The photosynthetic rate of water stressed leaves ofPrimula palinuriwas reduced drastically by stomatal closure, not by limitations imposed on the capacity of the photosynthetic apparatus, when water loss exceeded 20 of the water content of turgid leaves. The sudden decrease in phtosynthesis was not observed when the lower epidermis of the leaves had been removed. In these ‘stripped’ leaves, inhibition of photosynthesis increased only gradually during the wilting caused by increasing water stress and was complete when the relative water content was as low as 20 compared with the initial value. This corresponded to a water potential of about-40 bar. The light intensity at which half-maximum rates of photosynthesis were observed decreased as stress increased. In intact leaves photosynthesizing in the presence of CO2, light scattering, which is a measure of thylakoid energization, increased steeply during stomatal closure. The observed increase corresponded to the light scattering level measured in the absence of CO2. When the lower epidermis was removed, no sudden increase in thylakoid energization could be observed during dehydration. Thylakoid energization remained high even at low water potentials. It decreased drastically only below a relative water content of 20. Irrespective, of the extent of water stress, CO2fixation of stripped leaves increased when the oxygen content of air was reduced from 21 to 2. Usually the transition from 21 to 2 O2was accompanied by increased thylakoid energization. The increase in energization was more pronounced below than above a relative water content of 50. The data show that energy-dissipating photorespiratory CO2turnover in the in tercellular space of water-stressed leaves whose stomata are closed decreases only slowly as water stress increases. Respiratory CO2production by leaves in the dark was even more resistant to water stress than photosynthesis. It was still significant at water potentials as low as-80
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