Development of a thermal time model for the quantification of dormancy loss in Aesculus hippocastanum seeds

Abstract

The effects of temperature on dormancy loss, germination and viability were investigated in seeds of Aesculus hippocastanum L. harvested over a 4-year period. Release from embryo dormancy in freshly harvested seeds was manifest in two phases of morphological growth: initially, when the seed lot was only partially released, axis emergence resulted primarily 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 extension) at 16°C was a function of pre-treatment period at 2-11°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°C to 16°C for two seed lots harvested in separate years. Dormancy periods for individual seeds within both seed lot populations can be described by cumulative normal distributions; the predicted standard deviation of chilling units below the ceiling temperature (i.e. thermal time) was 186°C d. Visible germination occurred during the process of stratification at 2°C, starting after 21-25 weeks. By contrast, three years of hy-drated seed storage at 16°C, which was a non-permissive 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 of the physiological processes in recalcitrant seeds for the development of improved storage methods are discussed.

Direct seeding of ash (Fraxinus excelsior L.) and sycamore (Acer pseudoplatanus L.): The effects of sowing date, pre-emergent herbicides, cultivation, and protection on seedling emergence and survival

“…The soil temperature data were also used to calculate the accumulation of chill units to determine the quantity of dormancy-breaking chilling that was available after each sowing date. The exact temperature response for dormancy breakage of ash and sycamore seed is unknown, but published studies on other deeply dormant species (Seeley and Damavandy, 1985;Pritchard et al, 1996) show that dormancy is broken most rapidly at temperatures between 0 and 5 8C, with effectiveness decreasing at warmer temperatures. In this study, temperatures between 0 and 5 8C were assumed to be equally effective at accumulating a chill unit for every hour's exposure, the rate of accumulation was proportionately decreased to zero as temperatures increased from 5 to 10 8C (no soil temperatures below 0 8C were recorded in this study).…”

Temperature effects on dormancy levels and germination in temperate forest sedges (Carex)

“…Consequently, the most simple explanation, that low temperatures relieve and high temperatures induce dormancy, must give way to more complex concepts of temperature control for dormancy changes. It may be concluded that threshold temperatures derived from germination at a single germination test condition are not sufficient to characterize the dormancy changes of a seed population in the field, or in the laboratory after application of a range of stratification temperatures ͑Pritchard et al 1996. It is conceivable that individual seeds have different limits for dormancy change and the use of static threshold temperatures for dormancy changes would therefore be problematic.…”

Using thermal time models to predict the impact of assisted migration on the synchronization of germination and shoot emergence of oak (Quercus robur L.)

“…In this study, the impact of assisted migration on regeneration and synchronisation with the seasons is assessed using thermal time models. Thermal time models have previously been developed for Q. robur (Pritchard and Manger 1990) but also for other recalcitrant species including Aesculus hippocastanum (Pritchard et al 1996(Pritchard et al , 1999 and Castanea sativa (Pritchard and Manger 1990). These have focussed on germination i.e.…”