Responses to water stress in an ABA‐unresponsive hybrid poplar (<i>Populus koreana</i>×<i>trichocarpa</i> cv. Peace) III. Consequences for photosynthetic carbon assimilation

Abstract

SUMMARY Net CO 2 assimilation rates and chlorophyll a fluorescence were measured on leaves of an ABA‐unresponsive poplar ( Populus trichocarpa × koreana cv. Peace) during a period of drought. Stomata of fully expanded leaves of ‘Peace’ partly closed during drought, while those of Populus euramericana cv. Robusta closed almost completely. The measured data were used to calculate total light‐driven electron flows and to derive an estimate of a CO., mole fraction in the chloroplasts. Two major results were obtained, (i) In well watered plants, photosynthesis operated at lower CO 2 mole fractions in the chloroplasts of cv. Peace than in those of cv. Robusta, whereas CO 2 mole fractions in the sub stomatal spaces were of the same order of magnitude. We concluded that a higher resistance to CO 2 influx in the mesophyll contributed to the lower net assimilation rates in ‘Peace’, (ii) Drought induced an important decrease in CO 2 availability in ‘Robusta’ but not in ‘Peace’. This suggested that reduced CO 2 influx was a major cause of the limitation of net CO 2 assimilation during drought in ‘Robusta’, but not in ‘Peace’, where drought probably reversibly reduced the apparent carboxylation efficiency of Rubisco. Measurements of O 2 evolution under saturating CO 2 supported this view, as photosynthesis decreased in draughted plants of ‘Peace’ but not of ‘Robusta’. Moreover, estimates of the 13 C/ 12 C isotope ratio in the leaves of both cultivars showed drought‐related decreases in discrimination, which, in the case of ‘Peace’, could only be explained by changes in activity of the photosynthetic carbon reduction cycle.