Axial variation in conduit dimensions in angiosperm leaves and conifer needles - a review

The tip-to-base widening of xylem conduits is a well-documented feature of plant hydraulic architecture at the level of stems and branches. This anatomical pattern, predicted by hydraulic optimization theory (WBE model) minimizes the increase in hydraulic resistance associated with increasing conductive path length. Because the WBE model is not directly applicable to leaves, which function as near two-dimensional, laterally leaky vascular networks, an extended WBE framework was later developed to account for additional aspects of leaf vascular organization. This review focuses on only one component of that broader design - leaf level tip-to-base conduit widening. To date, only seven studies have directly quantified tip-to-base changes in conduit diameter within leaves or needles. Unlike stem, the exponent characterizing conduit widening in leaves is not universal across vascular plant lineages, likely reflecting differences in the geometric and functional constraints governing leaf vascular networks. An additional five studies have addressed spatial variation across the leaf vein hierarchy or scale petiole conduit diameter with leaf size. This limits our understanding of the functional significance of leaf-level xylem structural changes and how this differs systematically among lineages. Hence, this review emphasizes the need to unify methodologies for analyzing leaf xylem structure through the inclusion of hydraulic path length measurements. Nevertheless, advancing the field requires not only incorporating leaf-level tip-to-base conduit widening, but also adopting integrative sampling strategies that capture multiple dimensions of leaf hydraulic organisation within plant water transport frameworks.