A comparison of the variable j and carbon-isotopic composition of sugars methods to assess mesophyll conductance from the leaf to the canopy scale in drought-stressed cherry

Conductance of CO2 across the mesophyll (Gm) frequently constrains photosynthesis (PN)but cannot be measured directly. We examined Gm of cherry (Prunus avium L.) subjected to severedrought using the variable J method and carbon-isotopic composition (?13C) of sugars from the centreof the leaf, the leaf petiole sap, and sap from the largest branch. Depending upon the location of theplant from which sugars are sampled, Gm may be estimated over scales ranging from a portion of theleaf to a canopy of leaves. Both the variable J and ?13C of sugars methods showed a reduction inGm as soil water availability declined. The ?13C of sugars further from the source of their synthesiswithin the leaf did not correspond as closely to the diffusive and C-isotopic discrimination conditionsreflected in the instantaneous measurement of gas exchange and chlorophyll-fluorescence utilised bythe variable J approach. Post-photosynthetic fractionation processes and/or the release of sugars fromstored carbohydrates (previously fixed under different environmental and C-isotopic discriminationconditions) may reduce the efficacy of the ?13C of sugars from leaf petiole and branch sap in estimatingGm in a short-term study. Consideration should be given to the spatial and temporal scales at whichGm is under observation in any experimental analysis.