On the minimum leaf conductance: its role in models of plant water use, and ecological and environmental controls.

Contents Summary 693 I. Introduction 693 II. Comparison of various definitions and measurement techniques of minimum conductance 694 III. Cuticular conductance 695 IV. Contribution of stomata 696 V. Environmental and ecological variation in minimum conductance 696 VI. Use of minimum conductance in models 698 VII. Conclusions 703 Acknowledgements 703 References 703 SUMMARY: When the rate of photosynthesis is greatly diminished, such as during severe drought, extreme temperature or low light, it seems advantageous for plants to close stomata and completely halt water loss. However, water loss continues through the cuticle and incompletely closed stomata, together constituting the leaf minimum conductance (gmin ). In this review, we critically evaluate the sources of variation in gmin , quantitatively compare various methods for its estimation, and illustrate the role of gmin in models of leaf gas exchange. A literature compilation of gmin as measured by the weight loss of detached leaves is presented, which shows much variation in this trait, which is not clearly related to species groups, climate of origin or leaf type. Much evidence points to the idea that gmin is highly responsive to the growing conditions of the plant, including soil water availability, temperature and air humidity - as we further demonstrate with two case studies. We pay special attention to the role of the minimum conductance in the Ball-Berry model of stomatal conductance, and caution against the usual regression-based method for its estimation. The synthesis presented here provides guidelines for the use of gmin in ecosystem models, and points to clear research gaps for this drought tolerance trait.

Climate drives vein anatomy in Proteaceae.

PREMISE OF STUDY: The mechanisms by which plants tolerate water deficit are only just becoming clear. One key factor in drought tolerance is the ability to maintain the capacity to conduct water through the leaves in conditions of water stress. Recent work has shown that a simple feature of the leaf xylem cells, the cube of the thickness of cell walls divided by the lumen width (t/b)(3), is strongly correlated with this ability. METHODS: Using ecologically, phylogenetically, and anatomically diverse members of Proteaceae, we tested the relationships between (t/b)(3) and climate, leaf mass per unit area, leaf area, and vein density. To test relationships at high phylogenetic levels (mostly genus), we used phylogenetic and nonphylogenetic single and multiple regressions based on data from 50 species. We also used 14 within-genus species pairs to test for relationships at lower phylogenetic levels. KEY RESULTS: All analyses revealed that climate, especially mean annual precipitation, was the best predictor of (t/b)(3). The variation in (t/b)(3) was driven by variation in both lumen diameter and wall thickness, implying active control of these dimensions. Total vein density was weakly related to (t/b)(3) but unrelated to either leaf area or climate. CONCLUSIONS: We conclude that xylem reinforcement is a fundamental adaptation for water stress tolerance and, among evergreen woody plants, drives a strong association between rainfall and xylem anatomy. The strong association between (t/b)(3) and climate cannot be explained by autocorrelation with other aspects of leaf form and anatomy that vary along precipitation gradients.

Leaf hydraulic vulnerability is related to conduit dimensions and drought resistance across a diverse range of woody angiosperms.

Hydraulic dysfunction in leaves determines key aspects of whole-plant responses to water stress; however, our understanding of the physiology of hydraulic dysfunction and its relationships to leaf structure and ecological strategy remains incomplete. Here, we studied a morphologically and ecologically diverse sample of angiosperms to test whether the water potential inducing a 50% loss in leaf hydraulic conductance (P50(leaf)) is predicted by properties of leaf xylem relating to water tension-induced conduit collapse. We also assessed the relationships between P50(leaf) and other traits considered to reflect drought resistance and ecological strategy. Across species, P50(leaf) was strongly correlated with a theoretical predictor of vulnerability to cell collapse in minor veins (the cubed ratio of the conduit wall thickness to the conduit lumen breadth). P50(leaf) was also correlated with mesophyll traits known to be related to drought resistance, but unrelated to traits associated with carbon economy. Our data indicate a link between the structural mechanics of leaf xylem and hydraulic function under water stress. Although it is possible that collapse may contribute directly to dysfunction, this relationship may also be a secondary product of vascular economics, suggesting that leaf xylem is dimensioned to avoid wall collapse.

Leaf hydraulics and drought stress: response, recovery and survivorship in four woody temperate plant species.

Efficient conduction of water inside leaves is essential for leaf function, yet the hydraulic-mediated impact of drought on gas exchange remains poorly understood. Here we examine the decline and subsequent recovery of leaf water potential (Psi(leaf)), leaf hydraulic conductance (K(leaf)), and midday transpiration (E) in four temperate woody species exposed to controlled drought conditions ranging from mild to lethal. During drought the vulnerability of K(leaf) to declining Psi(leaf) varied greatly among the species sampled. Following drought, plants were rewatered and the rate of E and K(leaf) recovery was found to be strongly dependent on the severity of the drought imposed. Gas exchange recovery was strongly correlated with the relatively slow recovery of K(leaf) for three of the four species, indicating conformity to a hydraulic-stomatal limitation model of plant recovery. However, there was also a shift in the sensitivity of stomata to Psi(leaf) suggesting that the plant hormone abscisic acid may be involved in limiting the rate of stomatal reopening. The level of drought tolerance varied among the four species and was correlated with leaf hydraulic vulnerability. These results suggest that species-specific variation in hydraulic properties plays a fundamental role in steering the dynamic response of plants during recovery.