Mapping of drought-induced changes in tissue characteristics across the leaf profile of Populus balsamifera.

Leaf architecture impacts gas diffusion, biochemical processes, and photosynthesis. For balsam poplar, a widespread North American species, the influence of water availability on leaf anatomy and subsequent photosynthetic performance remains unknown. To address this shortcoming, we characterized the anatomical changes across the leaf profile in three-dimensional space for saplings subjected to soil drying and rewatering using X-ray microcomputed tomography. Our hypothesis was that higher abundance of bundle sheath extensions (BSE) minimizes drought-induced changes in intercellular airspace volume relative to mesophyll volume (i.e. mesophyll porosity, θ) and aids recovery by supporting leaf structural integrity. Leaves of 'Carnduff-9' with less abundant BSEs exhibited greater θ, higher spongy mesophyll surface area, reduced palisade mesophyll surface area, and less veins compared with 'Gillam-5'. Under drought conditions, Carnduff-9 showed significant changes in θ across leaf profile while that was little for 'Gillam-5'. Under rewatered conditions, drought-induced changes in θ were fully reversible in 'Gillam-5' but not in 'Carnduff-9'. Our data suggest that a 'robust' leaf structure with higher abundance of BSEs, reduced θ, and relatively large mesophyll surface area provides for improved photosynthetic capacity under drought and supports recovery in leaf architecture after rewatering in balsam poplar.