Date: Mon, 30 Sep 1996 14:27:41 -0500 (CDT)
From: jdekker@iastate.edu (Jack Dekker)
To: rlo1@cornell.edu (Ralph L Obendorf)
Subject: Dormancy definition
Hi Seed People,
I have enjoyed the recent activity on the list, with special warm
thanks to Dr. Proton on explaining the pH hypothesis in seeds and dormancy
relief. The copious lit cites are great.
I also appreciated the questions and answers about defining
"thermodormancy". Sometimes I get very confused with all the definitions
seed people use. When I discuss seed dormancy with my students I sometimes
(partially in jest) define "seed dormancy" as the biology of what isn't.
By this I mean we define such an important set of phenomena in terms of
what we don't see (absence of germination). My question is how do we
define dormancy in a positive way, about what happens? Any takers?
Jack
Jack Dekker, 3214 Agronomy Hall
Iowa State University, Ames, Iowa 50011
TEL: (515)294-8229; FAX: (515)294-3163
E-Mail: (jdekker@iastate.edu)
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Date: Tue, 1 Oct 1996 08:36:43 -0400
To: SEED-BIOLOGY-L@cornell.edu
Sender: aak1@nysaes.cornell.edu (Anwar Khan)
Subject: Dormancy definition
Jack:
A simple definition of seed dormancy which may not satisfy everyone is as follows:
"A physiological state which prevents the embryo to generate an expansive force sufficient to remove the barrier(s) to embryo growth. This definition takes into account dormancy due to both reduced embryo growth potential as well as the barrier effect of embryo covering structures.
I hope this helps.
Anwar Khan
Dept. of Hort. Sci
NYSAES, Geneva, NY 14456
USA
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Date: Tue, 1 Oct 1996 11:40:11 bst
To: SEED-BIOLOGY-L@cornell.edu
Sender: "A J Murdoch, Reading University" (A.J.Murdoch@reading.ac.uk)
Subject: Dormancy definition
As a start, why not define "non-dormancy" positively??!! [The ability of a seed to germinate given air (ie oxygen), moisture and a suitable temperature for seedling growth]
Dr A.J. Murdoch, Department of Agriculture, The University
Earley Gate, PO Box 236, READING RG6 6AT, U.K.
TEL.: +44 (0)118 931 6746. FAX: +44 (0)118 931 8297
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Date: Sun, 06 Oct 96 18:41:00 PDT
To: SEED-BIOLOGY-L@cornell.edu
Sender: "Bradford, K. J." (kjbradford@ucdavis.edu)
Subject: Dormancy
I can weigh in briefly on the topic of definitions of dormancy. I can agree with Anwar Khan's operational definition related to embryo growth and mechanical restraints, since we know the dormancy state of an individual seed only by virtue of when and whether it actually completes germination (i.e., protrusion of the embryo through the covering layers). Embryo protrusion requires cell expansion, which requires water uptake, so I think it is reasonable to begin to quantify dormancy status in terms of the water potential thresholds required to stop embryo protrusion. This term includes both the embryo growth potential and the restraint of the enclosing tissues.
We have developed a relatively simple model that can account for the continuously variable nature of dormancy, its variation among members of a seed population and in response to environment, and the germination patterns evident during the imposition or release of dormancy. The model also assumes that seeds can shift their threshold water potential for germination (ie., either the growth potential or the restraint or both) in response to environmental conditions. Thus, our approach can also accommodate most of the points made by Dr. Vleeshouwers. Our approach suggests that there can be a range of 'dormancy' even among seeds that eventually germinate and also among the seeds that do not germinate (it is useful to review the paper by Gordon, 1973, The rate of germination, in Seed Ecology ed. by Heydecker on this point). This variation can be quantified in terms of the distribution of water potential thresholds, which has been shown to shift positive or negative as expected in response to environmental or hormonal influences, with predictable consequences for germination patterns. Thus, while we cannot determine the dormancy status of an individual seed until it actually germinates, we can characterize (via extrapolation from the model) the entire seed population as to its dormancy state even if only a relatively small fraction of the seeds actually completes germination. The model and its application to dormancy are described in detail in my chapter on Water relations in seed germination in the book Seed Development and Germination edited by J. Kigel and G. Galili (1995; published by Jack's namesake, Marcel Dekker). A discussion of the model specific to dormancy will appear in the book on Seed Dormancy edited by Greg Lang which will appear this fall (CAB, 1996).
To add a different twist to the discussion, a consequence of the hydrotime model is that one can also think of dormancy as altering the 'clock' by which a seed is keeping time. That is, as the conditions for 'breaking dormancy' are met (chilling, light, etc.), each individual seed becomes increasingly likely to complete germination in a shorter and shorter time (corresponds also with widening of the 'requirements' for germination). Alternatively, the deeper the dormancy, the longer it will take for germination to be completed, if it is completed at all (c.f. paper by Gordon cited above). This reciprocal time relationship is central to the hydrotime model, and suggests that an 'elastic' view of time from the seed's point of view, stretching or contracting in response the environmental signals it is receiving, might be a constructive way to conceive of dormancy. Experimental data from Bill Finch-Savage and Phil Allen and their coworkers seem to confirm that time can start or stop or shrink or stretch depending upon the environmental conditions (particularly water and temperature). This is, of course, just the thermal time or heat sums concept expanded to include other factors such as water, hormones, nutrients, light, etc., as all influencing seed germination behavior by modifying a biological clock. Thus, it is possible to conceive of all imbibed seeds as progressing toward their final objective of germinating, but the degree of dormancy either shrinks or stretches the time that it takes for them to achieve this objective. This is not necessarily a 'positive' concept of dormancy, but it is at least neutral.
The idea can be encapsulated in a couplet for you to consider: "Time runs at different rates for seeds in different states."
Kent Bradford
University of California at Davis, USA
kjbradford@ucdavis.edu
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Date: Mon, 7 Oct 1996 13:26:59 -0500 (CDT)
To: SEED-BIOLOGY-L@cornell.edu
Sender: jdekker@iastate.edu (Jack Dekker)
Subject: Dormancy and biology
The discussion about seed dormancy has sparked some very interesting comments from several of you and I would like to comment, as well as suggest some ideas about dormancy from my foxtail (a weed; Setaria spp.) research.
Anwar's definition of dormancy (physiological state preventing embryo expansion) sounds good as far as it goes. But what of hard seeded dormancy as in velvetleaf (a weed; Abutilon theophrasti) or tomato? Hard seededness seems not physiological, but mechanical restraint. What of factors promoting germination and/or germinability? Dormancy ("the biology of what isn't") fails to bring in the concept of tissues both promoting growth and inhibiting growth. In our foxtail work we find instances of both embryo envelope inhibition and promotion. We dissect seeds and perform germination assays on seed, caryopses (hull removed) and embryos (hull, endosperm removed). In most instances the hull prevents germination, in others the caryopsis tissues inhibit. But in other instances, those in which the naked isolated embryo fails to germinate, other embryos with the same history are led to germinate when surrounded by caryopsis tissues. It is appears from this that germinability is a consequence of the physiology, and physical aspects, of the several interacting tissues.
Leo has some excellent comments about dormany also. That seed dormancy is a continuous phenomena is true for our foxtails. His definition (seed characteristic defined by the environmental conditions that must be met to cause germination) is good as far as it goes. But it implies the embryo is somewhat passive, awaiting only the right conditions: dormancy defined by factors outside the seed. What of biology? Noel addressed this issue when he queried about the peach seedling that has slowed growth and produced a dwarf or rosette. In our foxtail work we observe a range of responses in seed tissues, many in which growth proceeds but becomes arrested in the middle of germination; in others a very slow rate of germination (vigor?). Related to Leo's definition is the original question about defining thermodormancy (and also osmotic controlled dormancy): defining dormancy in terms of what conditions it takes to cause germination. Dormancy defined in terms of the dose of GA, or anesthetics, or herbicides, all fall into this mode of defining seed dormancy. What of biology?
Kent has suggested a very exciting conceptual approach with his ideas about hydrotime. His concepts of looking at seed dormancy as a population phenomena are also very interesting and help explain much of the variation we see. I am uncomfortable with his comment that "we know the dormancy state of a seed only by virtue of when and whether it actually completes germination" In a strict sense he is correct for an individual seed. On a population level we have been able to quantify a wide array of germinability states in highly dormant foxtail seed. Careful control and characterization of the seeds (environmental) history is critical. By assaying the array of germination states in each of a foxtail seed's three compartments we are able to model the dormancy states of the population under those specific historical and environmental conditions. By array of germination states I include germination arrested at some stage prior to seedling growth, axis-specific germination (germination of only the shoot, only the root), abnormal germination states (e.g. scutellar germination), and a rich array of combinations of these.
Another confusion I have with defining germination in terms of environment stems from Kent's hydrotime comments about water uptake requirements. Foxtail seed imbibe water freely. With this weed species is may not be "reasonable to quantify dormancy status in terms of its water potential thresholds required to stop embryo protrusion". In his species this works, in others that freely imbibe yet remain dormant, maybe not.
We also have developed a model, or the beginnings of a model, from our foxtail work. I have been profoundly influenced in our detailed seed behavior observations by some concepts from Simpson (Seed dormancy in grasses) and from Trewavas. The former inflamed by imagination about the idea of interacting parts of a seed leading to either synchrony of action or asynchrony. The later author's influence by his analogy of seed dormancy with memory: that a seed is at all times a reflection and accumulation of its past history, its past environmental experiences. Foxtail seed consist of 3 compartments: embryo, embryo within caryopsis, embryo within caryopsis and hull. These 3 compartments each arise from a separate genome: parental tissues, zygote endosperm (3N) and zygote embryo (2N). It is an illusion to see the this seed as a unit, and individual organism. It is an interacting group of tissues, each of which has undergone separate selection over evolutionary time. This difference in selection and adaptation has led to different strategies of germination regulation for each compartment. The individual compartments act independently of each other. When all three are germinable, the seed as a unit becomes more likely to germinate. If one or more compartments has reduced germinability, dormancy results. From this model, and extensive empirical evidence, germination can be defined by the synchronous germinability of the embryo within each of the 3 compartments; and that dormancy can be defined by the asynchronous germinability of the embryo within each of these 3 compartments. A more complete description can be found in:
Dekker, Dekker, Hilhorst and Karssen. 1996. Weedy adaptation in Setaria spp.. IV Changes in the germinative capacity of S. faberii embryos with development from anthesis to after abscission. Amer. J. Botany 83(8):979-991.
Advocating for the devil,
Jack
Jack Dekker, 3214 Agronomy Hall
Iowa State University, Ames, Iowa 50011
TEL: (515)294-8229; FAX: (515)294-3163
E-Mail: jdekker@iastate.edu
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Date: Thu, 10 Oct 1996 13:51:52 -0400 (EDT)
From: Bill Ebener (104076.363@compuserve.com)
To: SEED-BIOLOGY-L@cornell.edu
Subject: dormancy
all,
have been "following" this great thread - please continue - would like to share all comments presented on this topic with a group of ca seed analysts - particularly interested in the discussions on thermodormancy as it might relate to lettuce production in the desert. your comments have been greatly appreciated - a tremendous learning resource!!! thanks!!!
bill ebener
harris moran seed company
po box 3091
modesto ca 95353 usa
p. 209-549-5255
f. 209-526-6436
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Date: Thu, 10 Oct 1996 14:12:03 -0400 (EDT)
From: "Bradford, K. J." (kjbradford@ucdavis.edu)
To: SEED-BIOLOGY-L@cornell.edu
Subject: Dormancy
Response to Jack Dekker's comments:
I have enjoyed the discussion you sparked, and your recent comments. Just
wanted to respond to your comment that the water potential threshold
concept might not apply to seeds that quickly and fully imbibe. In fact,
all of the seeds we have worked with (except wild rice) also imbibe
quickly and fully. 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.
Also, the model doesn't care whether the water potential is decreased, or
the threshold is increased--either one can prevent radicle emergence. 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. As you point
out, it will require the coincidence of thresholds for all the tissues
involved in order for germination to actually be completed.
Kent Bradford
University of California at Davis, USA
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Date: Mon, 14 Oct 1996 09:02:19 -0400 (EDT)
To: SEED-BIOLOGY-L@cornell.edu
Sender: aak1@nysaes.cornell.edu (Anwar Khan)
Subject: Seed dormancy
To all "dormancy" enthusiasts
There is inherent difficulty in defining dormancy as correctly pointed out by Jack and others and I am delighted that it has generated some good ideas. I believe that the definition "a physiological state which prevents the embryo to generate an expansive force sufficient to remove the barriers to germination" is a good interim definition of dormancy. I also agree with Kent that "the shifts in threshold water potential" may reflect the dormant state. To understand dormancy, however, it will be essential to disassociate dormancy release/induction from germination or its inhibition. I believe that germination is simply a confirmation that seeds are no longer dormant. It does not tell us anything about the underlying causes of dormancy release. Induction/release of dormancy occurs under a wide variety of conditions, both stressful and nonstressful, while germination requires a very narrow set of conditions.
Research on understanding and quantifying dormancy must focus on the underlying hormonal, cellular or molecular pregerminative changes in seeds. A recent discovery with Grand Rapids lettuce seeds (a favorite material for the dormancy enthusiasts) that GA1 is synthesized upon red light irradiation (which releases dormancy) prior to radicle protrusion and is inhibited in absence of irradiation (which maintains the dormant state) (Toyomasu et al., Light effects on endogenous levels of gibberellins in photoiblastic lettuce seeds. J. Plant Growth Regul. 12: 85-90, 1993) may be a good beginning effort in this direction.
A workshop on "Quantification of Dormancy" was held last year at the ASHS conference in Montreal and the proceedings will be appear shortly in HortScience.
Anwar Khan
NYSAES, Cornell University
Geneva, NY 14456 USA
Fax: 315 787 2320
Tel: 315 787 2247
E-mail: aak1@nysaes.cornell.edu
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Date: Mon, 14 Oct 1996 09:06:41 -0400 (EDT)
To: SEED-BIOLOGY-L@cornell.edu
Sender: leo vleeshouwers (Leo.Vleeshouwers@STAFF.TPE.WAU.NL)
Subject: dormancy
Dear Jack,
A response to your comment that the definition of dormancy I gave "implies
the embryo is somewhat passive, awaiting only the right conditions: dormancy
defined by factors outside the seed".
This is not what I wanted to say. Rather the contrary.
In my view, dormancy does not depend on factors outside the seed. At least not on the present environment of the seed. Dormancy is an internal factor, a seed characteristic. However, whether or not a seed responds by germinating is dependent on its present environment (together with its degree of dormancy: do the two match or not?).
In order to germinate, seeds do not passively await the right conditions. They continuously adjust their degree of dormancy, as a reaction to the environment they experience. This goes back to conditions experienced during seed development and maturation at the mother plant. Free after Trewavas: the dormancy state of a seed is at all times a reflection and accumulation of its past history, its past environmental experiences.
In popular words: seeds buried in the seed bank continuously and actively adjust how choosy they are as to the conditions in which they will germinate. Thus, they play an important role in the timing of their own germination. This to avoid germination at a seemingly favourable time, when germination is possible, but not survival to reproduction.
Leo Vleeshouwers
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Date: Mon, 14 Oct 1996 10:52:06 -0500 (CDT)
To: SEED-BIOLOGY-L@cornell.edu
Sender: jdekker@iastate.edu (Jack Dekker)
Subject: Comments on dormancy
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?
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?
For Leo, I agree with you, as I did before. A new student of mine came up with a thought last summer at the pub that has captured my imagination. The student's background is in physics. After I tried to explain our 3 compartment model, along with some of our work looking at annual germinability cycling in the seed bank, he compared seeds to vectors. That each seed can be seen at any instance in time as having a direction (toward germination, toward dormancy) and a momentum (getting there quickly, going real slow). What environment (or GA, or moisture, or any external influence) does is change the seed's direction and momentum. The result will always be a reflection of the seed's starting direction and momentum as it reacts with the new external force. Maybe this relates to some of Henk Hilhorst's ideas he explained to me about different sensitivities of seeds to similar external influences at different times in a seed's life cycle resulting in different seed responses (e.g. at one time increased temperature causes one affect, at another time a different affect).
Lastly, mother plant may be sexist. Many plant flowers are perfect and their seeds arise from the parental plant.
Jack
Jack Dekker, 3214 Agronomy Hall
Iowa State University, Ames, Iowa 50011
TEL: (515)294-8229; FAX: (515)294-3163
E-Mail: (jdekker@iastate.edu)
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Date: Mon, 14 Oct 1996 15:06:18 -0400 (EDT)
To: SEED-BIOLOGY-L@cornell.edu
Sender: rdkeys@unity.ncsu.edu
Subject: Re: Seed dormancy
> To: SEED-BIOLOGY-L@cornell.edu
> Sender: aak1@nysaes.cornell.edu (Anwar Khan)
> Subject: Seed dormancy
>
> To all "dormancy" enthusiasts
Well it seems we are coming out of the woodwork, so to speak..... and that is nice. Consider it a feature of this electronic whizbang era. We are ``enthusiasts''!
> There is inherent difficulty in defining dormancy as correctly pointed out
> by Jack and others and I am delighted that it has generated some good
> ideas. I believe that the definition "a physiological state which prevents
> the embryo to generate an expansive force sufficient to remove the barrers
> to germination" is a good interim definition of dormancy.
Interim..... hmmm, probably so.
I sense, from the flow in the group, that we need to somewhat rethink dormancy, not only in a simplistic sense, where a simple definition will suffice, but also in a more complex sense, where a more defined set of criteria need to be considered.
For example, we might consider dormancy to be the switch that turns on or off processes leading to germination. Yet, that simplistic approach is inadequate in that it represents too instantaneous a picture frozen in time. And time, relatively speaking, is a very important aspect of dormancy.
Another post very enlighteningly pointed out the non-instantaneous aspect or fluidity found in dormancy, as relating to changes in growth substances that allow dormancy to be overcome. Such fluidity can be rather complex in that there may be many different factors, the sum of which together determine the position of that switch on or off. This is a time factor. Seasonal periodicities, as for example, in weed seeds would be important.
Somehow our definition needs to handle both sets of circumstances --- a) the instantaneous on/off and b) the fluidity over time, to be what I would consider a more rigorous definition of dormancy.
Perhaps our ponderings here will help to do that.
> I also agree with
> Kent that "the shifts in threshold water potential" may reflect the dormant
> state. To understand dormancy, however, it will be essential to
> disassociate dormancy release/induction from germination or its inhibition.
I would agree on the separation of dormancy and germination, in the strictest sense. Yet, they must be considered together in many instances, since at least some of the preparatory work of imbibition, prior to any germination will be done during dormancy. We probably should not assume that dormancy shuts down everything, but that only a few key processes may be stopped. I would be of the opinion that only in the case of a hard seed coat, where no uptake of water would have occurred, would the preparatory work not have been done, and all processes were stopped. In other cases, where some water uptake did occur, then the levels of water present might have effect on or reflect the dormancy state.
> I believe that germination is simply a confirmation that seeds are no
> longer dormant. It does not tell us any thing about the underlying causes
> of dormancy release. Induction/release of dormancy occurs under a wide
> variety of conditions, both stressful and nonstressful, while germination
> requires a very narrow set of conditions.
Good points. By the time we can measure something indicative of germination having begun or having taken place, dormancy is a thing of the long-gone past. But, does not the modulation of dormancy also expand the window of permitted conditions under which germination may occur? Thus, what would be a narrow set of germination conditions for nondormant seed in, for example, the effect of temperature, would be widened for seed which had been given a treatment to overcome dormancy. I see this in the Amaranthus retroflexus seed that I am working with, when normally some percentage of the seed will germinate at a given temperature. Yet, when alternating temperature of a few cycles is given or a few wetter/drier cycles of imbibition are given, the seeds will germinate over a much wider range of temperatures, and to a higher percentage of germination. In this case, the set of conditions had been widened. In other cases, such as the afterripening of tree seeds, the seeds would not be expected to germinate at the near-frozen temperature of afterripening. In this latter case, the release of dormancy occurs outside the set of conditions suited to germination. Perhaps as we gain insight into the particular biochemistry associated with dormancy release this will be resolved.
> Research on understanding and quantifying dormancy must focus on the
> underlying hormonal, cellular or molecular pregerminative changes in seeds.
> A recent discovery with Grand Rapids lettuce seeds (a favorite material for
> the dormancy enthusiasts) that GA1 is synthesized upon red light
> irradiation (which releases dormancy) prior to radicle protrusion and is
> inhibited in absence of irradiation (which maintains the dormant
> state)(Toyomasu et al., Light effects on endogenous levels of gibberellins
> in photoiblastic lettuce seeds. J. Plant Growth Regul. 12: 85-90, 1993) may
> be a good beginning effort in this direction.
Most interesting. I will have to scurry to the stacks and check this out. Back in earlier days (1972), we never seemed to have the right methodology nor the right bathtub chemistry to do this well. A lot of speculation was the end result. If GA1 was synthesized upon red light irradiation, then the next question would be what did far-red light do after the red light treatment. If synthesis to a certain level was required over time, then a far-red light treatment might be expected to suppress the GA1 synthesis up to a point in time, but then after that point in time dormancy release and subsequent germination would be committed to go. The best we could do back then was note that ethylene plus carbon dioxide plus gibberellin plus kinetin maximized release from dormancy. In our hands, the effect was such that all the hormones were required, to at least some degree, and that ethylene and carbon dioxide maximized the effect. Without ethylene and carbon dioxide, the effect of gibberellin was nil. It was great coffee-break fun to speculate how the combination of growth regulators were associated in the breaking of dormancy. Several workers have reported the association of light and gibberellin action, or substitutions or similar effects. Perhaps Toyomasu, et al's work will shed some light on this, in the GA aspect. Yet, remains cytokinins, ethylene, and carbon dioxide in the picture, before our understanding is complete. It seems we are still chasing that balanced mix of magic hormones to overcome dormancy. But, little by little we gain the knowledge.
> A workshop on "Quantification of Dormancy" was held last year at the ASHS
> conference in Montreal and the proceedings will be appear shortly in
> HortScience.
Interesting. Noted.
> Anwar Khan
> NYSAES, Cornell University
> Geneva, NY 14456 USA
> Fax: 315 787 2320
> Tel: 315 787 2247
> E-mail: aak1@nysaes.cornell.edu
Robert D. Keys
Seed Research
Department of Crop Science
NC State University
Raleigh, NC 27695-7620.
919-515-4071