Decline of leaf hydraulic conductance with dehydration: relationship to leaf size and venation architecture.

Christine Scoffoni,Athena Mckown,Michael Rawls,Hervé Cochard,Lawren Sack

doi:10.1104/pp.111.173856

Christine Scoffoni, Athena Mckown + Show 3 more

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https://doi.org/10.1104/pp.111.173856

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Abstract

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 (K(leaf)) represents the capacity of the transport system to deliver water, allowing stomata to remain open for photosynthesis. Previous studies showed that K(leaf) relates to vein density (vein length per area). Additionally, venation architecture determines the sensitivity of K(leaf) to damage; severing the midrib caused K(leaf) 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 K(leaf), 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 K(leaf), 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.

Highlights

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
When major veins were reduced by 90% in crosssectional conductivity to simulate the dysfunction of conduits due to embolism, the smaller leaves with greater major vein density showed a lesser impact on total xylem and whole-leaf hydraulic conductance per leaf area (Kx and Kleaf; i.e. a lower percentage loss of conductance [PLC]; Fig. 1A)
Leaf and whole-plant drought resistance would be conferred by a higher major vein density, which is generally associated with small leaf size (Dunbar-Co et al, 2009; McKown et al, 2010)

Summary

Introduction

Leaves vary enormously in their size and their venation architecture, of which one major function is to replace water lost to transpiration. 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. We hypothesized that higher major vein density, by providing transport pathways around embolized major veins, would confer tolerance of Kleaf to dehydration, i.e. more negative leaf water potential values (Cleaf) at 50% and 80% loss of Kleaf (P50 and P80, respectively) Such a role for leaf venation could be important in the optimization of leaf size. We compared theoretical results with experimentally measured relationships among leaf hydraulic vulnerability, leaf size, venation architecture, and other aspects of leaf drought tolerance for species diverse in leaf form and drought sensitivity

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