Date: Wed, 3 Jun 1998 16:36:03 -0400 From: tjw@chos001.cho.ge.com (Tim Williams Ext. 5529) To: cp@opus.hpl.hp.com Message-Id: <aabcdefg1900$foo@default> Subject: Re: Stratification
>>2. The change in the seed is chemical in nature. It is NOT the
>> imbibing of water and oxygen. Water and oxygen reach their max
>> levels in the seed during the first few days.<
>
>
> You miss the point ,you do not get the chemical reaction without the
>intake of water and oxygen. The imbibition of water initiates the enzymatic
>processes that results in the breackdown of stored compounds for their
>mobilization to various sites within the seed. This chemical reaction takes
>more then a few days.
>
>
Steve,
The point I was trying to make is that dormancy in seed requiring
stratification is not because of lack of water or oxygen, but because
an enzymatic reaction must occur. This reaction can only occur in the
presence of water, oxygen, and the correct temperature. Temperature
is required because the protein is only in the active configuration at
temperatures somewhere below 50 F.
There is a very large group of species in which the seeds require only
the uptake of water and oxygen. These are the seed which must be
scarified. Once the seed coat is breached and water uptake begins,
release from dormancy can take as little as several minutes. This is
quite different from stratification where release requires a chemical
modification of the seed and not just the uptake of water and oxygen.
No chemical change occurs in seed requiring scarification.
The limiting factor in release from dormancy by stratification is the
time required for the reaction to occur, 2 to 3 months. If you see
release from dormancy in shorter times, it is not due to stratification,
but some other mechanism.
Given the evidence that freezing overnight is as good as prolonged
cold treatment for Sarr., I put forth the proposal that Sarr. seed do
not need stratification, but rather that they need scarification -
a fundamentally different dormancy mechanism. Stratification requires
2 to 3 months, any shorter time has no effect. Scarification is
accomplished by freezing overnight. And you are right, freezing
overnight does nothing for seeds that require stratification.
>
>Try telling that to species with epicotyl dormancy.
>
No one said this was simple. Species with epicotyl dormancy have at
least two dormancy mechanisms that must be satisified sequentially
and in the proper order. And epicotyl dormancy is not the most
complicated mechanism known.
Tim Williams
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