Osmoregulation in cotton in response to water stress. II. Leaf carbohydrate status in relation to osmotic adjustment

Diurnal changes in tissue water potential components, photosynthesis, and specific leaf carbohydrates were examined in water stress-adapted and nonadapted cotton plants. Adapted plants exhibited lower daily minimum leaf water potentials and maintained turgor to lower leaf water potentials than nonadapted plants. Because of this turgor maintenance, photosynthesis continued in adapted plants at leaf water potentials that inhibited photosynthesis in nonadapted plants. Adapted plants exhibited lower rates of photosynthesis than did nonadapted plants when leaves were fully turgid. The inhibition was not due to stomatal restriction of CO2 diffusion because leaf conductances of nonadapted and adapted leaves were similar at high leaf water potentials. Adapted plants had more glucose than nonadapted plants, the pattern of glucose accumulation depending on leaf age. Sucrose accumulation in response to decreasing water potential also depended on leaf age. Adapted young leaves exported sucrose, whereas nonadapted leaves of the same age accumulated sucrose at the same leaf water potential. Older leaves of both adapted and nonadapted plants accumulated sucrose as plants became stressed during the day. Old, fully expanded leaves from adapted plants contained up to 5 times more starch than did nonadapted leaves, although the kinetics of starch accumulation and degradation were similar in adapted and nonadapted leaves. In young leaves, adaptation did not affect starch accumulation. When adapted plants were destarched by 80 hours darkness, they became "de-adapted" plants. In nonadapted and de-adapted plants, photosynthesis, leaf conductance, and leaf turgor responded identically to declining leaf water potentials. The data implicate starch in the regulation at cellular nonosmotic volume and, thus, osmotic adjustment.