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_IAS) and aids recovery by supporting leaf structural integrity.
• Leaves of ‘Carnduff-9’ with less abundant BSEs exhibited greater θ_IAS, 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_IAS across leaf profile while that was little for ‘Gillam-5’. Under rewatered conditions, drought-induced changes in θ_IAS 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_IAS, and relatively large mesophyll surface area provides for improved photosynthetic capacity under drought and supports recovery in leaf architecture after rewatering in balsam poplar.