Thylakoid membrane stability in drought-tolerant and drought-sensitive plants.

The stress stability of membranes from two drought-tolerant plants (Craterostigma plantagineum andCeterach officinarum) was compared with that of a drought-sensitive plant (Spinacia oleracea) in model experiments. Thylakoids from these plants were exposed to excessive sugar or salt concentrations or to freezing. All stresses caused loss of membrane function as indicated by the loss of cyclic photophosphorylation or the inability of the membranes to maintain a large proton gradient in the light. However, loss of membrane functions caused by osmotic dehydration in the presence of sugars was reversible. Irreversible membrane damage during freezing or exposure to salt was attributed mainly to chaotropic solute effects. The sensitivity to different stresses was comparable in thylakoid membranes from tolerant and sensitive plants indicating that the stress tolerance of a plant can hardly be attributed to specific membrane structures which would increase membrane stability. Levels of membrane-compatible solutes such as sugars or amino acids, among them proline, were much higher in the drought-tolerant plants than in spinach. Isolated thylakoids suspended in solutions containing an excess of sugars remained functional after dehydration by freeze-drying. This indicates that membrane-compatible solutes are important in preventing membrane damage during dehydration of poikilohydric plants.