Pectins as Brakes? Their Potential Implication in Adjusting Mesophyll Conductance Under Water Deficit and Salt Stresses

Water and salt stresses reduce net CO₂ assimilation (<i>A</i>_N) primarily by restricting stomatal conductance (<i>g</i>_s) and mesophyll conductance (<i>g</i>_m), while altering leaf structure, anatomy, and cell wall composition. Although some reports observed relationships between these modifications and <i>g</i>_m, in others they remain less clear. Here, we compiled data on studies in which major cell wall components (cellulose; C, hemicellulose; H; pectins; P) were determined with photosynthetic, structural and anatomical features, obtaining a dataset presenting distinct species subjected to both stresses. Among parameters previously reported to affect <i>g</i>_m (leaf mass per area: <i>LMA</i>; chloroplast surface area exposed to intercellular air spaces per unit of leaf surface area: <i>S</i>_c/<i>S</i>; fraction of intercellular air spaces: <i>f</i>_ias; cell wall thickness: <i>T</i>_cw), pectins and the P/(C + H) ratio were the unique consistently varying in salt- and water-stressed plants. Despite no single trait correlated with <i>g</i>_m, it was positively linked with [P/(C + H) × <i>S</i>_c/<i>S</i> × <i>f</i>_ias]/[<i>T</i>_cw × Lignin × <i>LMA</i>] in studies in which all parameters were tested, suggesting that distinct traits may exert antagonistic influences on <i>g</i>_m. Although further experiments are needed to reinforce our findings, we hypothesize that increases in pectins under stress could limit larger <i>g</i>_m declines, improving <i>g</i>_m/<i>g</i>_s ratio and water use efficiency (<i>WUE</i>).