Coordination of stem and leaf hydraulic conductance in southern California shrubs: a test of the hydraulic segmentation hypothesis
New Phytologist (2014) doi: 10.1111/nph.12850
Alexandria L. Pivovaroff1, Lawren Sack2 and Louis S. Santiago1
1Department of Botany and Plant Sciences, University of California, 2150 Batchelor Hall, Riverside, CA 92521, USA; 2Department of Ecology and Evolutionary Biology, University of California, 621 Charles E. Young Drive South, Los Angeles, CA 90095-1606, USA
Coordination of water movement among plant organs is important for understanding plant water use strategies. The hydraulic segmentation hypothesis (HSH) proposes that hydraulic conductance in shorter lived, ‘expendable’ organs such as leaves and longer lived, more ‘expensive’ organs such as stems may be decoupled, with resistance in leaves acting as a bottleneck or ‘safety valve’.
We tested the HSH in woody species from a Mediterranean-type ecosystem by measuring leaf hydraulic conductance (Kleaf) and stem hydraulic conductivity (KS). We also investigated whether leaves function as safety valves by relating Kleaf and the hydraulic safety margin (stem water potential minus the water potential at which 50% of conductivity is lost (Ψstem Ψ50)). We also examined related plant traits including the operating range of water potentials, wood density, leaf mass per area, and leaf area to sapwood area ratio to provide insight into whole-plant water use strategies.
For hydrated shoots, Kleaf was negatively correlated with KS, supporting the HSH. Addition- ally, Kleaf was positively correlated with the hydraulic safety margin and negatively correlated with the leaf area to sapwood area ratio.
Consistent with the HSH, our data indicate that leaves may act as control valves for species with high KS, or a low safety margin. This critical role of leaves appears to contribute impor- tantly to plant ecological specialization in a drought-prone environment.