DEVELOPMENT OF A THERMAL TIME MODEL FOR THE QUANTIFICATION OF DORMANCY LOSS IN AESCULUS-HIPPOCASTANUM SEEDS

The effects of temperature on dormancy loss, germination and viability were investigated in seeds of Aesculus hippocastanum L. harvested ove r a 4-year period. Release from embryo dormancy in freshly harvested s eeds was manifest in two phases of morphological growth: initially, wh en the seed lot was only partially released, axis emergence resulted p rimarily from cotyledonary petiole extension without radicle extension ; subsequently, when the seed lot was totally released, axis emergence of all seeds was followed immediately by extension to > 1 cm through growth of the radicle. Germination (axis emergence and radicle extensi on) at 16 degrees C was a function of pre-treatment period at 2-11 deg rees C. The rate of dormancy loss (probit germination d(-1)) increased linearly below a ceiling temperature for the chilling response; this temperature was estimated to vary from 13 degrees C to 16 degrees C fo r two seed lots harvested in separate years. Dormancy periods for indi vidual seeds within both seed lot populations can be described by cumu lative normal distributions; the predicted standard deviation of chill ing units below the ceiling temperature (i.e. thermal time) was 186 de grees C d. Visible germination occurred during the process of stratifi cation at 2 degrees C, starting after 21-25 weeks. By contrast, three years of hydrated seed storage at 16 degrees C, which was a non-permis sive temperature for dormancy loss, resulted in little preemergence of the axis during stratification; approximately one third of the seeds remained germinable. The implications of these quantitative analyses o f the physiological processes in recalcitrant seeds for the developmen t of improved storage methods are discussed.