Viable seeds that do not imbibe water and thus fail to germinate in an apparently favorable environment are commonly termed impermeable or hard seed. This physical, exogenous dormancy is especially common in species of the Fabaceae. The ecological significance of hard seed includes the ability to rapidly recolonize burnt areas after fire and to withstand ingestion by animals and birds. Advantages and problems that hard seed cause in agriculture are discussed. Species from different families with impermeable seeds appear to have in common a layer of macrosclerid cells that form a palisade layer in the testa. The term strophiole and its contradictory use in botanical literature are discussed. Genetic factors and environmental conditions both affect the proportion of impermeable seeds produced. Methods of artificially softening impermeable seeds include acid and solvent, soaking, mechanical scarification, pressure, percussion, freezing, heating, and radiation treatments that can result in a change in germination from less than 20% in some untreated species up to 90% or more in treated species. Natural softening involves high temperatures and temperature fluctuations and the degree of desiccation of the seed. The mechanism of water impermeability is related to the testa and is thought to involve waterproofing substances including wax, lignin, tannin, suberin, pectin, and quinone derivatives. The hilum acts as a hygroscopic valve that prevents water uptake but allows water loss to occur at low relative humidities in some species. The strophiole is an area of weakness in the testa of some Papilionoideae while the chalaza region has been determined as an area of weakness inPisum andGossypium. The water impermeable status of some species is reversible at a seed moisture content greater than 10%. The hard seed of a species can be described both in terms of the amount and the degree of impermeability.

Water impermeable seed dormancy

(1) Effects of seed size and seedling morphology on the establishment of six monocarpic perennials were examined in glasshouse experiments. Both withinand between-species comparisons of seed-size effects were made as seed weight varied by more than two orders of magnitude among species and by 3 to 20-fold within a species. Experiments were conducted in four ground-cover types: bare soil, litter cover, vegetated, and vegetated plus litter. (2) In vegetated cover, emergence of the two small-seeded species, Verbascum thapsus and Oenothera biennis, was significantly lower than in litter and bare soil. In contrast, emergence of the medium (Dacus carota and Dipsacus sylvestris) and large-seeded species (Tragopogon dubius and Arctium minus), was not significantly reduced in the presence of vegetation. Although the rate of emergence of all six species was significantly reduced in vegetated cover, there were no between species differences in seedling emergence rates in any cover type. (3) Relative growth rates of all six species were significantly lower in vegetated cover compared with litter and bare soil and the effect was greatest on the small-seeded species. At the end of the experiment, seedling weight in vegetated cover was positively correlated with seed weight. In non-competitive cover types (litter and bare soil), seedling weight was independent of initial seed weight. (4) Relative growth rates of seedlings in non-competitive cover types were inversely related to seed size. In bare soil and litter, the small-seeded species had relative growth rates twice those of the large-seeded species. In vegetated cover this pattern was reversed; the species with large seeds had the highest relative growth rates. (5) The growth form of a seedling did not have any effect on its probability of establishment in any cover type. Species with different growth forms and similar seed sizes had equal emergence, survival, and relative growth rates in all four cover types. (6) Within species differences in seed size had a significant effect on seedling growth in non-competitive cover, but had no effect on seedling growth in competitive cover. Thus, both withinand between-species differences in seed size had similar effects on seedling establishment success in different types of ground cover.

Effects of seed size and growth form on seedling establishment of six monocarpic perennial plants

Endosperm has been studied from a variety of vantage points: evolution, role in seed development and germination, genetics, physiology, and biochemistry. This tissue represents a renewable, biodegradable source of materials; much effort has been directed to improve its use in feed and food making as well as its refinement to secondary products such as oils and plastics. Although there is a vast literature dealing with each of these topics, we still have a remarkably superficial understanding of most of them. There has been revitalized interest in understanding the endosperm in relation to seed-specific developmental processes. lnformation from these studies could provide a basis for developing more efficient approaches for plant improvement and use. Recent advances in molecular biology have created the possibility for detailed study of many of the genetic and molecular mechanisms involved in endosperm development. This research could conceivably lead to answers to many basic questions in developmental biology as well as to new tools that enhance practical uses of endosperm. In this review, it is not our intention to present a comprehensive overview of what is known about endosperm. Rather, we have chosen to summarize some of the research that has been done, making note of comprehensive reviews and pointing out important questions that remain open to scientific inquiry.

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Endosperm Origin, Development, and Function

To investigate the role of stored and neosynthesized mRNAs in seed germination, we examined the effect of α-amanitin, a transcriptional inhibitor targeting RNA polymerase II, on the germination of nondormant Arabidopsis seeds. We used transparent testa mutants, of which seed coat is highly permeable, to better ascertain that the drug can reach the embryo during seed imbibition. Even with the most permeable mutant (tt2-1), germination (radicle protrusion) occurred in the absence of transcription, while subsequent seedling growth was blocked. In contrast, germination was abolished in the presence of the translational inhibitor cycloheximide. Taken together, the results highlight the role of stored proteins and mRNAs for germination in Arabidopsis and show that in this species the potential for germination is largely programmed during the seed maturation process. The α-amanitin-resistant germination exhibited characteristic features. First, this germination was strongly slowed down, indicating that de novo transcription normally allows the synthesis of factor(s) activating the germination rate. Second, the sensitivity of germination to gibberellic acid was reduced 15-fold, confirming the role of this phytohormone in germination. Third, de novo synthesis of enzymes involved in reserve mobilization and resumption of metabolic activity was repressed, thus accounting for the failure in seedling establishment. Fourth, germinating seeds can recapitulate at least part of the seed maturation program, being capable of using mRNAs stored during development. Thus, commitment to germination and plant growth requires transcription of genes allowing the imbibed seed to discriminate between mRNAs to be utilized in germination and those to be destroyed.

https://hal.archives-ouvertes.fr/hal-01508727/document

The Effect of α -Amanitin on the Arabidopsis Seed Proteome Highlights the Distinct Roles of Stored and Neosynthesized mRNAs during Germination

Ethylene is an important component of the gaseous environment, and regulates numerous plant developmental processes including seed germination and seedling establishment. Dormancy, the inability to germinate in apparently favorable conditions, has been demonstrated to be regulated by the hormonal balance between abscisic acid (ABA) and gibberellins (GAs). Ethylene plays a key role in dormancy release in numerous species, the effective concentrations allowing the germination of dormant seeds ranging between 0.1 and 200 μL L(-1). Studies using inhibitors of ethylene biosynthesis or of ethylene action and analysis of mutant lines altered in genes involved in the ethylene signaling pathway (etr1, ein2, ain1, etr1, and erf1) demonstrate the involvement of ethylene in the regulation of germination and dormancy. Ethylene counteracts ABA effects through a regulation of ABA metabolism and signaling pathways. Moreover, ethylene insensitive mutants in Arabidopsis are more sensitive to ABA and the seeds are more dormant. Numerous data also show an interaction between ABA, GAs and ethylene metabolism and signaling pathways. It has been increasingly demonstrated that reactive oxygen species (ROS) may play a significant role in the regulation of seed germination interacting with hormonal signaling pathways. In the present review the responsiveness of seeds to ethylene will be described, and the key role of ethylene in the regulation of seed dormancy via a crosstalk between hormones and other signals will be discussed.

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Ethylene, a key factor in the regulation of seed dormancy.

Knowledge of the mature seed and its germination is of importance in the study of seed formation Indeed, embryogeny and germination are extensions of each other separated by a period of relative metabolic inactivity called quiescence, and are essentially different phases of the continuing process of embryo growth and development Obviously, the nature of the pre-quiescence embryo bears very heavily on early germination behaviour and its control, and answers to questions relating to germination behaviour are increasingly being sought during the period of seed development

J. V. Jacobsen

Papers

The Seed: Germination