Mechanism of water‐stress induced cavitation in conifers: bordered pit structure and function support the hypothesis of seal capillary‐seeding

Sylvain Delzon,Cyril Douthe,Anna Sala,Herve Cochard

doi:10.1111/j.1365-3040.2010.02208.x

Abstract

Resistance to water-stress induced cavitation is an important indicator of drought tolerance in woody species and is known to be intimately linked to the anatomy of the xylem. However, the actual mechanical properties of the pit membrane are not well known and the exact mode of air-seeding by which cavitation occurs is still uncertain. We examined the relationship between cavitation resistance and bordered pit structure and function in 40 coniferous species. Xylem pressure inducing 50% loss of hydraulic conductance (P50, a proxy for cavitation resistance) varied widely among species, from −2.9 to −11.3 MPa. The valve effect of the pit membrane, measured as a function of margo flexibility and torus overlap, explained more variation in cavitation-resistance than simple anatomical traits such as pit membrane, pit aperture or torus size. Highly cavitation resistant species exhibited both a high flexibility of the margo and a large overlap between the torus and the pit aperture, allowing the torus to tightly seal the pit aperture. Our results support the hypothesis of seal capillary-seeding as the most likely mode of air-seeding, and suggest that the adhesion of the torus to the pit border may be the main determinant of cavitation resistance in conifers.

Highlights

Conifers are a very important component of terrestrial ecosystems and occur from dry woodlands to the vast boreal forests
We examine whether bordered pit structure and function relates to cavitation resistance across conifer species
Values of P50 were significantly different between families (F = 28.10, P < 0.0001) and within families for Pinaceae (F = 29.16, P < 0.0001) and Cupressaceae (F = 24.98, P = 0.0001).We found a strong correlation between P50 and S (log transformed data, r = -0.88, P < 0.0001 (Fig. 4), showing that the rate of embolism decreases as cavitation resistance increases and that species with P50 lower than -6 MPa always embolize very slowly

Summary

Introduction

Conifers are a very important component of terrestrial ecosystems and occur from dry woodlands to the vast boreal forests. A major cause of xylem cavitation appears to be air leakage from embolized conduits through inter-conduit pits (Crombie, Hipkins & Milburn 1985; Cochard, Cruizat & Tyree 1992; Jarbeau et al 1995). According to this air-seeding hypothesis, cavitation occurs when air bubbles are sucked from a non-functional air filled conduit into a functional conduit through the interconduit pits. Greater cell expansion during vessel maturation often results in wider pit area, larger membrane pores and, lower cavitation resistance (Wheeler et al 2005; Christman, Sperry & Adler 2009)

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