Photosynthesis and Transpiration as a Function of Gaseous Diffusive Resistances in Orange Leaves

Abstract

Abstract The CO 2 and H 2 O vapour exchange of single attached orange, Citrus sinensis (L.), leaves was measured under laboratory conditions using infrared gas analysis. Gaseous diffusive resistances were derived from measurements at a saturating irradiance and at a leaf temperature optimum for photosynthesis. Variation in leaf resistance (within the range 1.6 to 60 s cm ‐1 ) induced by moisture status, or by cyclic oscillations in stomatal aperture, was associated with changes in both photosynthesis and transpiration. At low leaf resistance (ri less than 10 s cm ‐1 ) the ratio of transpiration to photosynthesis declined with reduced stomatal aperture, indicating a tighter stomatal control over H 2 O vapour loss than over CO 2 assimilation. At higher leaf resistance (ri greater than 10 s cm ‐1 ) changes in transpiration and photosynthesis were linearly related, but leaf resistance and mesophyll resistance were also positively correlated, so that strictly stomatal control of photosynthesis became more apparent than real. This evidence, combined with direct measurements of CO 2 diffusive resistances (in a ‐O 2 gas stream) emphasised the presence of a significant mesophyll resistance; i.e. , an additional and rate limiting resistance to CO 2 assimilation over and above that encountered by H 2 O vapour escaping from the leaf.