Salicaceae endophyte inoculation alters stomatal patterning and improves the intrinsic water use efficiency of Populus trichocarpa

Abstract

Microorganisms may symbiotically enhance plant resilience to water stress by altering their hosts’ physiology and anatomy at the leaf level. Bacterial and yeast endophytes, isolated from the Salicaceae family, can improve the intrinsic water use efficiency (iWUE) of cultivated poplar (Populus) under water deficits by lowering stomatal conductance (gsw) while maintaining carbon assimilation (Anet). However, the relevance of stomatal anatomy underlying the reductions in gsw remains unclear, and examining these traits may illuminate the way endophytes boost host acclimation to water stress. We hypothesized that endophyte inoculation could change host stomatal anatomy, and this would relate to decreases in gsw. In two greenhouse studies, we subjected Salicaceae endophyte-inoculated and uninoculated Populus trichocarpa to well-watered and water-deficit treatments. We examined the changes of individual stomatal traits and related the composition of these parameters, termed stomatal patterning, to leaf-gas exchange under light saturation. Regardless of soil moisture status, inoculation improved iWUE at light saturation in experiment I from preserving Anet and lowering gsw; however, increases in iWUE were more pronounced during the water deficit. Drops in gsw corresponded to underlying shifts in stomatal patterning, and inoculated plants tended to have smaller, more compact stomata and greater anatomical maximum stomatal conductance (gsmax) relative to the control. Experiment II resulted in no changes in stomatal morphology from inoculation. Our results indicate that inoculation with Salicaceae endophytes can alter stomatal density and size relationships, leading to lower gsw and greater iWUE. However, improving the water-use efficiency of cultivated plants through endophytes likely relies on successful colonization of the host which may depend on the context of environmental conditions. Future efforts may quantify specific strains to examine a direct relationship between endophytes and host physiology and anatomy.