Front Range Seed Analysts
1993 Seed Forum Volume 7 Number 2
Hard Seeds
The Viability Conundrum by Annette Logan
In our laboratory, we have a monthly gathering called the Journal Club. Participants read a selected article ahead of time and then we all talk about it for an hour or so. Last Friday, the topic was hard seeds. We talked about everything: anatomy, causes, abnormalities and even philosophy of viability. At every turn in the conversation, I learned something new. People have been studying hard seededness for at least 100 years now. I was seeking the depth of my ignorance and I may have found it.
As a novice analyst a few years back, the situation was simple: 1. Some
kinds of seeds have hard seed coats that inhibit water uptake and germination.
2. All such seeds are viable according to AOSA Rules. A couple of years
into my seed testing career I listened to Dr. Nancy Vivrette describe the
radical (to me) notion that not all hard seeds were alive. This was shocking.
After all, if it was in the Rules it was true
wasn't it? Naive no more, I began to read old AOSA proceedings.
I went through waves of paranoia when I signed legume test reports. I even
presented a passionate argument to my supervisor that We should clip EVERYTHING.
Coworkers helped me back from the brink with some solid logic along with
threats of making me do all the legumes.
I have learned that clipping or scarifying is a gamble. one of four things can happen to a clipped seed: 1. normal germination. 2. abnormal germination. The seedling tries to emerge from the artificial hole and mangles itself. 3. the seed rots. 4. the seed swells, but does not germinate due to some other kind of dormancy. Acid scarification was disastrous the two times I tried it. Everything died. I know others have tried it and it works for some seeds but I am reluctant to try it again without more specific protocols in hand. I've also tried cold shocks, heat shocks, soaking and humidifying. You can germinate some of the seeds some of the time but you can't germinate all of the seeds all of the time.
What are the philosophies of hard seed viability? Back in 1940, AOSA had a chart which designated 100% of alfalfa, peas, beans, smooth vetch, crimson clover and Korean lespedeza; 50% of red clover, and 33% of other leguminous seeds were viable. W. 0. Whitcomb in Montana, Anna Lute in Colorado and several other analysts did dozens of studies to determine the field worthiness of hard seeds. Unfortunately, none of them had the benefit of TZ testing to back up their ideas. Occasionally, they would clip or scarify, but frequently questions would remain about whether abnormalities were artifacts or not. The proceedings in 1935 mention the use of an electric pencil to provide a sterile and quick scarification. Have any SF readers tried this? If so, please share your experience. I have tried a hobby drill. A drill is useful for the TZ of large seeds but it caused thick seeded large seeds to rot during the germination.
Some time in the late 1940's AOSA switched its policy to read that all hard seeds were to be considered viable. The reasoning is unclear but it was likely because of the following: 1. Whitcomb's studies, mostly with alfalfa, showed that most hard seeds were viable. 2. Pressure for uniformity of testing methods. 3. Improved conditioning of seed lots. (smaller, immature, yet hard seeds were removed) 4. Increased breeding to remove hard seededness. However, the problem did not go away. Even as late as the 19701s, the Iowa State Seed Laboratory applied percentage rules for hard seed viability to a few small legumes. In the 1980's and 1990's, the question of hard seed viability has come back to haunt us because of the mushrooming use of wild, undomesticated seeds in the revegetation and wildflower industries.
Dr. Nancy Vivrette has a hypothesis that the proportion of normal to abnormal seedlings in the readily germinable portion of the sample can be applied to the hard seeds left over at the end of the test. However, this won't work if 100% of the seeds remain hard. I had a sample of Bonamia grandiflora (an endangered species, convolvulaceae family) which remained 100% hard after two weeks (two replicates had a 24 hour liquid nitrogen exposure prior to germination). I clipped one rep and TZ'd the rest by clipping, imbibing on a moist blotter and immersing seeds in TZ solution. The clipped replicate was difficult to evaluate because of partially decayed cotyledons. I had to determine if the decay was caused by clipping artifacts or by primary disease. The TZ's showed some dead spots that were evidently not caused by clipping. The results showed nearly 50% of the hard seeds were viable. I was very glad I did not report 100% viability. Another discovery was the lack of correlation between viability and floatation (or sinking). I had also tried a hot water treatment (with no effect) and separated the floaters from the sinkers. Not all floaters were bad and not all sinkers were good.
During our Journal Club discussion we discussed the following:
I. Hard seededness is sometimes produced after the seed is shed and
dried down. Germination tests immediately after shedding often show higher
prompt germination than after the seed has been dried down and stored for
a few months. It is possible that mechanical damage or infection could
occur before the hard seed is fully formed. The abnormality or infection
would be "locked in" after the hard seed forms.
2. Hard seededness in cowpeas can be turned "off" or "on" merely by
a temperature change. Could this could be caused by seed coat contraction?
3. The seed coat is maternal tissue and the embryo is the product of
fertilization. since seed coat maturity is independent of embryo maturity,
immature hard seeds are possible.
4. A compromise (for our lab) in the "to clip or not to clip" would
be to clip or scarify samples of wild types, and go by the AOSA rules for
commercial varieties. (NSSL has this flexibility because our lab does not
do testing for labeling purposes. The testing objective at the NSSL is
to estimate viability prior to or during storage. Deviations from AOSA
rules are made to increase accuracy at the expense of lab to lab standardization).
5. Hard seed formation and breakdown is different for every species.
Evolutionarily, hard seededness is a "convergent" characteristic. Even
though Fabaceae, Convolvulaceae, Caryophyllaceae, and other families have
hard seededness in common, it is because the characteristic has adaptive
value not because the families are closely related. The characteristic
evolved independently within these families and thus the seed coat structure
is entirely different. Microscopic features which indicate seedcoat breakdown
will be different for every species. One strategy for breaking hard seededness
will never work for all species.