Drought Tolerance in Two Mosses: Correlated with Enzymatic Defence Against Lipid Peroxidation

Drought-induced changes in the activities of superoxide dismutase(SOD) and catalase, level of lipid peroxidation, and membranepermeability (solute leakage) have been studied in two mosses,the drought-tolerant Tortula ruralis and the drought-sensitiveCratoneuron filicinum. In T. ruralis the activities of SOD andcatalase increase during slow drying. The level of lipid peroxidationconsequently declines. On subsequent rehydration the enzymeactivities decline and the level of lipid peroxidation risesgradually to normal levels. The leakage of preloaded ⁸⁶Rb onrehydration of slowly dried T. ruralis is similar to that inturgid moss, i.e. leakage of about 20% of tissue ⁸⁶Rb. WhenT. ruralis is subjected to rapid drying there is no change inthe enzyme activities or in lipid peroxidation. However, whenthis moss is rehydrated there is a large immediate increasein lipid peroxidation. Half of the tissue ⁸⁶Rb is leaked intothe bathing medium during the first hour of rehydration. Butwithin the next hour, when SOD and catalase activities haveincreased to high levels, lipid peroxidation quickly declinesto a level lower than that in the turgid control moss, and the⁸⁶Rb leaked earlier is partly reabsorbed indicating that membranerepair is well underway. On prolonged rehydration the enzymeactivities decline and the level of lipid peroxidation risesgradually to reach normal levels found in control turgid moss.In the case of drought-sensitive C. filicinum the activitiesof SOD and catalase decline during drying as well as duringsubsequent rehydration. There is a rapid increase in lipid peroxidationduring rehydration and most of the preloaded ⁸⁶Rb leaks intothe bathing medium irreversibly. The changes in lipid peroxidationduring drying and subsequent rehydration of both the mossesappear to coincide in time with the reported changes in O₂ uptake,indicating that the drought-induced membrane damage may be dueto free radical-induced lipid peroxidation which is known torequire active O₂ uptake. Furthermore, there appears to be agood correlation between an ability of the tissue to controllipid peroxidation and its ability to retain solutes. It issuggested that ability of plant tissues to mobilize enzymaticdefence against uncontrolled lipid peroxidation may be an importantfacet of their drought tolerance.