The Role of Maturation Drying in the Transition from Seed Development to Germination

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Abstract
During mid-development (25–40 d after pollination: DAP) of the castor bean seed the amount of abscisic acid (ABA) increases in both the endosperm and the embryo, declining substantially thereafter until there is little present in the mature dry (60 DAP) seed. Premature desiccation of the seed at 35 DAP also leads to a major decline in ABA within the embryo and endosperm. Partial water loss from the seed at 35 DAP which, like natural and premature desiccation, leads to subsequent germination upon return of the seed to full hydration, causes a much smaller decline in ABA levels. In contrast, ABA declines substantially in the non-dried (hydrated) control at 35 DAP, but the seeds do not germinate. Hence, a clear negative correlation between ABA content and germinability is not observed. Both drying, whether natural or imposed prematurely, and partial drying decrease the sensitivity of the isolated embryo to exogenous ABA by about 10-fold. The protein synthetic response of the castor bean embryo exposed to 0.1 mol m−3 ABA following premature desiccation exhibits some similarity to the response of the non-dried developing embryo—in both cases the synthesis of some developmental proteins is enhanced by ABA, and germination is suppressed. Germination of mature seeds is also suppressed by 0.1 mol m−3 ABA, but the same developmental proteins are not synthesized. In the cotyledons of prematurely-desiccated seed, some proteins are hydrolysed upon imbibition in 0.1 mol m−3 ABA, a phenomenon that occurs also in the cotyledons of similarly treated mature embryos, but not in developing non-dried embryos. Hence the embryo exhibits an ‘intermediate’ response upon rehydration in 0.1 mol m−3 ABA following premature desiccation; viz. some of the responses are developmental and some germinative. Following natural or imposed drying, the isolated embryo becomes relatively insensitive to 0.01 mol m−3 ABA: germination is elicited and post-germinative reserve breakdown occurs in the radicle and cotyledons. The reduced sensitivity of the embryo to ABA as a consequence of desiccation may be an important factor in eliciting the switch to germination and growth within the whole seed.