (1971) also found that fruit number per
plant was inversely correlated with fruit soluble
solids. This indicates that assimilate supply during
the fruit growth period is critical to determination
of fruit quality. Thus factors that reduce the canopy
photosynthesis rate, such as the extent of exposure of
the leaf canopy to light, may also influence fruit
quality and yield. Knavel (1991) found, for instance,
that muskmelon cultivars
13
12
s
0
11 0
Night temperature
0
10
Leaf area
0
N 9
~i
8 Fruit load
7
42 43 44 45 46 47 48 49 50 51
Maturation time (days)
Fig. 9.11. Influence of fruit maturation time on fruit
soluble solids content for glasshousegrown muskmelon.
Maturation time was varied by manipulations of night
temperature, plant leaf area or fruit load per plant
in separate experiments (Welles and Buitelaar,1988).
with large, overlapping leaves and short internodes
had fewer fruits with low soluble solids content
compared to conventional vining types. Measurements
indicated that about 50% of the plants' leaf area was
shaded by other leaves, compared to 30% for a
conventional muskmelon line. Nerson and Paris (1987)
found that yield per plant among three muskmelon lines
differing in growth habit was proportional to
internode length. These results would indicate that
muskmelon is source-limited during the period when the
fruits are growing rapidly. Short-term carbohydrate
translocation studies with field-grown muskmelons
(Hughes et al., 1983) confirm this for plants with
small fruits. The degree of carbohydrate depletion
from source leaves depended on the proximity of the
leaf to the fruit, with the most distant leaves
retaining significantly more than those near the
fruit.
The demands for the production of high quality fruit
may also constrain the degree to which the fruit
harvest period can be shortened or synchronized in a
particular field. At plant densities at which more
than one fruit is produced per plant, it may be
difficult for several fruits to be growing rapidly at
the same time, without reduction in growth rate or
assimilate accumulation rate in the individual fruit.
When grown at wide spacing, muskmelon plants produce
two distinct flushes of fruit (McGlasson and Pratt,
1963). Attempts have been made to have more fruit
ripening simultaneously by selecting for morphological
types producing several branches simultaneously (Paris
et al., 1986). While these new types have produced a
more concentrated yield, problems of small fruit size
and low soluble solids must still be overcome (Paris
et al., 1988).
PHYSIOLOGICAL DISORDERS
PRECOCIOUS FEMALE FLOWERING
Under the cool conditions prevalent in temperate
regions during planting,
the development rate of male flowers of Cucurbita
pepo is inhibited more
than that of female flowers (Rylski and Aloni, 1990).
This can result in the
precocious development to anthesis of female flowers,
and a lack of fruit set
because of a dearth of open male flowers. The problem
is especially pro-
nounced in some hybrid cultivars of summer squash
that flower early in the
season. The disorder is less pronounced in some
open-pollinated viny culti-
vars, but first anthesis of all flowers is relatively
late in these lines (H.C.
Wien, 1990, Ithaca, NY, unpublished data).
Applications of GA 4+7 as flower
buds become visible can hasten male flower
development to anthesis, indi-
cating that flower development may be under similar
hormonal control as
flower differentiation in cucurbits.
SUDDEN WILT OF MUSKMELONS
The disorder is also called vine collapse, crown
blight and late collapse by
researchers in different parts of the world, and
refers to the rapid wilting of
plants just as the fruits are beginning to develop
netting, and the vines have
covered the ground (litter, 1995). Within days, the
entire field may be
affected, and vines may not recover. A range of
pathogenic organisms have
been associated with the disorder, and may be the
primary causes of the
observed symptoms. The causal organisms associated
with the disease dif-
fer in different melon-growing regions. For instance,
in New York, cucum-
ber mosaic virus (CMV) and Fusarium wilt have been
implicated (Bauerle,
1971; MacNab, 1971; Zitter, 1995). In California,
Pythium ultimum has been
linked to the disorder, and soil fumigation has
delayed development of
symptoms (Munnecke et al., 1984). In Israel, Nitzany
(1966) caused vine col-
lapse under cool conditions by inoculating with CMV
and Pythium. Miller
R
and co-workers (1995) implicated a number of fungal
pathogens in causing
vine collapse of melons in Texas.
The presence of rapidly growing fruits on the plants
appeared to be key
to development of the vine collapse symptoms on the
plants in a number of
these cases. Periods of cool, cloudy weather during
this growth stage, fol-
lowed by hot, sunny conditions increased incidence of
the disorder (litter,
1995). Although direct experimental evidence is
lacking, it is thought that
during the rapid fruit growth stage, demand for
assimilates of the growing
fruit is so high that root growth is reduced. If at
the same time, pathogens
attack the root system, or reduce the plant's capacity
to produce assimilates,
root death may result. A third factor that could
further exacerbate the situa-
tion is adverse weather conditions that would reduce
photosynthetic rates
and reduce root function through waterlogging of the
soil. Collaborative
13
12
s
e
~n 11 O
_-v Night temperature
0
10
Leaf area 0
9
ti
8 Fruit load
7
42 43 44 45 46 47 48 49 50 51
Maturation time (days)
Fig. 9.11. Influence of fruit maturation time on fruit
soluble solids content for glasshousegrown muskmelon.
Maturation time was varied by manipulations of night
temperature, plant leaf area or fruit load per plant
in separate experiments (Welles and Buitelaar, 1988).
with large, overlapping leaves and short internodes
had fewer fruits with low soluble solids content
compared to conventional vining types. Measurements
indicated that about 50% of the plants' leaf area was
shaded by other leaves, compared to 30% for a
conventional muskmelon line. Nerson and Paris (1987)
found that yield per plant among three muskmelon lines
differing in growth habit was proportional to
internode length. These results would indicate that
muskmelon is source-limited during the period when the
fruits are growing rapidly. Short-term carbohydrate
translocation studies with field-grown muskmelons
(Hughes et al., 1983) confirm this for plants with
small fruits. The degree of carbohydrate depletion
from source leaves depended on the proximity of the
leaf to the fruit, with the most distant leaves
retaining significantly more than those near the
fruit.
The demands for the production of high quality fruit
may also constrain the degree to which the fruit
harvest period can be shortened or synchronized in a
particular field. At plant densities at which more
than one fruit is produced per plant, it may be
difficult for several fruits to be growing rapidly at
the same time, without reduction in growth rate or
assimilate accumulation rate in the individual fruit.
When grown at wide spacing, muskmelon plants produce
two distinct flushes of fruit (McGlasson and Pratt,
1963). Attempts have been made to have more fruit
ripening simultaneously by selecting for morphological
types producing several branches simultaneously (Paris
et al., 1986). While these new types have produced a
more concentrated yield, problems of small fruit size
and low soluble solids must still be overcome (Paris
et al., 1988).
PHYSIOLOGICAL DISORDERS w
th
PRECOCIOUS FEMALE FLOWERING
Under the cool conditions prevalent in temperate
regions during planting,
the development rate of male flowers of Cucurbita
pepo is inhibited more
than that of female flowers (Rylski and Aloni, 1990).
This can result in the T'
precocious development to anthesis of female flowers,
and a lack of fruit set it
because of a dearth of open male flowers. The problem
is especially pro- h'
nounced in some hybrid cultivars of summer squash
that flower early in the
season. The disorder is less pronounced in some
open-pollinated viny culti- d
vars, but first anthesis of all flowers is relatively
late in these lines (H.C. r'
Wien, 1990, Ithaca, NY, unpublished data).
Applications of GA4+7 as flower 1(
buds become visible can hasten male flower
development to anthesis, indi- s'
cating that flower development may be under similar
hormonal control as g
flower differentiation in cucurbits. n
SUDDEN WILT OF MUSKMELONS
T
The disorder is also called vine collapse, crown
blight and late collapse by s
researchers in different parts of the world, and
refers to the rapid wilting of 7
plants just as the fruits are beginning to develop
netting, and the vines have c
covered the ground (litter, 1995). Within days, the
entire field may be
affected, and vines may not recover. A range of
pathogenic organisms have s
been associated with the disorder, and may be the
primary causes of the t
observed symptoms. The causal organisms associated
with the disease dif- r
fer in different melon-growing regions. For instance,
in New York, cucum-
ber mosaic virus (CMV) and Fusarium wilt have been
implicated (Bauerle, t
1971; MacNab, 1971; Zitter, 1995). In California,
Pythium ultimum has been
linked to the disorder, and soil fumigation has
delayed development of 1
symptoms (Munnecke et al., 1984). In Israel, Nitzany
(1966) caused vine col-
lapse under cool conditions by inoculating with CMV
and Pythium. Miller
and co-workers (1995) implicated a number of fungal
pathogens in causing
vine collapse of melons in Texas.
The presence of rapidly growing fruits on the plants
appeared to be key
to development of the vine collapse symptoms on the
plants in a number of
these cases. Periods of cool, cloudy weather during
this growth stage, fol-
lowed by hot, sunny conditions increased incidence of
the disorder (litter,
1995). Although direct experimental evidence is
lacking, it is thought that
during the rapid fruit growth stage, demand for
assimilates of the growing
fruit is so high that root growth is reduced. If at
the same time, pathogens
attack the root system, or reduce the plant's
capacity to produce assimilates,
root death may result. A third factor that could
further exacerbate the situa-
tion is adverse weather conditions that would reduce
photosynthetic rates -'`'
and reduce root function through waterlogging of the
soil. Collaborative
work between physiologists and pathologists will be
needed to test these theories and overcome this costly
disorder.
HOLLOW HEART OF WATERMELON
This disorder is characterized by the separation of
the inner parts of the fruit into distinct segments,
leaving hollow areas at harvest maturity. Hollow heart
occurs more often in the first-formed fruit on the
plant, as a result of excess nitrogen fertilization,
and delayed harvests (Kano, 1993). The disorder is
more prevalent under conditions of rapid fruit growth
rate, when the rind is expanding more rapidly than the
inner regions of the fruit (Sinnott, 1939; Kano,
1993). Ways of avoiding the condition include
selection of less susceptible cultivars, and using
cultural practices that moderate fruit growth rate and
final fruit size. These include adequate plant
populations, moderate levels of nitrogen, and prompt
harvests.
BITTER FRUIT IN SUMMER SQUASH
The sporadic occurrence of bitter fruit in plantings
of zucchini and other summer squash types has caused
serious medical problems in a few cases. The ingestion
of as little as 3 g of such fruit can cause nausea,
stomach cramps and diarrhoea (Herrington, 1983).
Consumption of bitter squash was responsible for 22
cases of food poisoning in Australia, and occasional
similar incidents in the USA (Herrington, 1983; Rymal
et al., 1984). The bitterness is caused by
cucurbitacins, tetracyclic triterpenes that occur
naturally in the family Cucurbitaceae (Rymal et al.,
1984). These compounds can occur in all parts of the
plant, although concentrations tend to be highest in
the roots (Rehm et al., 1957a,b). Plants may have
intensely bitter fruits, but non-bitter leaves or
cotyledons (Rymal et al.,1984). Concentrations of
cucurbitacins may be several times higher in the
placental region of the fruit, compared to the
pericarp or the rind (Jaworski et al., 1985). Thus the
bitter fruit of summer squash would be potentially
more dangerous than those of mature squash or pumpkin,
in which the placenta is not eaten.
The origin of these occasional plants producing bitter
fruit is not exactly known, but they are thought to
have arisen from chance outcrosses to bitterfruited
wild types or gourds during seed production, or
through mutations. In some cases where bitter fruit
could be traced to individual plants, the plant and
mature fruit characteristics did not match that of the
cultivar, indicating that genetic change had occurred
(Rymal et al., 1984).
CONCLUDING REMARKS
The cucurbit vegetables are a unique group of species
that have fascinated plant researchers for many years.
By virtue of their large seeds, they begin
growth rapidly, and achieve an efficient light
intercepting plant canopy earlier than most herbaceous
plants. They are aided in this by large, planophile
leaves borne on rapidly growing stems, even though
assimilation rates are not higher than most other
herbaceous crops with C-3 assimilatory pathway
(Bruggink and Heuvelink, 1987).
Reproductive growth has received much attention in
these crops, particularly the factors determining the
gender of the flowers. In spite of considerable
research, however, the hormonal controls of sex
expression have only been detailed for cucumber, with
much still to be done on crops like watermelon. It is
particularly noteworthy that some hormones such as
ethylene seem to have opposite effects on watermelon
as on cucumber (Christopher and Loy, 1982). A greater
understanding of the mechanisms of watermelon sex
expression may need to be preceded by the discovery of
a wider range of flower genotypes similar to those
that aided the investigations of cucumber sex
expression physiology.
Control of sex expression in cucumber appears to act
primarily at the non-sexual floral primordia stage
that allows either male or female or perfect flowers
to develop. In Cucurbita, a second control determines
if the male or female primordia reach anthesis or not.
Under cool conditions, male flowers are inhibited in
development (Rylski and Aloni, 1990), while
temperatures of 32/21°C (day/night) inhibit female
flowers but allow males to develop (H.C. Wien, 1996,
Ithaca, NY, unpublished data). Little is known about
the hormonal controls of these processes, and whether
endogenous gibberellins and ethylene are involved.
Rapid growth also characterizes the development of the
fruit of many cucurbit vegetables. While the giant
pumpkin cultivars of Cucurbita maxima present the most
striking example of this, with growth rates of more
than 11 kg (fresh weight) per day, glasshouse cucumber
fruits have been shown to gain more than 200 g per day
for short periods (Langevin, 1993; Marcelis, 1993).
Such a massive transfer of assimilates to one or more
reproductive structures on the plant make it likely
that growth of other plant parts will be curtailed
during this period. Cucurbit vegetable crops are known
for the strong inhibition of vegetative, and
particularly root growth after flowering, leading to a
cyclic development of fruits, and in extreme cases, to
collapse of the plants due to pathogen attack of the
weakened plant (de Stitger, 1969; Zitter, 1995). The
factors that control the movement of assimilates to
fruits, rather than to other parts of the plants are
at present poorly understood. Hormonal signals
presumably make the developing fruit a strong sink for
assimilates, but the nature and control of these
hormonal signals needs more investigation. Perhaps the
incentive of breaking the 1000-pound pumpkin fruit
size barrier (prize money in 1995 was $US 50,000) will
help stimulate the research.
Further work is also needed to compare the capacity of
the cucurbit plant to produce assimilates with the
demands of assimilatory products by the rapidly
growing fruits. Such studies, carried out by Pharr and
co-workers (1985) with cucumbers, need also to be done
with other cucurbit vegeta
bles, to understand the limits of productivity, both
in terms of fruit yields and of fruit quality factors
such as sweetness. It may be that higher yields of
fruits with acceptable soluble solids can only be
achieved by improvements
in the rates of photosynthesis, or more efficient
respiration and translocation
mechanisms. We have much to learn before such
statements can be made
with certainty.
ACKNOWLEDGEMENTS
I am grateful for the helpful suggestions of Drs
Richard Robinson and Scott NeSmith for improvement of
this chapter. Thanks to Dr Martin Goffinet for
providing Fig. 9.2.
REFERENCES
Acock, B., Acock, M.C. and Pasternak, D. (1990)
Interactions of CO Z enrichment and temperature on
carbohydrate production and accumulation in muskmelon
leaves. Journal of the American Society of
Horticultural Science 115, 525-529.
Atsmon, D. and Galun, E. (1960) A morphological study
of staminate, pistillate and hermaphrodite flowers in
Cucumis sativus L. Phytomorphology 10,110-115.
Atsmon, D. and Tabbak, C. (1979) Comparative effects
of gibberellin, silver nitrate and
aminoethoxyvinylglycine on sexual tendency and
ethylene evolution in the cucumber plant (Cucumis
sativus L.). Plant and Cell Physiology 20, 1547-1556.
Bauerle, W.L. (1971) Effect of sudden wilt disease on
the physiology of the muskmelon (Cucumis melo L. var.
reticulatus). Doctoral thesis, Cornell University,
Ithaca NY.
Beyer, E.M. Jr and Morgan, P .W. (1969) Time sequence
of the effect of ethylene on
transport, uptake and decarboxylation of auxin. Plant
and Cell Physiology 10,
787-799.
Beyer, E.M. Jr and Quebedeaux, B. (1974) Parthenocarpy
in cucumber: mechanism of action of auxin transport
inhibitors. Journal of the American Society of
Horticultural Science 99, 385-390.
Bianco, VV and Pratt, H.K. (1977) Compositional
changes in muskmelons during development and in
response to ethylene treatment. Journal of the
American Society of Horticultural Science 102,
127-133.
Bohn, G.W. and Davis, G.N. (1964) Insect pollination
is necessary for the production of muskmelons (Cucumis
melo v. reticulatus). Journal of Apicultural Research
3, 61-63.
Bohn, G.W. and Mann, L.K. (1960) Nectarless, a
yield-reducing mutant character in muskmelon.
Proceedings of the American Society of Horticultural
Science 76, 455-459.
Bruggink, G.T and Heuvelink, E. (1987) Influence of
light on the growth of young tomato, cucumber and
sweet pepper plants in the greenhouse: effects on
relative growth rate, net assimilation rate and leaf
area ratio. Scientia Horticulturae 31,161-174.
Cantliffe, D.J., Robinson, R.W. and Shannon, S. (1972)
Promotion of cucumber fruit set and development by
chlorflurenol. HortScience 7, 416-418.
100 Placenta
E
a~
Middle
ii, wall
E
a
a
~
U
Epidermis
10
1 10 100
Ovary diameter (mm)
Fig. 9.7. The relationship of ovary diameter to
diameter of individual cells in the placenta, middle
wall and epidermis of the growing fruit of
`Connecticut Field' pumpkin (Cucurbita pepo). Both
ovary and cell diameters are plotted on a logarithmic
scale (Sinnott, 1939).
period of cell division and cell enlargement to purely
cell enlargement takes place at about anthesis, and
occurred later in outer than in the inner layers of
the fruit. At fruit maturity, cells are largest in the
innermost layers of the fruit, and may be loosely
arranged or even torn apart. Epidermal cells are
small, closely packed, and in some cultivars, have
thickened cell walls to form a hard shell.
Similar patterns of cell division and enlargement were
found for other cultivated cucurbits (Kano et al.,
1957; Sinnott, 1939). In watermelon (Citrullus
lanatus), cell enlargement of the innermost fruit
tissues continued until the cells had attained an
astonishing 350,000-fold increase from their size at
the end of the cell division stage.
Fruit growth in Cucurbita pepo is characterized by an
initial log-linear (exponential) phase, followed by a
gradual growth rate decrease (Sinnott, 1945). A
comparison of cultivars ranging in fruit size from 40
to 7000 cm3 indicated that fruit growth rates varied
little, but the larger-fruited types had longer growth
durations (Fig. 9.8).
The expansion of cucumber fruits has also been found
to consist of initial exponential growth, followed by
a gradual decline (Tazuke and Sakiyama, 1984;
Marcelis,1992b). Increase in fresh weight was closely
correlated to volume growth, which in turn could be
accurately predicted from length and circumference
measurements.
Fruit growth rates can be profoundly affected by the
influence of the rest of the plant, and by
environmental factors. Marcelis and Baan HofmanEijer
(1993) showed that parthenocarpic greenhouse-grown
cucumbers had maximum fruit growth rates three times
higher when one rather than five fruits were
developing on the plant at the same time. Fruit growth
rate
366 H.C. Wien
8
7
E E
6
0
5 •
• R2 = 0.836"
c m 0 4
14 16 18 20 22 24 26 28 30 32 34
Fruit growth duration (days)
Fig. 9.8. The influence of fruit growth duration from
10 mm ovary diameter to maturity on final fruit volume
(logarithmic scale) in Cucurbita pepo (Sinnott, 1945).
increased with higher temperatures most markedly with
single-fruited plants, but reached maximum growth
rates at 25°C in plants with five fruits. Increasing
assimilate supply with higher irradiance also enhanced
fruit growth rates (Marcelis, 1993). The higher
assimilate levels resulted in increased number and
size of fruit cells, if higher light was given early
in fruit development. Later applications of high light
increased cell size only. These studies point out the
need to conduct fruit growth studies under uni
form environmental conditions, and with plants of
similar fruit load..
Considerable research has been devoted to the study of
the biochemistry and enzymology of fruit growth,
tracing the changes in fruit carbohydrates during
development. The principal translocated carbohydrate
of the cucurbit vegetables is the raffinose
polysaccharide stachyose (Webb and Gorham, 1964;
Hughes and Yamaguchi, 1983; Pharr et al., 1985). Once
it reaches the fruit peduncle, this transport sugar is
thought to be transformed into sucrose and hexose
sugars in muskmelon and cucumber (Handley et al.,
1983; Hubbard et al., 1989). Gross and Pharr (1982)
found that cucumber fruit peduncles contain the
necessary enzymes to convert stachyose to sucrose.
Peduncle extracts of Cucurbita moschata, watermelon
and muskmelon also had similar capabilities.
In the early stages of growth, muskmelon fruit sucrose
levels tend to be low, with soluble sugars made up
almost exclusively of glucose and fructose (McCollum
et al., 1988; Hubbard et al., 1989). It is thought
that the high levels of fruit acid invertase prevent
sucrose accumulation. During later stages of fruit
growth, acid invertase activity drops, and sucrose
phosphate syn
The Cucurbits: Cucumber, Melon, Squash and Pumpkin
367
thase (SPS) enzyme activity increases (Hubbard et al.,
1989). At the same time, sucrose levels also rise,
until they make up nearly 50% of the fruit's soluble
sugars. Hubbard et al. (1989) found that SPS activity
correlated well with sucrose concentration at fruit
harvest, when they compared melon cultivars with
contrasting fruit sucrose content. Although reducing
sugars make up between 2 and 3% of fruit fresh weight
of cucumber and melon during development, starch
levels of these fruits are less than 1% in both
species (Schaffer et al., 1987). It is therefore not
possible to increase fruit
sugar content of melons once the fruit has been
detached from the plant'
(Bianco and Pratt, 1977). For optimum fruit quality,
harvest should take
place as close as possible to the time of maturity of
the fruit.,
The changes in carbohydrate content of the fruits of
Cucurbita and of watermelon have not been intensively
investigated. As in muskmelon, watermelon fruit show
earlier increases in reducing sugars than in sucrose
during development (Porter et al., 1940). At maturity,
the fruit typically has
10% total sugars, of which about 35% is sucrose. If
the fruit is allowed to
become overmature on the vine, or stored at room
temperature, the propor-
tion of sucrose increases to around 65% (Porter et
al.,1940). Total sugars, and
soluble solids increase in the fruit until maturity
(Mizuno and Pratt, 1973).
Little is known about the enzyme systems responsible
for these changes in
the fruit.
FACTORS AFFECTING PRODUCTIVITY
Yield production in the annual herbaceous vegetable
crops of the Cucurbitaceae is affected both by factors
that influence overall plant productivity, and those
that determine the partitioning of assimilates to
reproductive tissue. As with other vegetable crops
such as tomato and pepper, crop responses have been
worked out in detail for glasshouse production
systems, in which temperatures, light and C02 levels
can be regulated. Accordingly, information is most
complete for the climatic controls needed for optimum
growth and yield of the gynoecious parthenocarpic
cucumber grown in the glasshouses of Northern Europe
and North America.
Productivity also involves issues of the timing and
concentration of harvests. In pickling cucumber, where
the fruits are harvested at a young stage, much effort
has been expended to devise production systems and
develop genetic types that give high yields in a short
harvest period. Some of these trials will be described
below.
Fruit quality is an important criterion in the
production of muskmelon, watermelon and winter squash.
Production systems must provide conditions which allow
fruit to develop acceptable sweetness and taste, and
the size characteristic of the cultivar.
GLASSHOUSE CUCUMBER
In the European production system, cucumbers typically
are sown in January and February, and bear fruit
during spring and summer. Considerable experimentation
has determined that mean air temperatures of 18-24°C
are optimum for greatest yield accumulation (brews ct
al., 1980; Liebig, 1980b; Slack and Hand, 1980). As
temperature increased, stem extension rate
accelerated, and time to first harvest declined (Krug
and Liebig, 1980). Plants started bearing earliest
under high temperatures, but had a shorter harvest
duration, and a reduced total yield (Liebig, 1980b).
Variation of day and night temperature about the mean
had no effect on earliness and early yield (Slack and
Hand, 1980; Grimstad and Frimanslund, 1993), but
profoundly influenced stem length (Krug and Liebig,
1980; Grimstad and Frimanslund, 1993) (see section on
germination and seedling growth above). At night
temperatures of 18°C or lower, earliness and
productivity was boosted by increasing soil
temperature (brews et al., 1980).
The response of the cucumber crop to temperature can
be modified by the light conditions under which the
crop is grown. Under the limiting light energy levels
of midwinter, stem extension and earliness of cropping
was maximal at 21°C, with no further increase as
temperature was raised beyond this mark (Heij, 1980).
Earliness of yield production and marketable yield at
a given air temperature was further boosted by
increases in light levels (Liebig, 1980a,b) (Fig.
9.9).
Increasing ambient C02 level for glasshouse cucumber
has become the standard practice particularly when
glasshouse vents are closed.
50
45
RADIATION
(1500 WWh m2 day-1)
40-
00__
35 ' O- - _ 2000 Wh
o--0..__._...0
30
3000 Wh
25
20
18 20 22 24 26 28
Temperature (°C)
Fig. 9.9. Influence of growing temperature on time to
harvest of the first fruits of glasshouse cucumbers
grown under three levels of supplementary radiation
(Liebig, 1980b).
Concentrations of 700-1000 frl I-1 COZ are commonly
used, and have been found to increase yields from
20-43% (Hand, 1984; Kimball, 1986). In situations
where temperature control requires ventilation, CO Z
supplementation becomes more difficult. Under warm
spring and fall conditions of North Carolina, Peet and
Willits (1987) found that enrichment periods as short
as 4.5 h still increased cucumber yields by 27%. A
larger benefit could be obtained, however, by use of a
passive rock storage cooling system that allowed the
vents to be kept closed for more than 8 h per day.
An alternative means of maintaining elevated COZ
levels in the glasshouse is to produce cucumbers on
bales of decomposing straw. Growers in Britain found
that this reduces the need for carbon dioxide
supplementation because the gas is being released by
the medium, and keeps COZ levels at nearly 1000 ul 1-1
(Hand, 1984).
FIELD-GROWN CUCUMBER
The climatic optima for growing cucumbers in the field
have not been intensively studied, but broadly confirm
the findings of the glasshouse research. Lorenz and
Maynard (1980) state the optimum temperature of the
crop to be 18-24°C. In establishing a heat unit system
to predict the timing of harvests, Perry et al. (1986)
found that a base temperature of 15.5 and a maximum
temperature of 32°C worked best. Perry and Wehner
(1990) found that these limits were better predictors
of harvest dates for pickling cucumbers than for
slicers in trials conducted in three locations over 3
years. Their results imply that fruit growth rates
during the later stages of development are influenced
by factors other than temperature. This was confirmed
by the research of Marcelis (1993), who showed the
dependence of fruit growth on assimilate supply,
particularly the presence of previously set fruits on
the plant.
The influence of growing fruit on a plant on the
development of younger fruit and further vegetative
growth has been thought to limit cucumber yields.
McCollum (1934) showed the inhibiting influence was
strongest in seeded fruits until the time the
seedcoats hardened. Parthenocarpic fruits had a much
weaker inhibitory effect. Denna (1973) extended these
observations with comparisons of parthenocarpic
glasshouse cultivars that were pollinated or allowed
to set seedless fruits. Among 12 cultivars, the
seedless plants produced 17% more fruit than their
seeded counterparts. Denna also pointed out that seeds
comprised a significant amount of the total plant dry
weight, namely 10% for the glasshouse cultivars, and
20% for two pickling cucumber lines (Table 9.5). From
that, one would expect the yield depression from the
first-formed fruits to be even stronger in seeded
cultivars.
The causes for the inhibition have not been clarified,
but two major factors are thought to be involved. The
first is competition for limited assimilates, and the
priority of reproductive structures for these (Pharr
et al., 1985; Marcelis, 1992a). Pharr et al. (1985)
calculated, for instance, that one actively
Table 9.5. Total plant dry weight and the percentage
of that dry matter in the principal plant components
for two parthenocarpic and two pickling cucumber
cultivars (Denna, 1973).
Total dry
weight Per cent dry weight
Cultivar Fruit condition g plant-' Fruit Seed Vine
Uniflora A Seedless 166 62 - 38
Seeded 108 52 11 37
Toska Seedless 177 57 - 43
Seeded 88 49 9 42
MSMR15 Fruitless 122 - - 100
Seeded 105 34 20 46
GSMR15 Fruitless 11_1 - - 100
Seeded 89 31 20 49
growing cucumber fruit required the photosynthetic
output of 40% of the
plant canopy. During the growth of parthenocarpic
cucumbers, the inhibi-
tion of vegetative growth is most marked during
periods of rapid fruit
growth (Marcelis, 1992a). The greater inhibitory
effect of seeded fruits may
be due to the fact that seeds contain 32% fat and
comprise 20% of fruit dry
weight (Winton and Winton, 1935; Denna, 1973).
Hormonal factors are also likely to be involved in
the competition
between old and younger fruits, and the fruits and
vegetative growth.
Schapendonk and Brouwer (1984) found for instance,
that just starving a
cucumber plant of carbohydrates by defoliation will
reduce fruit growth
rate, but is not sufficient to cause the reproductive
tissue to become necrotic.
They used this as indirect evidence that hormonal
influences are involved in
the fruit-induced growth inhibition. It is also
likely that hormonal activity in
the developing fruit allows that organ to become a
stronger sink for assimi-
lates than other tissues. Although concrete data on
these points have so far
been lacking amongst the cucurbit vegetables,
considerably more is known
a with regard to other vegetable crops (see Chapter
5).
Several strategies have been suggested to overcome
the intraplant com-
petitive effects in order to increase the yield
potential of pickling cucum-
bers. Increasing the number of female flowers per
plant by use of the
gynoecious trait (Tasdighi and Baker, 1981) was
positively correlated with
- increased yields. A genetic trait which increased
the number of female flow-
ers per node from one to three or four was, however,
not successful in
changing fruit number per plant (Uzcategui and Baker,
1979). Others have
crossed pickling cucumbers with C. sativus var.
hardwickii, which tends to
produce many small, seeded fruits per plant (Lower et
al., 1982).
Unfortunately, this species has several undesirable
traits such as photope-
riod sensitivity, small fruit size and chilling
temperature sensitivity.
Morphological traits such as determinate growth habit
and the 'little leaf'
trait, which confers increased branch numbers have
also been advocated as
25
Fruit
20 O Leaf blades
Petioles
o Stem
Q 15
a~
a
F 10
3
5
0
20 40 60 80 100 120 140 160 180 200220
Plant density (1000s ha-1)
Fig. 9.10. Effect of plant density on dry weight of
fruits and vegetative parts of pickling cucumbers at
48 days after planting (Wielders and Price, 1989).
means of increasing pickling cucumber yields,
particularly for once-over, mechanical harvesting
(Staub et al., 1992).
It is doubtful, however, if morphological
modifications of the plant are going to overcome the
inverse relationship between yield per plant and plant
density. As cucumber plants are crowded closer
together, the leaf area, branching and fruit number
per plant are reduced (Cantliffe and Phatak, 1975;
Widders and Price, 1989). Widders and Price (1989)
pointed out the remarkably close relation between dry
matter partitioned to fruits and to leaves as plant
populations are varied from 5 to 20 m-2 (Fig. 9.10).
This supports the findings of Pharr et al. (1985) of
the heavy demand that a rapidly growing cucumber fruit
makes on current photosynthesis. So unless
morphological changes increase photosynthetic rate, or
increase the proportion of the leaf canopy
intercepting light, alterations in branching habit and
leaf size are unlikely to increase fruit yield.
In the development of pickling cucumber production
systems using once-over mechanical harvesting, many
studies have focused on the population density needed
for high yield. Some of these have shown increased
yields as populations were raised to 50 or even 64
plants m-2 (Cantliffe and Phatak, 1975; O'Sullivan,
1980). At such high densities, only half the plants
produced fruit, however (O'Sullivan, 1980), indicating
that plant populations were in excess of what was
needed. In addition, weed control and harvesting
problems with these dense stands have limited their
use (Lower and Edwards, 1986). As a result,
populations of around 15 plants m-2 have generally
been adopted for once-over harvests in recent years
(Staub et al., 1992).
Another approach to increasing cucumber yields at
lower densities has been to delay pollination and thus
fruit set, and allowing the plants to develop more
nodes with female flowers. Connor and Martin (1970)
excluded bees from cucumber plots for the first 11
days of bloom, and achieved a 100% yield increase.
Fruit quality was also improved by the delayed fruit
set, a characteristic in common with late-fruiting
monoecious cultivars (Wehner and Miller, 1985). From a
practical standpoint, cucumber fields would need to be
entirely free of male flowers to achieve such delayed
fruiting, a situation which has been difficult to
accomplish. Many gynoecious cultivars are derived from
crosses between gynoecious by monoecious parents, and
produce significant numbers of male flowers,
especially under long daylengths and high temperatures
(Connor and Martin, 1971; Lower and Edwards, 1986)
(see section on flower differentiation above). The
percentage of female flowers can be increased
significantly in crosses of gynoecious by
hermaphrodite lines, and lines homozygous for the
gynoecious trait have also been developed by use of
ethylene inhibiting chemicals such as silver nitrate
(Lower and Edwards, 1986). Nevertheless, delayed
planting of pollinator lines (Connor and Martin,
1970), or use of the fruit setting chemical
chlorflurenol (see section on hormonal regulation of
fruit set, above), have not been adopted into
commercial practice, likely because of practical and
regulatory constraints.
COMBINING YIELD AND FRUIT QUALITY IN CUCURBIT
VEGETABLES
To be classed as 'marketable', muskmelon, watermelon
and some types of squash must attain a minimum level
of soluble solids, as well as a size and external
appearance characteristic of the cultivar. Sugars make
up about 85% of the soluble solids in muskmelon
(Porter et al., 1940). For muskmelon, soluble solids
must exceed 8% to be eligible for shipping out of
California (Peirce, 1987). Similarly, watermelon
fruits should have sugar content of 10% or greater to
be considered acceptable (Peirce, 1987).
Maintaining quality of fruit harvested while at the
same time increasing yield has generally not been
successful using higher plant densities (Zahara, 1972;
Mendlinger, 1994). As yields of fruit increase,
soluble solids percentage and fruit size declines.
Welles and Buitelaar (1988) found that any factor
which shortened the period from flowering to fruit
maturity reduced soluble solids content of the fruit
(Fig. 9.11). Increasing the night temperature,
reducing leaf area, and increasing the number of fruit
per plant all reduced the maturation period of the
fruit, and simultaneously lowered fruit quality. Davis
and Schweers (1971) also found that fruit number per
plant was inversely correlated with fruit soluble
solids. This indicates that assimilate supply during
the fruit growth period is critical to determination
of fruit quality. Thus factors that reduce the canopy
photosynthesis rate, such as the extent of exposure of
the leaf canopy to light, may also influence fruit
quality and yield. Knavel (1991) found, for instance,
that muskmelon cultivars
13
12
s
0
11 0
Night temperature
0
10
Leaf area
0
N 9
~i
8 Fruit load
7
42 43 44 45 46 47 48 49 50 51
Maturation time (days)
Fig. 9.11. Influence of fruit maturation time on fruit
soluble solids content for glasshousegrown muskmelon.
Maturation time was varied by manipulations of night
temperature, plant leaf area or fruit load per plant
in separate experiments (Welles and Buitelaar,1988).
with large, overlapping leaves and short internodes
had fewer fruits with low soluble solids content
compared to conventional vining types. Measurements
indicated that about 50% of the plants' leaf area was
shaded by other leaves, compared to 30% for a
conventional muskmelon line. Nerson and Paris (1987)
found that yield per plant among three muskmelon lines
differing in growth habit was proportional to
internode length. These results would indicate that
muskmelon is source-limited during the period when the
fruits are growing rapidly. Short-term carbohydrate
translocation studies with field-grown muskmelons
(Hughes et al., 1983) confirm this for plants with
small fruits. The degree of carbohydrate depletion
from source leaves depended on the proximity of the
leaf to the fruit, with the most distant leaves
retaining significantly more than those near the
fruit.
The demands for the production of high quality fruit
may also constrain the degree to which the fruit
harvest period can be shortened or synchronized in a
particular field. At plant densities at which more
than one fruit is produced per plant, it may be
difficult for several fruits to be growing rapidly at
the same time, without reduction in growth rate or
assimilate accumulation rate in the individual fruit.
When grown at wide spacing, muskmelon plants produce
two distinct flushes of fruit (McGlasson and Pratt,
1963). Attempts have been made to have more fruit
ripening simultaneously by selecting for morphological
types producing several branches simultaneously (Paris
et al., 1986). While these new types have produced a
more concentrated yield, problems of small fruit size
and low soluble solids must still be overcome (Paris
et al., 1988).
PHYSIOLOGICAL DISORDERS
PRECOCIOUS FEMALE FLOWERING
Under the cool conditions prevalent in temperate
regions during planting,
the development rate of male flowers of Cucurbita
pepo is inhibited more
than that of female flowers (Rylski and Aloni, 1990).
This can result in the
precocious development to anthesis of female flowers,
and a lack of fruit set
because of a dearth of open male flowers. The problem
is especially pro-
nounced in some hybrid cultivars of summer squash
that flower early in the
season. The disorder is less pronounced in some
open-pollinated viny culti-
vars, but first anthesis of all flowers is relatively
late in these lines (H.C.
Wien, 1990, Ithaca, NY, unpublished data).
Applications of GA 4+7 as flower
buds become visible can hasten male flower
development to anthesis, indi-
cating that flower development may be under similar
hormonal control as
flower differentiation in cucurbits.
SUDDEN WILT OF MUSKMELONS
The disorder is also called vine collapse, crown
blight and late collapse by
researchers in different parts of the world, and
refers to the rapid wilting of
plants just as the fruits are beginning to develop
netting, and the vines have
covered the ground (litter, 1995). Within days, the
entire field may be
affected, and vines may not recover. A range of
pathogenic organisms have
been associated with the disorder, and may be the
primary causes of the
observed symptoms. The causal organisms associated
with the disease dif-
fer in different melon-growing regions. For instance,
in New York, cucum-
ber mosaic virus (CMV) and Fusarium wilt have been
implicated (Bauerle,
1971; MacNab, 1971; Zitter, 1995). In California,
Pythium ultimum has been
linked to the disorder, and soil fumigation has
delayed development of
symptoms (Munnecke et al., 1984). In Israel, Nitzany
(1966) caused vine col-
lapse under cool conditions by inoculating with CMV
and Pythium. Miller
R
and co-workers (1995) implicated a number of fungal
pathogens in causing
vine collapse of melons in Texas.
The presence of rapidly growing fruits on the plants
appeared to be key
to development of the vine collapse symptoms on the
plants in a number of
these cases. Periods of cool, cloudy weather during
this growth stage, fol-
lowed by hot, sunny conditions increased incidence of
the disorder (litter,
1995). Although direct experimental evidence is
lacking, it is thought that
during the rapid fruit growth stage, demand for
assimilates of the growing
fruit is so high that root growth is reduced. If at
the same time, pathogens
attack the root system, or reduce the plant's capacity
to produce assimilates,
root death may result. A third factor that could
further exacerbate the situa-
tion is adverse weather conditions that would reduce
photosynthetic rates
and reduce root function through waterlogging of the
soil. Collaborative
13
12
s
e
~n 11 O
_-v Night temperature
0
10
Leaf area 0
9
ti
8 Fruit load
7
42 43 44 45 46 47 48 49 50 51
Maturation time (days)
Fig. 9.11. Influence of fruit maturation time on fruit
soluble solids content for glasshousegrown muskmelon.
Maturation time was varied by manipulations of night
temperature, plant leaf area or fruit load per plant
in separate experiments (Welles and Buitelaar, 1988).
with large, overlapping leaves and short internodes
had fewer fruits with low soluble solids content
compared to conventional vining types. Measurements
indicated that about 50% of the plants' leaf area was
shaded by other leaves, compared to 30% for a
conventional muskmelon line. Nerson and Paris (1987)
found that yield per plant among three muskmelon lines
differing in growth habit was proportional to
internode length. These results would indicate that
muskmelon is source-limited during the period when the
fruits are growing rapidly. Short-term carbohydrate
translocation studies with field-grown muskmelons
(Hughes et al., 1983) confirm this for plants with
small fruits. The degree of carbohydrate depletion
from source leaves depended on the proximity of the
leaf to the fruit, with the most distant leaves
retaining significantly more than those near the
fruit.
The demands for the production of high quality fruit
may also constrain the degree to which the fruit
harvest period can be shortened or synchronized in a
particular field. At plant densities at which more
than one fruit is produced per plant, it may be
difficult for several fruits to be growing rapidly at
the same time, without reduction in growth rate or
assimilate accumulation rate in the individual fruit.
When grown at wide spacing, muskmelon plants produce
two distinct flushes of fruit (McGlasson and Pratt,
1963). Attempts have been made to have more fruit
ripening simultaneously by selecting for morphological
types producing several branches simultaneously (Paris
et al., 1986). While these new types have produced a
more concentrated yield, problems of small fruit size
and low soluble solids must still be overcome (Paris
et al., 1988).
PHYSIOLOGICAL DISORDERS w
th
PRECOCIOUS FEMALE FLOWERING
Under the cool conditions prevalent in temperate
regions during planting,
the development rate of male flowers of Cucurbita
pepo is inhibited more
than that of female flowers (Rylski and Aloni, 1990).
This can result in the T'
precocious development to anthesis of female flowers,
and a lack of fruit set it
because of a dearth of open male flowers. The problem
is especially pro- h'
nounced in some hybrid cultivars of summer squash
that flower early in the
season. The disorder is less pronounced in some
open-pollinated viny culti- d
vars, but first anthesis of all flowers is relatively
late in these lines (H.C. r'
Wien, 1990, Ithaca, NY, unpublished data).
Applications of GA4+7 as flower 1(
buds become visible can hasten male flower
development to anthesis, indi- s'
cating that flower development may be under similar
hormonal control as g
flower differentiation in cucurbits. n
SUDDEN WILT OF MUSKMELONS
T
The disorder is also called vine collapse, crown
blight and late collapse by s
researchers in different parts of the world, and
refers to the rapid wilting of 7
plants just as the fruits are beginning to develop
netting, and the vines have c
covered the ground (litter, 1995). Within days, the
entire field may be
affected, and vines may not recover. A range of
pathogenic organisms have s
been associated with the disorder, and may be the
primary causes of the t
observed symptoms. The causal organisms associated
with the disease dif- r
fer in different melon-growing regions. For instance,
in New York, cucum-
ber mosaic virus (CMV) and Fusarium wilt have been
implicated (Bauerle, t
1971; MacNab, 1971; Zitter, 1995). In California,
Pythium ultimum has been
linked to the disorder, and soil fumigation has
delayed development of 1
symptoms (Munnecke et al., 1984). In Israel, Nitzany
(1966) caused vine col-
lapse under cool conditions by inoculating with CMV
and Pythium. Miller
and co-workers (1995) implicated a number of fungal
pathogens in causing
vine collapse of melons in Texas.
The presence of rapidly growing fruits on the plants
appeared to be key
to development of the vine collapse symptoms on the
plants in a number of
these cases. Periods of cool, cloudy weather during
this growth stage, fol-
lowed by hot, sunny conditions increased incidence of
the disorder (litter,
1995). Although direct experimental evidence is
lacking, it is thought that
during the rapid fruit growth stage, demand for
assimilates of the growing
fruit is so high that root growth is reduced. If at
the same time, pathogens
attack the root system, or reduce the plant's
capacity to produce assimilates,
root death may result. A third factor that could
further exacerbate the situa-
tion is adverse weather conditions that would reduce
photosynthetic rates -'`'
and reduce root function through waterlogging of the
soil. Collaborative
work between physiologists and pathologists will be
needed to test these theories and overcome this costly
disorder.
HOLLOW HEART OF WATERMELON
This disorder is characterized by the separation of
the inner parts of the fruit into distinct segments,
leaving hollow areas at harvest maturity. Hollow heart
occurs more often in the first-formed fruit on the
plant, as a result of excess nitrogen fertilization,
and delayed harvests (Kano, 1993). The disorder is
more prevalent under conditions of rapid fruit growth
rate, when the rind is expanding more rapidly than the
inner regions of the fruit (Sinnott, 1939; Kano,
1993). Ways of avoiding the condition include
selection of less susceptible cultivars, and using
cultural practices that moderate fruit growth rate and
final fruit size. These include adequate plant
populations, moderate levels of nitrogen, and prompt
harvests.
BITTER FRUIT IN SUMMER SQUASH
The sporadic occurrence of bitter fruit in plantings
of zucchini and other summer squash types has caused
serious medical problems in a few cases. The ingestion
of as little as 3 g of such fruit can cause nausea,
stomach cramps and diarrhoea (Herrington, 1983).
Consumption of bitter squash was responsible for 22
cases of food poisoning in Australia, and occasional
similar incidents in the USA (Herrington, 1983; Rymal
et al., 1984). The bitterness is caused by
cucurbitacins, tetracyclic triterpenes that occur
naturally in the family Cucurbitaceae (Rymal et al.,
1984). These compounds can occur in all parts of the
plant, although concentrations tend to be highest in
the roots (Rehm et al., 1957a,b). Plants may have
intensely bitter fruits, but non-bitter leaves or
cotyledons (Rymal et al.,1984). Concentrations of
cucurbitacins may be several times higher in the
placental region of the fruit, compared to the
pericarp or the rind (Jaworski et al., 1985). Thus the
bitter fruit of summer squash would be potentially
more dangerous than those of mature squash or pumpkin,
in which the placenta is not eaten.
The origin of these occasional plants producing bitter
fruit is not exactly known, but they are thought to
have arisen from chance outcrosses to bitterfruited
wild types or gourds during seed production, or
through mutations. In some cases where bitter fruit
could be traced to individual plants, the plant and
mature fruit characteristics did not match that of the
cultivar, indicating that genetic change had occurred
(Rymal et al., 1984).
CONCLUDING REMARKS
The cucurbit vegetables are a unique group of species
that have fascinated plant researchers for many years.
By virtue of their large seeds, they begin
growth rapidly, and achieve an efficient light
intercepting plant canopy earlier than most herbaceous
plants. They are aided in this by large, planophile
leaves borne on rapidly growing stems, even though
assimilation rates are not higher than most other
herbaceous crops with C-3 assimilatory pathway
(Bruggink and Heuvelink, 1987).
Reproductive growth has received much attention in
these crops, particularly the factors determining the
gender of the flowers. In spite of considerable
research, however, the hormonal controls of sex
expression have only been detailed for cucumber, with
much still to be done on crops like watermelon. It is
particularly noteworthy that some hormones such as
ethylene seem to have opposite effects on watermelon
as on cucumber (Christopher and Loy, 1982). A greater
understanding of the mechanisms of watermelon sex
expression may need to be preceded by the discovery of
a wider range of flower genotypes similar to those
that aided the investigations of cucumber sex
expression physiology.
Control of sex expression in cucumber appears to act
primarily at the non-sexual floral primordia stage
that allows either male or female or perfect flowers
to develop. In Cucurbita, a second control determines
if the male or female primordia reach anthesis or not.
Under cool conditions, male flowers are inhibited in
development (Rylski and Aloni, 1990), while
temperatures of 32/21°C (day/night) inhibit female
flowers but allow males to develop (H.C. Wien, 1996,
Ithaca, NY, unpublished data). Little is known about
the hormonal controls of these processes, and whether
endogenous gibberellins and ethylene are involved.
Rapid growth also characterizes the development of the
fruit of many cucurbit vegetables. While the giant
pumpkin cultivars of Cucurbita maxima present the most
striking example of this, with growth rates of more
than 11 kg (fresh weight) per day, glasshouse cucumber
fruits have been shown to gain more than 200 g per day
for short periods (Langevin, 1993; Marcelis, 1993).
Such a massive transfer of assimilates to one or more
reproductive structures on the plant make it likely
that growth of other plant parts will be curtailed
during this period. Cucurbit vegetable crops are known
for the strong inhibition of vegetative, and
particularly root growth after flowering, leading to a
cyclic development of fruits, and in extreme cases, to
collapse of the plants due to pathogen attack of the
weakened plant (de Stitger, 1969; Zitter, 1995). The
factors that control the movement of assimilates to
fruits, rather than to other parts of the plants are
at present poorly understood. Hormonal signals
presumably make the developing fruit a strong sink for
assimilates, but the nature and control of these
hormonal signals needs more investigation. Perhaps the
incentive of breaking the 1000-pound pumpkin fruit
size barrier (prize money in 1995 was $US 50,000) will
help stimulate the research.
Further work is also needed to compare the capacity of
the cucurbit plant to produce assimilates with the
demands of assimilatory products by the rapidly
growing fruits. Such studies, carried out by Pharr and
co-workers (1985) with cucumbers, need also to be done
with other cucurbit vegeta
bles, to understand the limits of productivity, both
in terms of fruit yields and of fruit quality factors
such as sweetness. It may be that higher yields of
fruits with acceptable soluble solids can only be
achieved by improvements
in the rates of photosynthesis, or more efficient
respiration and translocation
mechanisms. We have much to learn before such
statements can be made
with certainty.
ACKNOWLEDGEMENTS
I am grateful for the helpful suggestions of Drs
Richard Robinson and Scott NeSmith for improvement of
this chapter. Thanks to Dr Martin Goffinet for
providing Fig. 9.2.
REFERENCES
Acock, B., Acock, M.C. and Pasternak, D. (1990)
Interactions of CO Z enrichment and temperature on
carbohydrate production and accumulation in muskmelon
leaves. Journal of the American Society of
Horticultural Science 115, 525-529.
Atsmon, D. and Galun, E. (1960) A morphological study
of staminate, pistillate and hermaphrodite flowers in
Cucumis sativus L. Phytomorphology 10,110-115.
Atsmon, D. and Tabbak, C. (1979) Comparative effects
of gibberellin, silver nitrate and
aminoethoxyvinylglycine on sexual tendency and
ethylene evolution in the cucumber plant (Cucumis
sativus L.). Plant and Cell Physiology 20, 1547-1556.
Bauerle, W.L. (1971) Effect of sudden wilt disease on
the physiology of the muskmelon (Cucumis melo L. var.
reticulatus). Doctoral thesis, Cornell University,
Ithaca NY.
Beyer, E.M. Jr and Morgan, P .W. (1969) Time sequence
of the effect of ethylene on
transport, uptake and decarboxylation of auxin. Plant
and Cell Physiology 10,
787-799.
Beyer, E.M. Jr and Quebedeaux, B. (1974) Parthenocarpy
in cucumber: mechanism of action of auxin transport
inhibitors. Journal of the American Society of
Horticultural Science 99, 385-390.
Bianco, VV and Pratt, H.K. (1977) Compositional
changes in muskmelons during development and in
response to ethylene treatment. Journal of the
American Society of Horticultural Science 102,
127-133.
Bohn, G.W. and Davis, G.N. (1964) Insect pollination
is necessary for the production of muskmelons (Cucumis
melo v. reticulatus). Journal of Apicultural Research
3, 61-63.
Bohn, G.W. and Mann, L.K. (1960) Nectarless, a
yield-reducing mutant character in muskmelon.
Proceedings of the American Society of Horticultural
Science 76, 455-459.
Bruggink, G.T and Heuvelink, E. (1987) Influence of
light on the growth of young tomato, cucumber and
sweet pepper plants in the greenhouse: effects on
relative growth rate, net assimilation rate and leaf
area ratio. Scientia Horticulturae 31,161-174.
Cantliffe, D.J., Robinson, R.W. and Shannon, S. (1972)
Promotion of cucumber fruit set and development by
chlorflurenol. HortScience 7, 416-418.
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