Using photorespiratory oxygen response to analyse leaf mesophyll resistance

Classical approaches to estimate mesophyll conductance ignore differences in resistance components for CO₂ from intercellular air spaces (IAS) and CO₂ from photorespiration (F) and respiration (R_d). Consequently, mesophyll conductance apparently becomes sensitive to (photo)respiration relative to net photosynthesis, (F + R_d)/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_d)/A ratios vary. This method was applied to tomato and rice leaves measured at five O₂ 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, CO₂, and O₂ levels, similar to response patterns of stomatal conductance to these variables. Largely due to different m values, the fraction of (photo)respired CO₂ 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 CO₂ compensation point estimates, support our method of effectively analysing mesophyll resistance.