Response of carbon dioxide fixation to water stress; parallel measurements on isolated chloroplasts and intact spinach leaves

Application of water stress to isolated spinach (Spinacia oleracea) chloroplasts by redutcion of the osmotic potentials of CO2 fixation media below -6 to -8 bars resulted in decreased rates of fixation regardless of solute composition. A decrease in CO2 fixation rate of isolated chloroplasts was also found when leaves were dehydrated in air prior to chloroplast isolation. An inverse response of CO2 fixation to osmotic potential of the fixation medium was found with chloroplasts isolated from dehydrated leaves--namely, fixation rate was inhibited at -8 bars, compared with -16 or -24 bars. Low leaf water potentials were found to inhibit CO2 fixation of intact leaf discs to almost the same degree as they did CO2 fixation by chloroplasts isolated from those leaves. CO2 fixation by intact leaves was decreased by 50 and 80% when water potentials were reduced from -7.1 to -9.6 and from -7.1 to -17.6 bars, respectively. Transpiration was decreased by only 40 and 60%, under the same conditions. However, correction for the increase in leaf temperature indicated transpiration decreases of 57 and 80%, similar to the relative decreases in CO2 fixation. Despite the 4-fold increase in leaf resistance to CO2 diffusion in the gas phase when the water potential of leaves was reduced from -6.5 to -14.0 bars, an additional increase of about 50% in mesophyll resistance was obtained. CO2 concentration at compensation also increased when leaf water potential was reduced.