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

Abstract

The kinetics of primary dormancy loss were investigated in seeds of horse chestnut (Aesculus hippocastanum L.) harvested in four different years. Freshly collected seeds from 1991 held for up to 1 year at temperatures between 2 °C and 42 °C exhibited two peaks in germination (radicle growth), representing a low temperature (2–8 °C) and a high temperature response (31–36 °C). Germination at 36 °C generally occurred within 1 month of sowing, but was never fully expressed in the seedlots investigated. At low temperatures (2–8 °C), germination started after around 4 months. Generally, very low levels of germination were observed at intermediate temperatures (11–26 °C). Stratification at 6 °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 °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 °C. Increased sensitivity to chilling was associated with warmer temperatures during the period of seed filling. The estimated base temperature for germination, Tb, for newly harvested seeds varied slightly between collection years, but was close to 25 °C. For all seedlots, Tb decreased by 1 °C every 6 d of chilling at 6 °C. This systematic reduction in Tb with chilling ultimately facilitated germination at 6 °C after dormancy release.