PHOSPHOLIPIDS AND PLANT MEMBRANE PERMEABILITY

SUMMARY Current views on membrane construction are presented as a basis for discussing certain modifications of structure that may render membranes leaky, allowing solutes to diffuse from them. Biophysical experiments on dehydrated membranes indicate that, when their water content is reduced below 20%, they no longer adopt the lipid bilayer conformation. When dry seeds, spores or lichens are placed in water, soluble cell constituents leak from them for a few seconds or minutes until membrane integrity is re‐established; the same may apply also to the liberation of solutes from pollen grains. Moreover, there are indications that ‘dry’ in this context means having less than about 20% water. Some plants are sensitive to chilling, their tissues becoming leaky when exposed to temperatures between 0 and 10°C. This chilling‐induced leakage, attributed to a phase change in all the phospholipids of which the membrane is composed occurs at a slightly lower temperature than the change in the activation energy of membrane‐bound enzymes. At senescence, leaf tissues become leaky and may eventually dry out. So much phospholipid disappears from senescing cotyledons of cucumber that there is no longer sufficient to fabricate complete and intact membranes around the cells. Treating cucumber cotyledons with iodoacetate also causes a loss of phospholipids, allowing solutes to leak out of the cells. It is possible that exposing seeds or mature plant tissues to oxygen at pressures of 1 or more atm renders them leaky because of lipid peroxidation.