Effects of Water Stress on the Ultrastructure of Leaf Cells of Sorghum bicolor

The subcellular changes which occurred in sorghum leaves during increasing water stress and subsequent rewatering are described. Stomata were closed, abscisic acid levels were elevated, and the amounts of starch in the bundle sheath chloroplasts were much reduced by - 14 bars leaf water potential. Swelling of the outer chloroplast membrane, and reorganization of the tonoplast to form small vesicles from the large central vacuole, occurred by a leaf water potential of - 37 bars. Complete structural disruption of the tonoplast, as previously described for maize was not found. On rewatering, large amounts of starch reappeared within three hours. These findings strengthen the hypothesis that maintenance of tonoplast integrity is an important factor in the ability of plants to withstand drought.