The Mechanism of Chill‐ and Drought‐hardening of Phaseolus Vulgaris Leaves

SUMMARY Drought‐hardening at 25° C, 40% R.H. by witholding water from the roots for 4 days is as effective in preventing chilling‐injury to the leaves of Phaseolus vulgaris as 4 days chill‐hardening at 12° C, 85% R.H. During chill‐hardening the degree of unsaturation of the fatty acids associated with the phospholipids increased and this was considered to prevent water loss and electrolyte leakage by lowering the temperature at which phase changes occur in the membrane lipids to below 5° C (Wilson and Crawford, 1974b). However, drought‐hardening at 25° C, 40% R.H. resulted in no increase in the degree of unsaturation of the phospholipids or glycolipids suggesting that lipid phase changes are not the primary cause of chilling‐injury. Furthermore, a re‐examination of the conditions under which water and electrolytes leak from the leaves of P. vulgaris indicates that phase changes in the membrane lipids of the plasmalemma and other organelles at 5° C are only of minor importance in determining the extent and speed of chilling‐injury. The primary cause of chilling‐injury to P. vulgaris leaves on transfer from 25° C, 85% R.H. to 5° C, 85% R.H. is leaf dehydration due to the opening of the stomata at a time when the permeability of the roots to water is low. The primary factor inducing hardening against chilling‐injury in P. vulgaris is a water stress and not low temperature per se.