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

The kinetics of primary dormancy loss were investigated in seeds of horse c hestnut (Aesculus hippo-castanum L.) harvested in four different years. Fre shly collected seeds from 1991 held for up to 1 year at temperatures betwee n 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 temperat ure response (31-36 degrees C), Germination at 36 degrees C generally occur red within 1 month of sowing, but was never fully expressed in the seedlots investigated. At low temperatures (2-8 degrees C), germination started aft er around 4 months. Generally, very low levels of germination were observed at intermediate temperatures (11-26 degrees C). Stratification at 6 degree s C prior to germination at warmer temperatures increased the proportion of seeds that germinated, and the rate of germination for all seedlots. Withi n a harvest, germination percentage (on a probit scale) increased linearly with stratification time and this relationship was independent of germinati on temperature (16-36 degrees C), However, inter-seasonal differences in th e increases in germination capacity following chilling were observed, varyi ng from 0.044 to 0.07 probits d(-1) of chilling at 6 degrees C, Increased s ensitivity to chilling was associated with warmer temperatures during the p eriod 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 ever y 6 d of chilling at 6 degrees C. This systematic reduction in T-b with chi lling ultimately facilitated germination at 6 degrees C after dormancy rele ase.