BIOPHYSICAL, PHYSIOLOGICAL AND BIOCHEMICAL PROCESSES REGULATING SEED-GERMINATION

In the physiological sense, germination begins with seed water uptake and ends with the initiation of elongation by the embryonic axis, usua lly the radicle. The driving forces and constraints on expansion by th e embryo are examined, particularly for seeds in which the embryo is s urrounded by endosperm and testa tissues that restrict growth. Models have been developed to predict germination based on thermal time, hydr otime and combined hydrothermal time. These population-based models in dicate that the timing of germination is closely tied to physiological ly determined temperature and water potential thresholds for radicle e mergence which vary among individual seeds in a population. The restra int imposed by tissues surrounding the radicle is a major determinant of the threshold water potential. Enzymatic weakening of these tissues is a key event regulating the timing of radicle emergence. Considerab le evidence suggests that endo-beta-mannanase is involved in this proc ess in a number of species, although it is doubtful that it is the sol e determinant of when radicle emergence occurs. Molecular and biochemi cal studies are revealing the complexity of events occurring in endosp erm and embryo cells associated with the completion of germination. Un ique permeability properties and the presence of enzymes associated wi th pathogen resistance suggest additional functional roles for the tis sues enclosing the embryo. The insights gained from physiology and mod elling are being extended by the application of molecular techniques t o identify and determine the function of genes expressed in associatio n with germination. Single-seed assay methods, in vivo reporters, spec ific modification of gene expression and mutagenesis will be critical technologies for advancing our understanding of germination.