Do Xylem Fibers Affect Vessel Cavitation Resistance?

Abstract

Possible mechanical and hydraulic costs to increased cavitation resistance were examined among six co-occurring species of chaparral shrubs in southern California. We measured cavitation resistance (xylem pressure at 50% loss of hydraulic conductivity), seasonal low pressure potential (P min ), xylem conductive efficiency (specific conductivity), mechanical strength of stems (modulus of elasticity and modulus of rupture), and xylem density. At the cellular level, we measured vessel and fiber wall thickness and lumen diameter, transverse fiber wall and total lumen area, and estimated vessel implosion resistance using (t/b) h 2 , where t is the thickness of adjoining vessel walls and b is the vessel lumen diameter. Increased cavitation resistance was correlated with increased mechanical strength (r 2 5 0.74 and 0.76 for modulus of elasticity and modulus of rupture, respectively), xylem density (r 2 5 0.88), and P min (r 2 5 0.96). In contrast, cavitation resistance and P min were not correlated with decreased specific conductivity, suggesting no tradeoff between these traits. At the cellular level, increased cavitation resistance was correlated with increased (t/b) h 2 (r 2 5 0.95), increased transverse fiber wall area (r 2 5 0.89), and decreased fiber lumen area (r 2 5 0.76). To our knowledge, the correlation between cavitation resistance and fiber wall area has not been shown previously and suggests a mechanical role for fibers in cavitation resistance. Fiber efficacy in prevention of vessel implosion, defined as inward bending or collapse of vessels, is discussed.Among vascular plants, water is transported through xylem under negative pressure. Xylem must withstand both the mechanical stresses associated with negative pressure as well as the risk of air entering the hydraulic pathway. Failure to do so may lead to cavitation of water columns and blockage of water transport. Failure may occur when gas is pulled into water-filled xylem conduits from gas-filled cells or intercellular spaces through pores in the xylem pit membrane in a process referred to as air-seeding (Zimmermann, 1983;Baas et al., 2004). Failure may also occur when negative pressures overcome the ability of the xylem conduit walls to resist implosion (i.e. inward bending or collapse; Carlquist, 1975;Hacke et al., 2001a;Donaldson, 2002;Cochard et al., 2004;Brodribb and Holbrook, 2005). Implosion may trigger cavitation or, in leaves, may restrict hydraulic transport by reducing conduit diameter (Brodribb and Holbrook, 2005). In addition to negative pressures, freezing can lead to failure when sap in the xylem freezes and the gas dissolved in the sap comes out of solution. This can lead to cavitation upon thawing if the bubbles do not go back into solution but instead expand (Yang and Tyree, 1992). The result of cavitation is embolism or gas blockage, which reduces hydraulic transport and can result in reduced stomatal conductance (Pratt et al., 2005), reduced photosynthesis (Brodribb and Feild, 2000), and dieback of branchlets (Rood et al., 2000;Da...

Hydraulic integration and shrub growth form linked across continental aridity gradients

“…Through asymmetric secondary growth that leads to fluted sapwood and in some cases to axis splitting (17,22,23), plants ensure complete hydraulic isolation of segments. This type of growth is associated with high wood density (Tables 2 and 3), with high implosion resistance of vessels, and therefore probably with high resistance to forming embolisms (6,7). An alternative strategy to reduce the degree of hydraulic integration seems to be making fewer and larger vessels and surrounding them with a matrix of thick-walled, nonconducting, and possibly gas-filled fibers (25).…”

“…This common, even daily, event (2-4) can lead to complete failure of the hydraulic system if runaway embolism occurs (5). Two of the three attributes by which plants' negative-pressure flow systems can be protected against failure, resistance and reparability, have been subjects of active research during the last decade (2)(3)(4)(6)(7)(8)(9)(10). The third attribute, redundancy, has received much less attention as an important drought adaptation but is emerging as a focus of research (11)(12)(13)(14).…”

Morphological traits and water use strategies in seedlings of Mediterranean coexisting species

“…Previous significant correlations between root morphology parameters as SRL and hydraulic conductivity in herbaceous and woody species have also been reported (Rieger and Litvin 1999;Pemán et al 2006), highlighting the role of root architecture in plant water status. Indeed, root structure has been related to the variability in physiological traits, including the vulnerability to cavitation in Mediterranean shrubs of California (Jacobsen et al 2005(Jacobsen et al , 2007a. M. communis was the species that displayed the lowest SRL, K As , and stomatal conductance (g s ) values but the highest K RRL .…”

Effect of Provenance and Climate on Xylem Anatomy of Haloxylon Ammodendron (C. A. Mey) Bunge in the Gurbantunggut Desert, China

“…The average temperature in January was negatively related to the wtF (P < 0.05; Table 5, Eq. 7), implying that wtF increased in order to enhance resistance to cavitation under cold stress (Jacobsen et al, 2005). The results of PCA indicated that the PC1 of Jinghe, with a maximum positive score, characterized the lowest precipitation and was in accordance with the highest values of dR, hR, wR, dV and wtV.…”

“…Fibers provide mechanical strength to steady stems and vessel conduction against implosion due to negative pressure (Sperry, 2003;Jacobsen et al, 2005). In the Gurbantunggut Desert, the maximum wind velocity of the year occurred from April to June.…”