Volume and osmotic potential changes in relation to inhibition of photosynthesis by water stress in intact leaves

It has been reported that for osmotically stressed leaf slices of a wide range of species, carbon dioxide saturated photosynthesis is uniformly inhibited by stress when water status is expressed as relative volume. Comparable data for intact leaves of a range of species are lacking. It is also unknown whether the same pattern of response applies to carbon dioxide limited photosynthesis. For these reasons responses of photosynthesis to carbon dioxide partial pressure were determined at 21% oxygen at high irradiance in intact leaves of five species as water deficits developed slowly in intact plants. Relative water content (volume) and total and osmotic water potentials were measured at each level of water stress. Three species adjusted osmotically such that volume remained unchanged over a range of water potentials. Regardless of whether volume was maintained, carbon dioxide saturated photosynthesis decreased as water potentials decreased. In contrast to the data for osmotically stressed leaf slices, relative volume, relative osmotic potential, and total water potential did not indicate a uniform level of inhibition of either carbon dioxide saturated or carbon dioxide limited photosynthesis across species. In some species the carbon dioxide compensation point increased with stress. The initial slope of photosynthesis versus substomatal carbon dioxide partial pressure was relatively less inhibited by stress than was the saturated rate. This difference was greater in some species than in others.