Role of Osmotic Potential Gradients during Water Stress and Leaf Senescence in Fragaria virginiana.

The physiological basis underlying differences in sensitivity of different aged leaves to water stress was investigated in Fragaria virginiana Duchesne. Differential susceptibility of only older leaves to water stress in the field during summer months appeared related to gradients in leaf osmotic potential within the plant and by an age dependency in the ability of leaves to adjust osmotically when challenged by periodic water deficits. Under greenhouse conditions, older leaves senesced invariably during an imposed water stress while control leaves of comparable age and stressed younger leaves remained green. Osmotic potentials of intermediate aged and younger leaves became approximately 1 to 2 bars lower after a single cycle of imposed stress and up to 10 bars lower after two cycles of stress. Pronounced gradients in leaf osmotic potential within individual whole plants were observed following two cycles of water stress that were significantly different from control values. Osmotic adjustment was dependent on leaf age with the greatest capacity for adjustment in the intermediate aged leaves. Loss of osmotic adjustment was rapid upon rewatering with a half-life of 4 days. An irreversible component of adjustment was observed, amounting to about 10% (or 2 bars) of the maximally adjusted state. This irreversible component could be accounted for in part by significant changes in cell size and other anatomical alterations in the leaf that affect cellular osmotic volume, and, hence, cellular water relations.