This review focuses mainly on eudicot seeds, and on the interactions between abscisic acid (ABA), gibberellins (GA), ethylene, brassinosteroids (BR), auxin and cytokinins in regulating the interconnected molecular processes that control dormancy release and germination. Signal transduction pathways, mediated by environmental and hormonal signals, regulate gene expression in seeds. Seed dormancy release and germination of species with coat dormancy is determined by the balance of forces between the growth potential of the embryo and the constraint exerted by the covering layers, e.g. testa and endosperm. Recent progress in the field of seed biology has been greatly aided by molecular approaches utilizing mutant and transgenic seeds ofArabidopsis thalianaand theSolanaceaemodel systems, tomato and tobacco, which are altered in hormone biology. ABA is a positive regulator of dormancy induction and most likely also maintenance, while it is a negative regulator of germination. GA releases dormancy, promotes germination and counteracts ABA effects. Ethylene and BR promote seed germination and also counteract ABA effects. We present an integrated view of the molecular genetics, physiology and biochemistry used to unravel how hormones control seed dormancy release and germination.

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Reactions regulating seed dormancy can proceed at water contents which are probably too low to permit metabolic activity. The loss of dormancy via afterripening of red rice. (Oryza sativa L.) seeds was examined as a representative case. Equilibration of seeds to various moisture contents showed that afterripening was most rapid at 6–14% moisture content (dry weight basis). Afterripening did not occur at > 18% moisture content and was severely inhibited at < 5% moisture content. Seed viability was greater than 95% for all treatments. Utilization of moisture isotherms to calculate water‐binding enthalpy values identified the optimal afterripening range as approximately the boundary between water‐binding region 1 and region 2. From these findings, it is suggested that afterripening may involve some oxidative reactions which are inhibited at lower water contents by the rising free‐energy and at the higher side by metabolic reactions.

Seed shattering is an important trait that distinguishes crop cultivars from the wild and weedy species. The genetics of seed shattering was investigated in this study to provide insights into rice domestication and the evolution of weedy rice. Quantitative trait locus (QTL) analysis, conducted in 2 recombinant inbred populations involving 2 rice cultivars and a weedy rice accession of the southern United States, revealed 3-5 QTLs that controlled seed shattering with 38-45% of the total phenotypic variation. Two QTLs on chromosomes 4 and 10 were consistent in both populations. Both cultivar and weedy rice contributed alleles for increased seed shattering. Genetic backgrounds affected both QTL number and the magnitude of QTL effects. The major QTL qSH4 and a minor QTL qSH3 were validated in near-isogenic lines, with the former conferring a significantly higher degree of seed shattering than the latter. Although the major QTL qSH4 overlapped with the sh4, the presence of the nonshattering single nucleotide polymorphism allele in the weedy rice accession suggested involvement of a linked locus or an alternative molecular genetic mechanism. Overlapping of several QTLs with those from earlier studies indicated that weedy rice may have been derived from the wild species Oryza rufipogon. Natural hybridization of rice cultivars with the highly variable O. rufipogon present in different geographic regions might be responsible for the evolution of a wide range of phenotypic and genotypic variabilities seen in weedy rice populations worldwide.

The effect of postharvest storage temperature (–15, 5, 20, and 30 C) on seed dormancy of red rice [Oryza sativa L. ‘strawhulled (SH)’], an annual species, was studied. Intact and dehulled (by hand) seeds were dormant at harvest. Intact seeds were nondormant (greater than 90% germination at 30 C) after dry storage at 20 or 30 C for 4 weeks after harvest. In intact seeds stored dry at 5 C, the degree of dormancy was dependent upon postharvest exposure time at 20 C prior to storage. Dormancy of intact seeds decreased at 5 C as this initial postharvest exposure to 20 C was increased from 2 to 7 days. This effect of initial 20 C exposure was independent of seed moisture content (11 to 12%). After dry storage of intact seeds at 5 C, dehulling promoted germination. Germination of such dehulled seeds increased with increasing storage time at 5 C up to 11 months when complete germination occurred. The response of seeds dehulled immediately after dry storage at 5 C was independent of prior exposure time (2 to 7 days) at 20 C. Storage at −15 C prevented all forms of dry-afterripening for 1 yr.