Osmoregulation in Cotton in Response to Water Stress : I. ALTERATIONS IN PHOTOSYNTHESIS, LEAF CONDUCTANCE, TRANSLOCATION, AND ULTRASTRUCTURE.

Cotton plants subjected to a series of water deficits exhibited stress adaptation in the form of osmoregulation when plants were subjected to a subsequent drying cycle. After adaptation, the leaf water potential coinciding with zero turgor was considerably lower than in plants that had never experienced a water stress. The relationship between leaf turgor and leaf water potential depended on leaf age.Nonstomatal factors severely limited photosynthesis in adapted plants at high leaf water potential. Nonetheless, adapted plants maintained photosynthesis to a much lower leaf water potential than did control plants, in part because of increased stomatal conductance at low leaf water potentials. Furthermore, adapted plants continued to translocate recently derived photosynthate to lower leaf water potentials, compared with control plants.Stress preconditioning modified cellular ultrastructure. Chloroplasts of fully turgid adapted leaves contained extremely large starch granules, seemed swollen, and had some breakdown of thylakoid membrane structure. In addition, cells of adapted leaves appeared to have smaller vacuoles and greater nonosmotic cell volume than did control plants.