LEAF WATER RELATIONS AND MAINTENANCE OF GAS-EXCHANGE IN COFFEE CULTIVARS GROWN IN DRYING SOIL

Plant water status, leaf tissue pressure-volume relationships, and photosynthetic gas exchange were monitored in five coffee (Coffea arabica L.) cultivars growing in drying soil in the field. There were large differences among cultivars in the rates at which leaf water potential (PSI-L) and gas exchange activity declined when irrigation was discontinued. Pressure-volume curve analysis indicated that increased leaf water deficits in droughted plants led to reductions in bulk leaf elasticity, osmotic potential, and in the PSI-L at which turgor loss occurred. Adjustments in PSI-L at zero turgor were not sufficient to prevent loss or near loss of turgor in three of five cultivars at the lowest values of midday PSI-L attained. Maintenance of protoplasmic volume was more pronounced than maintenance of turgor as soil drying progressed. Changes in assimilation and stomatal conductance were largely independent of changes in bulk leaf turgor, but were associated with changes in relative symplast volume. It is suggested that osmotic and elastic adjustment contributed to maintenance of gas exchange in droughted coffee leaves probably through their effects on symplast volume rather than turgor.