Physiological and Molecular Aspects of the Control of Dormancy and Germination in Developing Sorghum Caryopses

摘要

Preharvest sprouting is a major constraint for the production of sorghum (Sorghum bicolor) in the humid Pampa of Argentina. This crop matures during the end of summer and the beginning of the autumn, when environmental humidity is high; hence, susceptible genotypes are prone to sprouting. Nevertheless, variability exists for this trait: among white, tanninless sorghums, some are characterized as sprouting-susceptible and others as sprouting-resistant. Since the early 1990s, progress has been made towards the elucidation of the basis of preharvest sprouting resistance in sorghum by utilizing two genotypes with contrasting sprouting behaviour as an experimental system. The Redland B2 variety is susceptible to preharvest sprouting, while the IS 9530 variety is resistant. Following the acquisition of germination capacity throughout grain development of these two varieties, it was shown that developing seeds of the sprouting-susceptible Redland B2 acquire the capacity to germinate more rapidly, i.e. 20-25 days after pollination (DAP), than grains from sprouting-resistant IS 9530 (40-45 DAP) (Steinbach et al., 1995) (Fig. 2.1). However, when isolated from the entire grain and incubated in distilled water, the embryos of both varieties are able to germinate very rapidly from early stages of development (15-20 DAP). These results show that the inability to germinate during seed development is the result of dormancy imposed by the presence of the seed coat tissues (i.e. endosperm plus pericarp) that surround the embryo. This coat-imposed dormancy is the barrier preventing untimely germination.