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Re: Dormancy


Robins,

Norm Lesch has made a GREAT observation and told us about it in his post
of this AM.He pointed out that only SOME hosta seeds germinated under
normal warmth and moisture but that more within the same pod population
DID germinate after 4 weeks of refrigeration. Ran made similar
observations about his experiences in the past. I am not surprised at
these findings. In fact, I am delighted to hear of these careful
observations.

According to Plant Propagation, Principles and Practices by Hartman and
Kester, a classic Horticultural Textbook, plant physiologist/seed
technologist separate seeds into five Groups based upon their response
characteristics to  environmental conditions and handling methods used
to achieve germination. They are as follows:

   Group I... Seeds with hard, moisture impermeable coats preventing
water and gas absorption by the embryo.

   Group II...Seeds with dormant embryos that respond to chilling to
achieve germination (Several cold/warm cycles may be necessary for
germination).

   Group III...Seeds with a combination of impermeable seed coats and
dormant embryos.

   Group IV...Seeds containing chemical inhibitors

   Group V...Seeds that are dormant at harvest and become able to be
germinated upon dry storage. Dormancy may last only a few day,
disappearing with dry storage treatment followed by moisture and warmth.

Superimposed on these characteristics of seeds are a series of optimum
environmental conditions necessary  for germination, i.e. optimum
temperatures,light, moisture, depth of planting, planting media physical
characteristics, freedom from disease, humidity control, etc. I have
observed that hosta seed, if dried and stored for a short period of time
usually germinates well, both in darkness and in the presense of light.
The rate of germination (its rapidity) is often correlated to
temperatures. At high temperatures (85 degrees, for example, germination
is often less than a week. At 70 to 75 degrees germination usually
occurs in less than two weeks. The viability percentage of hosta seed is
not uniform because of the many variables that may exist. Among these
variables are genetic factors and all of the horticultural skill
variables.

As I try to understand what Norm has observed, I think that the genetic
variables are probably  more significant than we have formerly believed.
If seed within the same pod respond differently in terms of germination
rate and germination percentages because of response to vernalization,
then there must be differences due to genetic make-up within the seed in
the same pod. I am assuming that the cultural practices were not faulty.

I have always considered hosta seed to fall in the Group V category
because most seed seemed to usually germinate well  soon after drying
and a brief period of storage. Now it seems from Norm's observation that
SOME seeds may fall in the Groups II requiring chilling and Group IV,
seeds that have inhibiting chemicals. Dormancy is known to be due to
presence of Abscisic Acid (ABA), a natural plant hormone. Its presence
may be considered inhibiting to germination, thus perhaps identified as
being in Group IV. Chilling is believed to be the environmental
condition necessary to destroy the ABA in plant tissues... during the
winter dormancy period. Perhaps some seeds within a pod possess more ABA
than others because their genetic make-up is different. Perhaps colder
temperatures for longer periods of time are required to destroy the ABA
within such seeds. All of this reasoning is speculative of course, but
even this is necessary to try to understand what may be happening.  

Thanks, Norm and Ran for your observations. Perhaps we need to modify
our seed germinating methods somewhat as suggested by Beth
Arnold....when many seeds don't germinate, take out those seedlings that
have emerged and put the container into a cold environment for another
month. That's what I plan to do if I find anything "special" not
germinating.

Jim Hawes



As I try to understand what Norm has observed, I think that the genetic
variables are more significant than we have formerly believed. If seed
within the same pod respond differently in terms of germination rate
and  germination percentages because of   response to vernalization,
then there must be differences due to genetic make-up within the seed in
the same pod.

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