Seed development and maturation inPhaseolus vulgarisI. Ability to germinate and to tolerate desiccation

The onset and development of both the ability to germinate and to tolerate rapid enforced desiccation were investigated during the development and maturation of seeds of bean (Phaseolus vulgahs L.) at different temperatures and also after different slow-drying treatments. The onset of germinability occurred when seeds were less than half-filled in the absence of both a post-ovule abscission programme and water loss from the seeds. Maximum ability to germinate normally and maximum tolerance to rapid enforced desiccation to 14–16% moisture content did not occur until 2–23 d and 6–23 d after mass maturity (end of the seed-filling period), respectively. The slow-drying of immature seeds for 7 d ex planta before rapid enforced desiccation increased the ability to germinate and stimulated the onset of desication tolerance. Holding seeds moist for 7 d (during which time moisture content declined by <5%) had similar effects, but seed germination after rapid enforced desiccation was consistently greater in seeds first dried slowly than held moist. Comparisons between seeds less than half-filled dried slowly ex planta and full seeds undergoing maturation drying in planta showed that a similar (slow) rate of water loss over a 7 d period had a similar effect on the subsequent ability of seeds to tolerate rapid enforced desiccation. Thus, neither a post-ovule abscission programme nor loss of water were required for the onset of the ability to germinate in developing bean seeds, but both were required for the development of the ability to germinate and resistance to solute leakage, when rehydrated, after rapid enforced desiccation.