Seed Maturation, Germination, and Dormancy

For most plants the transition from seed to seedling is the transition from the stage of the life cycle most impervious to the stage most susceptible to the external environment. The latter phase of seed maturation, at least for most seeds (orthodox; Roberts, 1973), is largely a preparation to survive maturation-desiccation (Harada, 1997) which permits attainment of the quiescent state resistant to environmental perturbation and which makes seeds such excellent propagules. Desiccation is one of the most potent switches signalling the cessation of seed development and their entry into the germinative programme upon cellular rehydration (Bewley, 1995); this switch can be by-passed in certain aberrant circumstances (Harada, 1997) and, by definition, does not exist in recalcitrant seeds. The preparation for anhydrobiosis is a complex physiological event superimposed on a system already awe inspiring in its intricacy. Between 12,000–18,000 genes are expressed in the cotyledons, embryonal axes, and endosperm of a typical seed (Galau and Dure, 1981; Goldberg et al., 1981; Pernollet and Vailant, 1984). Although approximately 85% of these genes are expressed during various phases of seed development (Goldberg et al., 1981), this still leaves a considerable number of genes whose expression may control the events of seed maturation. There is some evidence that mechanisms having positive impact on seed longevity during the desiccated state and germinative vigour upon rehydration are put in place during this last phase of seed development. Upon maturation-desiccation, seeds can lie quiescent for a considerable period. Upon being provided with water, favourable temperature, oxygen, and in some cases, light, seeds can imbibe, increase their metabolism remarkably, and ultimately complete germination by protruding a portion of the embryo (typically, the radicle) through the seed covers. Many parallels exist across kingdoms regarding these attributes. The germination phase of the life cycle is enigmatic. There are usually no distinguishable changes in seed appearance or measurable metabolic events that are absolutely indicative of the completion of germination at some point in the future. The difficulty in identifying, a priori, which seeds will and which will not complete germination at some later point is compounded by the vast increase in metabolism that accompanies water uptake, even in seeds that will not complete germination due to dormancy. At least a part of the metabolic activity of seeds during germination is due to an integrated natural repair mechanism functioning to ameliorate some of the damage incurred by extreme desiccation and prolonged periods under unfavourable environmental conditions. Additional energy may be expended in up-regulating seed defence mechanisms to ward off invasive fungi and bacteria. In some seeds, the germinative phase is extended beyond the time necessary to repair cellular damage, prepare the embryo for elongation, and for eventual weakening of the seed covers to facilitate radicle protrusion. Such seeds are termed dormant and require additional time to either eliminate the inhibition, or synthesize/become sensitive to stimuli prompting radicle protrusion. Dormancy is a mechanism to delay the completion of germination of progeny to minimize catastrophic seedling mortality due to capricious environmental events. Hence, dormancy maximizes the probability of successful transfer of maternal genetic material to future generations.