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

Abstract

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.Int. J. Mol. Sci. 2020, 21, 1222 2 of 15 to directly measure the transport of CO 2 across the mesophyll layer to the site of carboxylation within the chloroplast envelope (termed mesophyll conductance: G m ); therefore, a number of methodologies have been developed to approximate G m . Such quantification of G m has demonstrated the importance of the movement of CO 2 across the mesophyll layer to P N and plant acclimation to changing growth conditions [1,3,8,9]. Indeed, the physical [10] and biochemical [11] factors influencing G m are key attributes in the development of more productive and/or drought-tolerant crops [12,13]. However, the methods used to estimate G m all involve certain assumption and aspects susceptible to error [14]. Moreover, as some methods require sensitive equipment in addition to standard gas exchange [15] or extended periods of measurement [16] they are not suited to use in the field. Here, we utilised the 'variable J' method involving simultaneous leaf gas exchange and chlorophyll fluorescence (Chl-Flr) [17] alongside analysis of the carbon isotopic composition (δ 13 C) of recently synthesised sugars [18,19] to characterise G m in cherry (Prunus avium) subject to drought.The assimilation of CO 2 during photosynthesis creates a diffusion gradient between the chloroplast and the internal leaf air-spaces; however, the conductance of CO 2 across the mesophyll is highly complex, involving gaseous and aqueous phases, the biochemistry of the mesophyll and physical resistances [14,20,21]. The physical structure of the mesophyll plays a major role in G m [22]; species with increased surface area [10] and lower distances between the air-space and chloroplas...

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