Altered ultrastructure of cells of sunflower leaves having low water potentials

Summary Desiccation-induced alterations in cell structure were investigated in sunflower (Helianthus annum L.) leaves using light and electron microscopy. Desiccation was imposed by withholding water from the tissue, and all tissue fixation was carried out under isosmotic conditions. In addition to shrinkage of the vacuoles and intercellular spaces caused by water loss, the significant features of cell desiccation were the appearance of lipid droplets and vesicles close to dictyosomes, and plasmalemma and/or tonoplast breakage in the mesophyll cells. Breakage was followed by massive loss of cell organelles except for the thylakoid membranes of the chloroplasts, which retained much of their integrity even in the air-dried state. Plasmalemma and tonoplast disruption began in a few cells at water potentials of — 15 bars (relative water contents of 47%) and went to completion below —26 bars (relative water contents less than 28%) in the leaf mesophyll. Typically in this tissue, net photosynthesis becomes zero and the tissue becomes increasingly incapable of full rehydration at water potentials below — 20 bars. By contrast, water potentials of — 26 bars had no detectable effects on the phloem tissue. Structural alterations were little influenced by the rapidity of desiccation (a few minutes to as long as four days). It was concluded that desiccation-induced changes in cell structure are tissue-specific and occur on a cell-by-cell basis rather than in all cells of a tissue at once. The concentration of the cytoplasm and the disruption of the plasmalemma and/or tonoplast seem to be central events in the alteration of cell ultrastructure by desiccation.This research was supported by NSF grant GB41314.