The influence of temperature,daylength and calendar date on cold tolerance of Rhododendron

Summary

Physical factors changing the acclimation and deacclimation processes that lead to cold (frost) tolerance and susceptibility in Rhododendron cv. Hatsurgiri are reported. Interactions between photoperiod and temperature which encouraged acclimation were studied by exposing detached leaves 0°C to –10°C, and assessing resulting injury using ion efflux and visual scoring. Combinations of short days (8 h) and exposure to 5°C resulted in the greatest cold tolerance. Exposure to long days (18 h) or 20°C reduced the cold tolerance of leaves. Application of each environmental factor, separately, increased cold tolerance compared with control tissues. When photoperiods were maintained at 12 h, exposing leaves to 5°C during the dark period enhanced cold tolerance compared with a 20°C dark period. Continuous 5°C applied during both light and dark periods encouraged greatest cold tolerance. Deacclimation from the cold tolerant state was accelerated by both higher temperature and longer photoperiods, but temperature was predominantly the more significant factor. Application of short days combined with low temperatures significantly delayed deacclimation. But even in this environment cold tolerance decreased over time, possibly through the action of endogenous annual rhythms. Cold tolerance changed with calendar time even when external environmental conditions were maintained at constant values and plant morphology was apparently unchanged.