Water status changes during development in relation to the germination and desiccation tolerance of Aesculus hippocastanum L. seeds

Seed growth characteristics of Aesculus hippocastanum were examined in detail during development from about 70 to 140 d after anthesis (DAA), mainly in 1988 and 1989. Mean fresh and dry weights increased for both the axis and the whole seed up to the time of peak seed fall at 135 DAA with no cessation before fruit abscission. Water per seed increased up to 100 DAA, after which no further increase occurred: moisture content declined for the embryonic axis and whole seed respectively from above 75 and 65% at 95 DAA to 65 and 50% at 130 DAA. At fruit shedding in 1990 water potential values of -1.2, -2.6 and -1.1 MPa were observed for the testa, cotyledon and axis tissues respectively; relevant sorption isotherms are presented. Decreases in seed moisture content during development were accompanied by increases in desiccation tolerance and in germinability, both reaching their maximum at the time of peak seed fall. At maturity, only about 10% viability was retained on drying seed to 20% moisture content: it is confirmed that the seeds are 'recalcitrant'. The exact relationship between moisture content and germination during development was dependent on the depth of dormancy, as judged by the period of chilling required; each duration of chilling at 2 degrees C within the range 3-12 weeks yields a curve of sigmoid shape. No germination occurred at 26 degrees C without chilling, but nearly full germination can be observed for samples collected at 6 weeks before maximum seed fall with 12 weeks chilling. The rate of moisture loss during desiccation at 15 degrees C and 15% rh becomes reduced during development. The ontogeny of these 'recalcitrant' seeds is compared with that of 'orthodox' seeded species and the implications of sigmoid-shaped curves for the relationship between seed moisture content and germination are considered.