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

Water and salt stresses reduce net CO₂ assimilation (A_N) primarily by restricting stomatal conductance (g_s) and mesophyll conductance (g_m), while altering leaf structure, anatomy, and cell wall composition. Although some reports observed relationships between these modifications and g_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 g_m (leaf mass per area: LMA; chloroplast surface area exposed to intercellular air spaces per unit of leaf surface area: S_c/S; fraction of intercellular air spaces: f_ias; cell wall thickness: T_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 g_m, it was positively linked with [P/(C + H) × S_c/S × f_ias]/[T_cw × Lignin × LMA] in studies in which all parameters were tested, suggesting that distinct traits may exert antagonistic influences on g_m. Although further experiments are needed to reinforce our findings, we hypothesize that increases in pectins under stress could limit larger g_m declines, improving g_m/g_s ratio and water use efficiency (WUE).