Trade-offs among vulnerability to xylem cavitation, water transport, and other physiological traits in temperate forest species

Catarina F. Moura, Hafiz Maherali, Cynthia J. Willson, and Robert B. Jackson

Department of Biology, Duke University, Durham, NC 27708, USA

 

Abstract

The vulnerability of xylem to cavitation plays a significant role in the hydraulic function of plants. Resistance to cavitation has been considered a major character involved in drought tolerance. We determined whether variation in cavitation vulnerability for a set of woody species was accompanied by a trade-off with other water transport and physiological traits. We evaluated the relationship between vulnerability to cavitation, expressed as the xylem tension at which 50% of conductivity was lost (Y50), saturated hydraulic conductivity (KH), stem-specific hydraulic conductivity (Ks) and leaf-specific hydraulic conductivity (KL) for 13 woody species (including seven oaks) of Central North Carolina. Relationships with other plant traits (specific leaf area, leaf nitrogen, photosynthesis and stomatal conductance) were also examined. Y50 varied from –0.92 to –7.12 MPa. Among the species, oaks (ring-porous wood) were the most vulnerable (mean Y50= -1.39 MPa) followed by diffuse-porous species (mean Y50= -2.24 MPa), and conifers (mean Y50 of 3.17 for pines and 7.12 MPa for eastern red cedar). Within the oaks, the association between Y50 and KH was strong (r2=0.62), and also strong between Y50 and KL (r2=0.65). There was evidence of a negative association between Y50 and KH (r2=0.67) across all species (excluding eastern red cedar). These findings, particularly those within Quercus, provide evidence of a trade-off between vulnerability to cavitation and hydraulic properties, suggesting that the evolution of increased resistance to xylem cavitation is associated with decreased hydraulic conductivity.

Introduction

Xylem hydraulic properties are important for the general function of plants as they exert a strong influence on water transport and therefore on the potential for carbon uptake (Zimmermann 1983). The occurrence air-emboli (cavitation) limits water transport. Resistance to cavitation therefore may be advantageous for drought-tolerant species. However, there may be trade-offs with other physiological traits.

Objectives

  • To examine whether there is a trade-off between hydraulic conductivity and resistance to cavitation (“efficiency vs. safety”)
  • To evaluate trade-offs among suites of hydraulic, gas exchange and leaf traits

Methods

Study sites: Duke Forest-Durham, NC and Sandhills-Fort Bragg, NC

Vulnerability to cavitation: The loss of hydraulic conductivity with decreasing xylem pressure was measured for six stems of each species using the centrifugal force method (Alder et al. 1997). Percent loss of conductivity (PLC=100(1-Kh/Kmax) was plotted against xylem pressure (vulnerability curve)

Field data: Midday leaf water potential measured with a pressure chamber and gas exchange data collected with a Li-cor 6400 between 9.00-12.00h on each day.

Statistical analysis: Vulnerability curves were fitted with an exponential sigmoidal function (PLC=100/(1+exp(a(Y-b)))) (Pammenter and Willigen 1998). Trade-offs evaluated with regression analysis. Both using S-Plus 2000 for Windows. Correlagram based on Pearson correlation using SPSS 10 for Windows

Field measurements were determined for all species except Quercus laevis and Pinus echinata during the summer of 2000

 

Table 1 - Physiological traits

Ymd, Midday water potential (MPa) gs, Stomatal conductance (mol m-2 s-1)
Y50, Estimated value of xylem tension at 50% loss in conductivity Ci/Ca, Intercellular-Ambient CO2 concentration ratio
KH, Saturated hydraulic conductivity (kg m MPa-1 s-1) WUE, Water use-efficiency (mmol mol-1)
Ks, Stem-specific hydraulic conductivity (kg m-1 MPa-1 s-1) Amass, Photosynthesis-mass based (mmol g-1 m-2 s-1)
KL, leaf-specific hydraulic conductivity (kg m-1 MPa-1 s-1) gsmass, conductance-mass based (mol g-1 m-2 s-1)
Al/As, Leaf-stem area ratio C/N, Carbon-Nitrogen ratio
Leaf size (cm2) Nmass, Nitrogen content-mass based (g g-1)
SLA, Specific Leaf Area (cm2/g) Narea, Nitrogen content-area based (g m-2)
A, Photosynthesis (mmol m-2 s-1) PNUE, Photosynthetic nitrogen use-efficiency (mmol g-1 s-1)

 

The response to decreasing pressure differed between species and functional groups (vulnerability curves of two species of each group).

  • Oaks tended to be more vulnerable
  • Conifers were the most resistant.
  • Diffuse-porous species had intermediate values of Y50.

    (bars with different letters are significantly different (p-value<0.0006); t-test with Bonferroni correction)

 

  • There was a strong negative relationship between KH and Y50 within the oaks (n=7) and across all species (n=12; J. virginiana not included). There was also a strong association between KL and Y50 within the oaks, however that was not observed among all species.
  • The significance of the relationship of KH vs Y50 across all species was likely driven by the group of oaks at one end and conifers at the other

 

  • Correlogram showing significant positive (green) and negative (red) correlations among traits (p<0.01)
  • The suites of hydraulic and gas exchange traits appeared to be fairly independent; leaf attributes had some interaction with gas exchange traits

 

Conclusions

  • There was evidence of a trade-off between the resistance to cavitation and saturated hydraulic conductivity across the set of woody species
     
  • The fact that this relationship was strong within closely related species (genus Quercus) suggests a common evolutionary pathway for hydraulic conductivity and resistance to cavitation
     
  • Resistance to cavitation does not seem to limit photosynthetic capacity and other gas exchange properties

Literature Cited

Alder, N. N., W. T. Pockman, J. S. Sperry, and Nuismer S. 1997. Use of centrifugal force in the study of xylem cavitation. Journal of Experimental Botany 48: 665-674
Pammenter, N. W., and Vander Willigen, C. 1998. A mathematical and statistical analysis of the curves illustrating vulnerability of xylem to cavitation. Tree Physiology 18: 589-593
Zimmermann M. H. 1983. Xylem Structure and the Ascent of Sap. Springer-Verlag. Berlin

Acknowledgements

We thank B. Beecham, M. Brinkley, and W. Cook for their assistance. This study was funded by a FCT-PRAXIS XXI Doctoral Fellowship to CFM and NSF funds to RBJ.

This poster was presented at the 2001 Ecological Society of America meeting.

Current Projects | Jackson Lab home

Last modified 30 August 2001