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

Abstract

Net CO₂ 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 CO₂ mole fraction in the chloroplasts. Two major results were obtained. (i) In well watered plants, photosynthesis operated at lower CO₂ mole fractions in the chloroplasts of cv. Peace than in those of cv. Robusta, whereas CO₂ mole fractions in the substomatal spaces were of the same order of magnitude. We concluded that a higher resistance to CO₂ influx in the mesophyll contributed to the lower net assimilation rates in ‘Peace’. (ii) Drought induced an important decrease in CO₂ availability in ‘Robusta’ but not in ‘Peace’. This suggested that reduced CO₂ influx was a major cause of the limitation of net CO₂ assimilation during drought in ‘Robusta’, but not in ‘Peace’, where drought probably reversibly reduced the apparent carboxylation efficiency of Rubisco. Measurements of O₂ evolution under saturating CO₂ supported this view, as photosynthesis decreased in droughted plants of ‘Peace’ but not of ‘Robusta’. Moreover, estimates of the ¹³C/¹²C 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.