Plant productivity and survival depends on maintenance of water transport through the hydraulic network, which is vulnerable to dysfunction with increasing water stress. While the majority of the flow through the network occurs by axial flow through xylem conduits, water flows radially between the xylem and through extraxylary tissue in absorbing roots and again at the terminal end through the leaf mesophyll. This radial hydraulic component (radial hydraulic conductance , krad) is a potential bottleneck where water must pass through living tissue or through the small pores in the cell wall matrix. Studies in angiosperms suggest that krad in leaves is one of the dominant resistors to water flow at the whole plant scale. In conifers leaf krad may be especially limiting as conifer leaves contain only a single central vein, excluding the possibility of compensation for low krad by increased vein density. We studied the coordination between krad and axial leaf and stem xylem hydraulic conductance (kxleaf and kxstem respectively), and vulnerability to water stress in the conifer genus Pinus, focusing on four species P. taeda L., P. virginia Mill., P. pungens Lamb., and P strobus L.). Vulnerability curves were constructed for shoots using a combination of a modified evaporative flux method, and the vacuum chamber method yielding curves for leaf elements (krad , kxleaf) and stem xylem (kxstem ).
Results/Conclusions
In all species krad accounted for 90% or more of shoot hydraulic resistance across the range of measured water potentials. We also found evidence for vulnerability segmentation in Pinus shoots with leaves more vulnerable to losses in hydraulic conductance then stems. P50, the xylem pressure associated with a 50% loss of hydraulic conductance was less negative in leaf elements (krad , kxleaf, -1.4 to -2.4 MPa ) than kxstem (-2.5 to -3.1 MPa) suggesting leaves may act as hydraulic fuses in Pinus. This study highlights the importance of leaf hydraulic conductance to understanding plant hydraulic function. Future studies to elucidate the physiological and anatomical determinants of leaf krad and its hydraulic vulnerability will increase our mechanistic understanding of tree responses to climate change.