Date: Wed, 16 Oct 1996 12:28:22 -0400
To: SEED-BIOLOGY-L@cornell.edu
Sender: aak1@nysaes.cornell.edu (Anwar Khan)
Subject: Seed dormancy
Points raised by Dr. Keys
You have raised some very interesting points which I would like to expand further for those interested in the differences between germination and dormancy. In addition I would take this opportunity to expound my own thinking on the subject. We seem to be on the same wavelength on several issues relating to seed dormancy.
Dormancy induction/release.
Unlike germination, dormancy induction/release occurs under a wide variety of conditions, including those that inhibit germination. There are several examples. a) You have pointed to dormancy release by moist chilling at 5C (nongerminating condition) in many seeds. In addition, there are numerous examples of annual dormancy/nondormancy pattern in weeds in response to germinating and nongerminating field temperatures b) Many light requiring seeds (lettuce, Rumex, Chenopodium) imbibed in low water potential (nongerminating conditions) media in darkness become dormant and the dormancy is prevented by irradiation and GA c) Dormancy in lettuce seeds is induced at 35C (nongerminating condition) in darkness and prevented by irradiation or GA. d) Dormancy is released in Rumex by a brief high temperature (40C) shift applied during dark imbibition, making the seeds germinable in darkness. e) Taylorson's work on anaesthetics in releasing dormancy and other 'drastic measures' which promote dormancy release. Unlike dormancy induction/release, radicle protrusion does not occur in most seeds imbibed at at 5C, 35C, 40C or in low water potential (e.g., -1.2MPa) media. In Chenopodium, dormancy can be released by moist-chilling at 5C and these seeds again become dormant on transfer to -1.2 MPa PEG solution.
Primary role of GA in dormancy release.
It seems to me that GA, together with irradiation and moist-chilling are the primary dormancy releasing agents. Dormancy induction/release is a reversible process, unlike germination. Germination is incidental to dormancy release or is simply a confirmation that seeds are no longer dormant. Your studies with lettuce on CO2 and ethylene and our studies with cytokinins and ethylene showed that germination could occur under stressful conditions but GA or irradiation must be provided in order to prevent dormancy induction, thus maximizing germination. We continue to believe that cytokinins and ethylene are stress alleviating agents and work on germination while GA works on dormancy release for which germination is not essential. The studies of Toyomasu et al. (1993) and their more recent study on the characterization of gibberellin responsive genes (Biosci. Biotech. Biochem. 59: 1846-1849, 1995) are consistent with our findings that inhibition of GA synthesis may be related to dormancy induction or maintenance (GA synthesis inhibitors induce dormancy in many seeds).
Increase/decrease in expansive force of the embryo.
It seems to me, this is central to dormancy release/induction. No doubt many internal and external factors interact with embryo in developing and reducing the expansive force. In seeds such as beans, carrot and tomato (and other solanaceous types and many cultivated species), GA or irradiation have little or no role in generating the expansive force of the embryo; dormancy (if any)is determined largely by the embryo covers, their weakening (by action of GA mediated enzymes) or mechanical disruption is associated with improved rate of germination. Thus, there is a fine line in separating dormancy release from germinability and a danger of mistaking germinability for dormancy release (which is essentially the function of the changes in the embryo) in cases where embryo plays a rather passive role and dormancy is due to covering structures. In seeds such as lettuce, embryo is more positively involved in dormancy regulation and a distinction between germination and dormancy release is clearly warranted.
Then, there is the question of seed integrity, which appears to be essential for embryo function. I am constantly reminded of the remarks of JD Watson in this context "that the normal environment of a cell from a multicellular organism is the intact organism, and that when we remove a cell from its normal cellular companions, we may so alter it that it is unable to function in the way that interests us". To stretch the analogy to seeds, it appears that the behavior of the embryo changes when the integrity of the seed is compromised by separating the embryo from the covering structures.
Anwar Khan
NYSAES, Cornell University
Geneva, NY 14456
FAX: 315-787-2320
E-Mail: aak1@nysaes.cornell.edu
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Date: Wed, 16 Oct 96 18:38:00 PDT
To: SEED-BIOLOGY-L@cornell.edu
Sender: "Bradford, K. J." (kjbradford@ucdavis.edu)
Subject: RE: Comments on dormancy
Jack:
Responses to your questions inserted into the text below:
Kent Bradford
>
>
>
>Here are some random comments and questions about the recent seed
>dormancy discussion.
>
>The first is directed to Kent, mostly due to my ignorance. I provide
> two of your statements with questions:
>
>1. "The water potential threshold at which germination is blocked is the
>operational way that the germination capacity is defined and quantified,
>but the value of the threshold also determines the speed of germination
>for seeds at any water potential, including pure water."
>I am not sure what you mean here. Do you mean there is an actual
>physiological (or physical) impediment to continued growth, or is the block
>metaphorical?" I would guess in my foxtail seed that are imbibed (but
>won't germinate) that water limitations are not blocking germination, but
>that some other inhibitory factor that results in water utilization is the
>problem. Or have I got this all muddled?
>
It is helpful to think of hydrotime in analogy to thermal time. The base temperature is the minimum temperature at which germination will occur, and can be quantified by germinating seeds at a range of temperatures to see what the minimum temperature is. However, the base temperature is still pertinent even when germination occurs at higher temperatures, since it is the reference temperature above which thermal time (degree days) is calculated (e.g., (T-Tb) x time = thermal time). In the same way, the base or threshold water potential defines the minimum water potential at which germination will occur, and can be determined by germinating seeds at a range of water potentials. However, it is relevant to germination at any water potential above that (including pure water) because it is the reference against which hydrotime is accumulated (e.g., (Y-Yb) x time = hydrotime). Since the total hydrotime required for germination is a constant, as the water potential increases above the threshold, the time to germination decreases in proportion. Thus, even when germination is occurring in pure water and the seeds are fully hydrated, their physiological water potential threshold still determines their behavior, even though water is not limiting, just as the base temperature determines germination rate even at the optimum temperature.
>2. "So I view dormancy as a case where the seed has physiologically
>increased its threshold to the point that emergence won't occur at whatever
>water potential the seed is at, even 0 MPa or full imbibition."
>Same query: do you mean actual resistance to water utilization or do you
>use "water threshold" here metaphorically (as in a reflection of other
>non-water related processes)? Is prevention of water use for growth the
>cause of, or just correlated with, dormancy? Are you implying water
>utilization thresholds are the mechanism of dormancy in seeds? Or have I
>got this all muddled?
>
If you look at the equation ((Y-Yb) x time), it makes no difference whether the seed water potential is reduced or the threshold is increased; in either case, germination will be slowed because the difference between Y and Yb is less and the time to radicle emergence will be increased. What I am suggesting happens with dormancy is that the value of Yb shifts up or down depending upon the dormancy state. If the threshold (Yb) increases to equal the external Y, then the time becomes infinite, or germination does not occur. If the water potential is 0 MPa, and the threshold is also 0 MPa, this means that germination does not occur on water under those conditions, which is the operational definition of dormancy. The threshold value is a physiological parameter under the control of the seed, i.e., it can increase or decrease in response to environmental (temperature, light, nitrate, etc.) or hormonal (GA, ABA, etc.) influences. This corresponds to the ability of seed dormancy status to change independently of whether it actually germinates or not. The threshold value determines the probability that a seed will germinate (and how fast) for a given seed water content. The value of Yb is the net result of the various factors that influence the ability of the embryo to grow through whatever tissues may be in its path, if there are any. Since the act of germination requires growth, which requires water for cell expansion, I think that water is important in the process, but its availability is obviously not the controlling factor, as dormant seeds don't germinate even when given plenty of water, as you noted.
What I like about this model is what I have called its "ecological rationality." Ecological factors act on the seed to shift its threshold, which determines whether and how fast the seed will germinate under a given condition. However, by acting through a water potential difference, whether germination actually occurs is intimately tied to the current water availability to the seed. Since water is the most critical factor in seedling survial, this mechanism allows the seasonal and climatic factors to act on the physiological propensity for germination (i.e., the threshold), while maintaining an immediate sensitivity to the most important factor in its current environment. If either the threshold is too high (some dormancy) or the water potential is too low (drought), germination is slowed or prevented, and the seeds have the chance to wait for a better opportunity. Even a seed that has fully satisfied its environmental requirements (i.e., has lost dormancy) needs to accurately sense its current environment for the probability of survival.
>
>Jack Dekker, 3214 Agronomy Hall
>Iowa State University, Ames, Iowa 50011
>TEL: (515)294-8229; FAX: (515)294-3163
>E-Mail: (jdekker@iastate.edu)
>
Kent Bradford
University of California at Davis
Davis, California USA
E-mail: "Bradford, K. J." (kjbradford@ucdavis.edu)
**************************************************
Date: 19 Oct 1996 13:20:23 -0500
To: SEED-BIOLOGY-L@cornell.edu
Sender: "Mike Foley" (foley@btny.purdue.edu)
Subject: Dormancy discussion
I have enjoyed the discussion on dormancy and regret my teaching load prohibits me from adding my comments (at hit time) as I view dormancy/germination from a physiological and genetic perspective. However, since we are covering Division Anthophyta ("flowering plants") in my Intro Botany class right now, I couldn't let Jack's comment on where seeds arise from go by (mother plant versus parental plant).
Parental plant may be politically correct but technically less than precise. The seed arises from the female gametophyte after fertilization (union of the sperm and egg) of the egg in the embryo sac by one of the male gametes. The zygote develops into the embryo, and the embryo, etc. is eventually enclosed in the seed coat and pericarp which were part of the ovule (integuments) and ovary wall in the megasporophyll (pistil) of the sporophyte generation. The issue to me is male versus female gametophyte rather than male or female sporophyte (or perfect or imperfect flowers).
I will have to ask the ladies and gentlemen in my course what they think (or maybe use it as an extra credit question).
Michael Foley
Department of Botany
Purdue University
West Lafayette, Indiana USA
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Date: Sat, 19 Oct 1996 12:30:46 -0400
To: SEED-BIOLOGY-L@cornell.edu
Sender: aak1@nysaes.cornell.edu (Anwar Khan)
Subject: Seed dormancy
To all on the list of Seed-L.
I thought you all should hear this very sad news. The passing away of a brilliant seed dormancy researcher. Hence I am transmitting the message from Greg Lang I just received.
Anwar Khan
NYSAES, Cornell University
Geneva, NY 14456, USA
Dear Colleagues,
It is my regret to pass along the sad information, of which I just became aware, that Dr. William J. VanDerWoude, one of the excellent speakers (Role of Phytochrome in Seed Dormancy) at the 1st International Symposium on Plant Dormancy, passed away this past summer due to complications during liver surgery. Bill was just 53 years old. His passing not only creates a void in his challenging area of plant dormancy research, but also robs the scientific community of a genuinely pleasant and thought-provoking colleague. It is a further tragedy that Bill was unable to finish preparation of his Symposium contribution for inclusion in the book, due not to his own health problems at the time, but to those of his wife who I believe was undergoing treatments for cancer in the year following the Symposium.
As for the book, CAB International finally sent me an invoice for the participant copies, so I presume it will be in the mail to everyone before the month is out.
Best regards,
Greg Lang