Kinetics of dormancy release and the high temperature germination response in Aesculus hippocastanum seeds

The kinetics of primary dormancy loss were investigated in seeds of horse chestnut (Aesculus hippo-castanum L.) harvested in four different years. Freshly collected seeds from 1991 held for up to 1 year at temperatures between 2 degrees C and 42 degrees C exhibited two peaks in germination (radicle growth), representing a low temperature (2-8 degrees C) and a high temperature response (31-36 degrees C), Germination at 36 degrees C generally occurred within 1 month of sowing, but was never fully expressed in the seedlots investigated. At low temperatures (2-8 degrees C), germination started after around 4 months. Generally, very low levels of germination were observed at intermediate temperatures (11-26 degrees C). Stratification at 6 degrees C prior to germination at warmer temperatures increased the proportion of seeds that germinated, and the rate of germination for all seedlots. Within a harvest, germination percentage (on a probit scale) increased linearly with stratification time and this relationship was independent of germination temperature (16-36 degrees C), However, inter-seasonal differences in the increases in germination capacity following chilling were observed, varying from 0.044 to 0.07 probits d(-1) of chilling at 6 degrees C, Increased sensitivity to chilling was associated with warmer temperatures during the period of seed filling. The estimated base temperature for germination, T-b for newly harvested seeds varied slightly between collection years, but was close to 25 degrees C. For all seedlots, T-b decreased by 1 degrees C every 6 d of chilling at 6 degrees C. This systematic reduction in T-b with chilling ultimately facilitated germination at 6 degrees C after dormancy release.