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 CO ₂ across the mesophyll ( G _m ) frequently constrains photosynthesis ( P _N ) but cannot be measured directly. We examined G _m of cherry ( Prunus avium L . ) subjected to severe drought using the variable J method and carbon-isotopic composition ( δ ¹³ C) of sugars from the centre of the leaf, the leaf petiole sap, and sap from the largest branch. Depending upon the location of the plant from which sugars are sampled, G _m may be estimated over scales ranging from a portion of the leaf to a canopy of leaves. Both the variable J and δ ¹³ C of sugars methods showed a reduction in G _m as soil water availability declined. The δ ¹³ C of sugars further from the source of their synthesis within the leaf did not correspond as closely to the diffusive and C-isotopic discrimination conditions reflected in the instantaneous measurement of gas exchange and chlorophyll-fluorescence utilised by the variable J approach. Post-photosynthetic fractionation processes and/or the release of sugars from stored carbohydrates (previously fixed under different environmental and C-isotopic discrimination conditions) may reduce the efficacy of the δ ¹³ C of sugars from leaf petiole and branch sap in estimating G _m in a short-term study. Consideration should be given to the spatial and temporal scales at which G _m is under observation in any experimental analysis.
Competing Interests: The authors declare no conflict of interest.