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 horsechestnut (Aesculus hippocastanum L.) harvested in fourdifferent years. Freshly collected seeds from 1991 held for up to 1 year attemperatures between 2C and 42C exhibited two peaks in germination (radiclegrowth), representing a low temperature (2-8°C) and a hightemperature response (31-36°C). Germination at 36°Cgenerally occurred within 1 month of sowing, but was never fully expressedin the seedlots investigated. At low temperatures (2-8°C),germination started after around 4 months. Generally, very low levels oftermination were observed at intermediate temperatures (11-26°C).Stratification at 6°C prior to germination at warmer temperaturesincreased the proportion of seeds that germinated, and the rate ofgermination for all seedlots. Within a harvest, germination percentage (ona probit scale) increased linearly with stratification time and thisrelationship was independent of germination temperature (16-26°C).However, inter-seasonal differences in the increases in germinationcapacity following chilling were observed, varying from 0.044 to 0.07probits d^-1 of chilling at 6°C. Increasedsensitivity to chilling was associated with warmer temperatures during theperiod of seed filling. The estimated base temperature for germination, Tb,for newly harvested seeds varied slightly between collection years but wasclose to 25°C. For all seedlots, Tb decreased by 1°C every6 d of chilling at 6°C. This systematic reduction in Tb withchilling ultimately facilitated germination at 6°C after dormancyrelease.