This review examines the presence of free and bound PAs during the life cycle of seeds. Although data are still rather scarce, PAs, present, but with an unknown function during embryogenesis, are accumulated in relatively high quantities in mature dry seeds of some species. PA concentration can be altered by the process of stratification, and PAs are synthesized in growing organs after germination, but it is also possible that the reserve organs synthesize and export some types of PA to the axis. This metabolism and sink-source relationship depend on the germination conditions and on the greater or lesser presence of enzymes related to synthesis (ADC, ODC, LDC and AdoMetDC) and degradation (DAO and PAO) of free PAs. The biochemical mechanism binding the PAs to a series of compounds of low and high molecular weight is currently unknown in seeds. The present study also considers intra- and intercellular compartmentation of free and bound PAs in growing axes and the physiological implications of the ethylene-production pathway in relation to seed behaviour.

Features of chemicals which break seed dormancy and the mechanisms of their action are discussed with reference to results obtained with red rice (Oryza sativa) and other relevant research. The significance of chemical structure, including the nature and location of functional groups, is considered, and the fact that similar structures can elicit different biochemical responses is illustrated with examples. As yet it is unclear whether changes in cell pH, highly correlated with the uptake of, and metabolism to, weak acids, and which some consider a possible trigger or marker in dormancy breaking, are causative, consequential or merely correlative. Uptake and metabolism of dormancy-breaking compounds is, or can be, very rapid and the importance of establishing the time course of dormancy-breaking, as opposed to germination events, is emphasized.

The role of free radical-induced damage as a cause of loss of vigour in seeds is by no means resolved. In this contribution, the effects of environmental treatments known to reduce viability rapidly were compared with the effects of long-term, low-temperature storage on germination, hypocotyl growth and free radical accumulation and lipid peroxidation in soybean seeds. Accelerated aging was achieved by incubating seeds at 35°C and 1% relative humidity over H2SO4 for up to 69 days in the light and in darkness. In contrast, seeds under long-term storage were maintained at 5°C and 6% moisture content in darkness for up to 6 years. At 35°C there were rapid and significant reductions in rates of seed germination and hypocotyl extension. Loss of viability and declining vigour were associated with increases in lipid peroxidation and free radical build-up but the latter, surprisingly, was largely confined to the testa rather than the cotyledon. Exposure to light greatly enhanced lipid peroxidation and increased organic free radical accumulation in the translucent testas of seeds, but not in the cotyledons. Similar responses to light were recorded in testas detached from seeds. These results show that in soybean the testa is a significant locus of free radical degenerative events induced by high temperature combined with low moisture.

Among the forage legumes somatic embryogenesis has been most studied in alfalfa (Medicago sativa L.) reflecting inherent breeding problems and the high demand for seeds of this species. Current methods of somatic embryogenesis are reviewed with emphasis on each of the 3 stages involved: the acquisition of competence, the induction of somatic embryogenesis, and somatic embryo development and maturation. The role of plant growth regulators is particularly stressed. Artificial seed production, the development of reduced-generation synthetics or hybrids and genetic transformation of alfalfa are considered. Research on somatic embryogenesis and plant transformation in a wide range of other forage legumes is examined but it is suggested that the application of artificial seed technology is unlikely in forage legumes other than alfalfa.

The effects of temperature on dormancy loss, germination and viability were investigated in seeds of Aesculus hippocastanum L. harvested over a 4-year period. Release from embryo dormancy in freshly harvested seeds was manifest in two phases of morphological growth: initially, when the seed lot was only partially released, axis emergence resulted primarily from cotyledonary petiole extension without radicle extension; subsequently, when the seed lot was totally released, axis emergence of all seeds was followed immediately by extension to >1 cm through growth of the radicle. Germination (axis emergence and radicle extension) at 16°C was a function of pre-treatment period at 2–11°C. The rate of dormancy loss (probit germination d−1) increased linearly below a ceiling temperature for the chilling response; this temperature was estimated to vary from 13°C to 16°C for two seed lots harvested in separate years. Dormancy periods for individual seeds within both seed lot populations can be described by cumulative normal distributions; the predicted standard deviation of chilling units below the ceiling temperature (i.e. thermal time) was 186°C d. Visible germination occurred during the process of stratification at 2°C, starting after 21–25 weeks. By contrast, three years of hy-drated seed storage at 16°C, which was a non-permissive temperature for dormancy loss, resulted in little pre-emergence of the axis during stratification; approximately one third of the seeds remained germinable. The implications of these quantitative analyses of the physiological processes in recalcitrant seeds for the development of improved storage methods are discussed.

Intact grains of barley (Hordeum distichum cv. Triumph) germinated rapidly in the dark or when exposed to brief daily light breaks in the temperature range 15–25°C, although germination proceeded less rapidly at low temperatures. Prolonged illumination (16 h/day) or continuous light inhibited germination of grains most effectively when conditions were less than optimal for germination, i.e. under low temperatures, restricted water, and low water potential induced by mannitol. Embryos isolated from grains responded in a similar way to light applied when the amount of water was restricted; water stress did not affect germination percentages in the dark or after one brief light break (3 min) but appreciably reduced the rate of germination. While light was only slightly inhibitory to embryos under optimal germination conditions, its inhibitory effect was intensified when other factors tending to delay germination were present. Water restriction also caused ABA contents in embryos to rise. Under conditions of water stress (water restriction or low osmotic potential), CaCl2 enhanced the inhibitory effect of light; however, addition of LaCl3 reversed this effect. The nature of light-induced inhibition of germination is discussed.