Using photorespiratory oxygen response to analyse leaf mesophyll resistance.

Classical approaches to estimate mesophyll conductance ignore differences in resistance components for CO from intercellular air spaces (IAS) and CO from photorespiration (F) and respiration (R). Consequently, mesophyll conductance apparently becomes sensitive to (photo)respiration relative to net photosynthesis, (F + R)/A. This sensitivity depends on several hard-to-measure anatomical properties of mesophyll cells. We developed a method to estimate the parameter m (0 ≤ m ≤ 1) that lumps these anatomical properties, using gas exchange and chlorophyll fluorescence measurements where (F + R)/A ratios vary. This method was applied to tomato and rice leaves measured at five O levels. The estimated m was 0.3 for tomato but 0.0 for rice, suggesting that classical approaches implying m = 0 work well for rice. The mesophyll conductance taking the m factor into account still responded to irradiance, CO, and O levels, similar to response patterns of stomatal conductance to these variables. Largely due to different m values, the fraction of (photo)respired CO being refixed within mesophyll cells was lower in tomato than in rice. But that was compensated for by the higher fraction via IAS, making the total re-fixation similar for both species. These results, agreeing with CO compensation point estimates, support our method of effectively analysing mesophyll resistance.