Seeds are a vital component of the world’s diet. Cereal grains alone, which comprise -90% of all cultivated seeds, contribute up to half of the global per capita energy intake. Not surprisingly then, seed biology is one of the most extensively researched areas in plant physiology. Even in relation to the topics reviewed here, a casual perusal of the Agricola database reveals that well over 5000 publications on seed germination and 700 on seed dormancy have appeared in the last decade. Yet we still cannot answer two fundamental questions: how does the embryo emerge from the seed to complete germination, and how is embryo emergence blocked so that seeds can be maintained in the dormant state? Obviously, with such a large literature on the subject, this review is far from comprehensive. Nevertheless, it provides both an overview of the essential processes that are associated with germination and a description of the possible impediments thereto that may result in dormancy. With the seed, the independence of the next generation of plants begins. The seed, containing the embryo as the new plant in miniature, is structurally and physiologically equipped for its role as a dispersa1 unit and is well provided with food reserves to sustain the growing seedling until it establishes itself as a self-sufficient, autotrophic organism. Because the function of a seed is to establish a new plant, it may seem peculiar that dormancy, an intrinsic block to germination, exists. But it may not be advantageous for a seed to germinate freely, even in seemingly favorable conditions. For example, germination of annuals in the spring allows time for vegetative growth and the subsequent production of offspring, whereas germination in similar conditions in the fall could lead to the demise of the vegetative plant during the winter. Thus, dormancy is an adaptive trait that optimizes the distribution of germination over time in a population of seeds. Seed dormancy is generally an undesirable characteristic in agricultural crops, where rapid germination and growth are required. However, some degree of dormancy is advantageous, at least during seed development. This is particularly true for cereal crops because it prevents germination of grains while still on the ear of the parent plant (preharvest sprouting), a phenomenon that results in major losses to the

/pdf/seed-germination-and-dormancy-33s4axu1ho.pdf

Seed Germination and Dormancy.

Aspects of Plant Intelligence

Over the past decades many studies have aimed at elucidating the regulation of seed dormancy and germination. Many hypotheses have been proposed and rejected but the regulatory principle behind changes in dormancy and induction of germination is still a ‘black’ box. The majority of proposed mechanisms have a role for certain plant hormones in common. Abscisic acid and the gibberellins are the hormones most frequently suggested to control these processes. The development of hormone-deficient mutants made it possible to provide direct evidence for the involvement of hormones in germination and dormancy related processes. In the present paper an attempt is made to assess the role of abscisic acid and gibberellins in the transitions between dormant and non-dormant states and germination. First a conceptual framework is presented in which the different states of dormancy and germination are defined in order to contribute to a solution of the semantic confusion about these terms that has existed since the beginning of seed physiology. It is concluded that abscisic acid plays a pivotal role during the development of primary dormancy and gibberellins are involved in the induction of germination. Changes in sensitivity to these hormones occur during changes in dormancy. Both synthesis of and responsiveness to the hormones are controlled by natural environmental factors such as light, temperature and nitrate.

https://link.springer.com/content/pdf/10.1007/BF00024561.pdf

Seed dormancy and germination: the role of abscisic acid and gibberellins and the importance of hormone mutants

Plant growth regulators are involved in major developmental transitions, such as flowering, embryogenesis, or dormancy, and in real-time responses to environmental conditions, such as adjustments in growth rates or stomatal conductance. In addition, it is generally the case that only specific cells or tissues in the plant are responsive to a particular regulatory factor. For example, only certain cells at or near the abscission zone express cellulase when exposed to ethylene (del Campillo et al., 1990); aleurone cells of cereal grains respond specifically to GA by synthesizing a-amylase (Jacobsen and Knox, 1973); stomatal guard cells are dramatically responsive to ABA (MacRobbie, 1991); and transcriptional responses to auxin vary markedly among different cell and tissue types (Gee et al., 1991). The wide range of hormonal responses evident among individual cell types and the different developmental and physiological time scales on which hormones act raise two questions that will be addressed here. (a) How is the variation in hormonal sensitivity or content among cells or tissues integrated into developmental patterns? (b) What is the relationship of hormonal sensitivity or content to the timing of developmental or physiological events? Recently, new insights into these fundamental questions have begun to form into a general model of plant hormone action. Our objective here is to outline a novel conceptual framework that integrates variation in both hormonal sensitivity and content with an expanded view of biological time.

http://www.plantphysiol.org/content/plantphysiol/105/4/1029.full.pdf

Sensitivity Thresholds and Variable Time Scales in Plant Hormone Action.

Secondary dormancy is predominantly associated with seed behaviour in soil seed banks. Periodic changes in secondary dormancy may explain seasonal emergence of weedy species. Temperature and possibly soil water potential appear to be the predominant factors that determine the annual cycling of dormancy. Dormancy cycling is paralleled by modulations of germination responsiveness to germination stimulants, such as light and nitrate and to the width of the germination temperature window. Membranes have been proposed to be the primary target for the perception of temperature. It is hypothesized that alterations in properties of cellular membranes are involved in the regulation of dormancy. Possible mechanisms are discussed.

The regulation of secondary dormancy. The membrane hypothesis revisite

Timing and memory processes in seed embryo dormancy – a conceptual paradigm for plant development questions

Signalling Plant Development

Many a mickle makes a muckle; network maxims may mitigate controversy.

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Timing and memory processes in seed embryo dormancy – a conceptual paradigm for plant development questions

Signalling Plant Development

Many a mickle makes a muckle; network maxims may mitigate controversy.