To study the role of the plastidial a-glucan phosphorylase in starch metabolism in the leaves of Arabidopsis, two independentmutant lines containing T-DNA insertions within the phosphorylase gene were identified. Both insertions eliminate the activityof the plastidial a-glucan phosphorylase. Measurement of other enzymes of starch metabolism reveals only minor changescompared with the wild type. The loss of plastidial a-glucan phosphorylase does not cause a significant change in the totalaccumulation of starch during the day or its remobilization at night. Starch structure and composition are unaltered. However,mutant plants display lesions on their leaves that are not seen on wild-type plants, and mesophyll cells bordering the lesionsaccumulate high levels of starch. Lesion formation is abolished by growing plants under 100% humidity in still air, butsubsequent transfer to circulating air with lower humidity causes extensive wilting in the mutant leaves. Wilted sectors die,causing large lesions that are bordered by starch-accumulating cells. Similar lesions are caused by the application of acute saltstress to mature plants.We conclude that plastidial phosphorylase is not required for the degradation of starch, but that it playsa role in the capacity of the leaf lamina to endure a transient water deficit.