Decline of Leaf Hydraulic Conductance with Dehydration: Relationship to Leaf Size and Venation Architecture

Across plant species, leaves vary enormously in their size and their venation architecture, of which one major function is to replace water lost to transpiration. The leaf hydraulic conductance (Kleaf) represents the capacity of the transport system to deliver water, allowing stomata to remain open for photosynthesis. Previous studies showed that Kleaf relates to vein density (vein length per area). Additionally, venation architecture determines the sensitivity of Kleaf to damage; severing the midrib caused Kleaf and gas exchange to decline, with lesser impacts in leaves with higher major vein density that provided more numerous water flow pathways around the damaged vein. Because xylem embolism during dehydration also reduces Kleaf, we hypothesized that higher major vein density would also reduce hydraulic vulnerability. Smaller leaves, which generally have higher major vein density, would thus have lower hydraulic vulnerability. Tests using simulations with a spatially explicit model confirmed that smaller leaves with higher major vein density were more tolerant of major vein embolism. Additionally, for 10 species ranging strongly in drought tolerance, hydraulic vulnerability, determined as the leaf water potential at 50% and 80% loss of Kleaf, was lower with greater major vein density and smaller leaf size (|r| = 0.85–0.90; P , 0.01). These relationships were independent of other aspects of physiological and morphological drought tolerance. These findings point to a new functional role of venation architecture and small leaf size in drought tolerance, potentially contributing to well-known biogeographic trends in leaf size.