Response of Photosynthesis and Dark-CO2-Fixation to Light, CO2and Temperature in Leaf Slices under Osmotic Stress

Photosynthesis and dark-CO2-fixation were measured in vacuum-infiltrated leaf slices from the mesophyte Spinacia oleracea and the Mediterranean xerophyte Arbutus unedo under hypertonic stress as a function of light-intensity, CO2-concentration and temperature, in the absence of stomatal control. Under hypertonic stress, photosynthesis and dark-CO2-fixation were inhibited in leaf tissue from both plants. 50% inhibition of photosynthesis in spinach occurred at about −3.0 MPa, and of dark-CO2-fixation at about −3.5 MPa. 50% inhibition of photosynthesis in Arbutus unedo was reached at about −4.0 MPa (sorbitol as osmoticum). In both plants, osmotic dehydration decreased the slope and the maximum of the CO2- and light-response curves. The slope of the CO2-response curve of dark-CO2-fixation was also decreased under hypertonic stress, but the inhibition of the maximal fixation rate was less obvious than for photosynthesis. Photosynthesis and dark-CO2-fixation differed significantly in their response to high temperature: under light- and CO2-saturation, photosynthesis of spinach leaf slices had a temperature optimum at about 37 °C, and it was nearly completely inhibited at 45 °C. The rate of dark-CO2-fixation, however, increased continuously up to 45 °C. Osmotic dehydration increased the resistance of photosynthesis to high temperatures.