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Articles

Response of Carbon Dioxide Fixation to Water Stress

Parallel Measurements on Isolated Chloroplasts and Intact Spinach Leaves ¹

^a Agricultural Research Organization, The Volcani Center, Bet Dagan, ^b Hebrew University of Jerusalem, Faculty of Agriculture, Rehovot, Israel

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.