Temperature and Aging Effects on Leaf Membranes of a Cold Hardy Perennial, Fragaria virginiana

The lipid composition of leaves of wild strawberry (Fragaria virginiana Duchesne) was analyzed throughout an annual growth cycle in the field. Cellular hardiness to temperature stress was assessed concomitantly by a solute leakage technique. Leaves were shown to be very sensitive to an applied temperature of -5 degrees C during the summer months but insensitive to a 35 degrees C treatment. This general pattern was also seen in young overwintering leaves but was reversed after a period of low-temperature hardening of these same leaves. Associated with cold hardening of the overwintering leaves was a twofold increase in the phospholipid content of the leaf membranes with a proportionately smaller increase in free sterols. The large increase in phospholipids presumably is due primarily to the proliferation of a sterol-poor membrane fraction, probably the endoplasmic reticulum. These quantitative changes in membrane material may be important in increasing freezing tolerance in the overwintering leaf cells by enhancing the overall capacity of the cell for plasma membrane and tonoplast extension through vesicle fusion using components from this endomembrane pool. Analysis of electron micrographs of hardened leaf cells showed an increase in vesiculated smooth endoplasmic reticulum and tonoplast membrane over nonhardened leaf cells, the latter resulting in an enhanced tonoplast surface area to vacuolar volume ratio. During this same period, no changes in the fatty acid or free sterol composition were detectable, suggesting that regulation of membrane fluidity via these components is not required for cold acclimation in this species. During aging and senescence of both the overwintering and the summer leaves, the cellular membranes remained functionally intact but became progressively more vulnerable to temperature stress. Free sterol content increased during this time. This feature may be related to the inability of the older leaves to withstand environmental stress. Increasing sensitivity of the cellular membranes to stress may, in turn, be causally related to the actual onset of senescence in these leaves, thus explaining why only the older leaves senesce when the plant is challenged by periodic environmental stress.