Genetics..
Lots of pages... May have to scan this again.
The Cucurbits: Cucumber, Melon,
Squash and Pumpkin
H.C. Wien
Department of Fruit and Vegetable Science, Cornell
University, 134A Plant
Science Building, Ithaca, New York 14853-5908, USA
© CAB INTERNATIONAL 1997. The Physiology o f Vegetable
Crops
The species in the family Cucurbitaceae that have been
used as vegetables have enriched and diversified the
diets of humankind for many centuries. For example,
cucumber (Cucumis sativus) is consumed as an
ingredient in salad, or in the pickled form. The fresh
fruits of muskmelon or cantaloupe (Cucumis melo) and
watermelon (Citrullus lanatus) are eaten as desserts,
while the fruit flesh of the squashes and pumpkins is
generally consumed after boiling or baking. The
immature fruit of summer squash or marrows (Cucurbita
pepo), and the mature fruit of all the principal
squash and pumpkin species (C. argyrosperma, C.
moschata, C. maxima, and C. pepo) are harvested and
used as food. Shoot tips of the cucurbit vegetables
are eaten as a cooked vegetable in Southern and
Eastern Africa. Seeds of watermelon and some squash
and pumpkin species are roasted and eaten as snacks,
or ground as an ingredient of sauces. Several
cultivars of C. pepo with hull-less seeds have been
developed that facilitate the food uses of the seeds.
With an oil and protein content of 46 and 34%,
respectively, these could be exploited as alternative
oil-bearing crops (Whitaker and Davis, 1962). In North
America, pumpkins and gourds (Cucurbita pepo) are
frequently grown solely for the ornamental value of
the fruits, in harvest displays and as part of the
Halloween celebration.
The major cultivated cucurbits can be classified into
new world and old world species with regard to their
origin. Cucumbers are thought to have been first
cultivated in India, where their use has been recorded
as long as 3000 years ago (Whitaker and Bemis, 1976).
Cucumis melo is thought to have arisen in the central
part of Africa, and have spread rapidly into Asia,
where many cultivars have since been selected. The
watermelon originated in the dry parts of southern
Africa, and also has South Asia as a second centre of
diversity. The new world species include all the
cultivated Cucurbita. Although the four most
economically important species may have had a common
ancestor in one location in the Americas, such
evidence is difficult to find. Each of these species
has been selected separately, so that interbreeding of
different species is for the most part difficult to
achieve. From archaeological records, the Cucurbita
species are amongst the most ancient of cultivated
crops in the Americas. Indeed, squash was one of the
principal components of the diet of the ancient Mayan
civilizations, together with beans and maize, dating
back as far as 10,000 years (Whitaker and Bemis,
1976).
At the present time, the cucurbit vegetables are
cultivated in all major regions of the world (Table
9.1) (FAO, 1994). Watermelon leads the production
figures both in terms of tonnage and land area (1.8
million ha). Production in Asia comprises half or more
of the total area devoted to each cucurbit vegetable
worldwide.
Our knowledge of the physiology of the cucurbits has
not grown in proportion to their importance in
production. Most effort has been devoted to cucumber,
followed by Cucumis melo, and watermelon in third
place. As in the case of Capsicuni (Chapter 7),
detailed understanding of the growth and productivity
of cucumber has come recently from the need to
optimize production in greenhouse environments. In
addition, intensive investigations into the hormonal
control of flower sex expression during the 1960s and
1970s facilitated the production of hybrid cultivars
of cucumber and squash. More recently, interest in
developing melon cultivars of better fruit quality has
spurred research efforts in fruit carbohydrate
metabolism. Altogether, the state of our physiological
knowledge of the cucurbit crops can be described as
rather uneven, with detailed understanding in some
areas, and others virtually unexplored. The latter is
particularly true of watermelon, squash and pumpkins,
perhaps due to the fact that their sprawling plant
habit make them difficult experimental subjects in
greenhouse, growth chamber and field.
Table 9.1. Production of the major cucurbit vegetables
in the world in 1993 (production: 1000 metric tons)
(FAO, 1994).
Region Watermelon Cucumber Melons Pumpkins and
squash
Asia 15,746 12,761 7726 3552
Europe 2447 2563 2367 1184
NC America 2069 971 1696 428
Africa 2036 400 878 926
South America 994 59 233 768
World 27,063 18,326 12,976 8019
The
GERMINATION AND SEEDLING GROWTH
The germination of cucurbit vegetable seeds requires
relatively warm temperatures (Lorenz and Maynard,
1980), and takes place within 3 or 4 days at 25-30°C.
For cucumber, the lower limit of germination has been
shown to be 11.5°C (Simon et al., 1976), while
germination of muskmelon and watermelon was low at
16°C (Nelson and Sharpies, 1980). Summer squash
(Cucurbita pepo) germination showed a lower threshold
between 5 and 10°C, and optimum germination between 30
and 35°C (NeSmith and Bridges, 1992). Considerable
effort has been expended on improving the capability
of cucumber seeds to germinate at low temperatures.
Soaking the seed in acetone solutions of fusicoccin
(0.5 mmol), or GA4+7 (a mixture of gibberellins 4 and
7) (1 mmol), was the most effective in stimulating
germination at 12°C (Nelson and Sharpies, 1980).
Similar growth regulator treatments had no significant
effect on rate or final percentage emergence in six
field plantings into cool soils, however (Staub et
al., 1987). Imbibing or pregerminating cucumber seeds
in water at 32°C before planting them in soil at 15°C
also did not shorten the time of emergence, although
plant stand was improved significantly (Staub et al.,
1986). These results indicate that several steps of
the germination and emergence process are limited by
low temperatures. Simon et al. (1976) conjectured that
the low temperatures may cause a denaturation of
proteins in the germinating seedling, resulting in
more severe damage the longer the exposure.
Considerable genetic differences exist in cucurbits in
capacity to germinate at low temperatures (Schulte and
Grote, 1974; Nerson et al., 1982; Nienhuis et al.,
1983). After four cycles of selection for germination
at 15°C, Nienhuis et al. (1983) improved low
temperature germination from 32 to 94%. This increase
was also correlated with improved germination at
higher temperatures. In muskmelon, Nerson et al.
(1982) showed 'bird's nest' cultivars developed in
Iran to have significantly better cold temperature
germination than standard viny and dwarf cultivars
developed in the USA. We do not know yet what
physiological processes differ among lines of either
species contrasting in cold temperature germination.
The low germination rate of cucumber at 15°C may be
partly due to seed dormancy (Nienhuis et al., 1983).
Freshly harvested seed failed to germinate at this
temperature, and remained dormant until it had been
stored for 84 days. Watts (1938) had also encountered
this phenomenon in 'Black Diamond' cucumber, and was
able to overcome the inhibition by removing the
seedcoat, or germinating the seed at 30°C. The
presence of a germination inhibitor in the testa was
further supported by the finding that soaking cucumber
seed in acetone significantly improved cold
temperature germination (Nelson and Sharpies, 1980).
These workers also showed that watermelon seed
germination at 16°C could be improved by washing the
seed for 2 h in water. While dormancy may play a role
in the reduced germination of fresh cucumber seed at
low temperatures, it must be emphasized that poor
germination under cool soil conditions is also a
problem with cucurbit seed that has passed the
apparent rest period.
Another approach to overcoming the growth-inhibiting
effects of cold soils is to graft cucumber onto
rootstocks of species less susceptible to cold root
temperatures (den Nijs, 1980a). This approach, now
widely practised in Japan and Korea, where about 80%
of greenhouse cucumbers are grafted, allows cucumber
and other cucurbits to be grown without soil heating.
Another benefit from the practice is the avoidance of
problems of soil-borne diseases, to which the
rootstock is not susceptible, and the improvement in
fruit quality that comes with the use of certain
rootstocks (Lee, 1994). More particularly with regard
to root growth in cold soils, Tachibana (1982, 1987)
found that Cucurbita ficifolia, one of the most common
rootstocks used for cucumber, maintains active growth,
water and nutrient uptake at 12 and 14°C (Fig. 9.1).
At these temperatures, these processes are sharply
reduced
120
110
100
°_.., 90 .v-~. _
80 0 v Fig-leaf gourd
3ucu r
ucu
0 70 .:. Cuc/gourd
---------------------------------------------------------------------
Pumpkin-growing FAQ: http://www.mallorn.com/lists/pumpkins/search.cgi
To sign-off this list, send email to majordomo@mallorn.com with the
message text UNSUBSCRIBE PUMPKINS
