Decoupled stem and leaf hydraulic conductance in California chaparral and coastal sage scrub plant species

Background/Question/Methods

           Plant hydraulic conductance measures how efficiently water flows through a plant, and can be used to describe ecological strategy variation with respect to plant water use.  Whole-plant hydraulic conductance, K_plant, can be broken down into components, i.e. soil, root, stem, and leaf hydraulic conductance.  Leaf hydraulic conductance, K_leaf, is a major determinant of K_plant, as leaves account for a significant percentage of resistance to water movement in plants.  We investigated whether native and maximum rates of K_leaf were correlated with stem hydraulic conductance, K_stem, or whether these traits were decoupled in chaparral and coastal sage scrub shrublands.  To answer these questions, we collected stem and leaf samples from the Santa Margarita Ecological Reserve in California.  Native and maximum K_leaf were measured using the evaporative flux method on excised leaves.  Native and maximum K_stem were measured on excised stem segments, which were attached to a hydraulics line with water flowing from an elevated source, through the stem, and onto a balance to record the flow rate.

Results/Conclusions

                      Results show that for species tested, maximum K_stem was greater than maximum K_leaf. However, few of these differences were statistically significant. 
However, few of these differences were statistically significant.  Overall, however, K_leaf was uncorrelated with K_stem, which is consistent with findings of other studies.  Even though stems and leaves are part of the same hydraulic pathway, decoupling of maximum K_leaf and K_stem could be caused by: 1) structural differences between stem and leaf xylem properties, 2) regulatory differences between stomatal control of water loss in leaves and hydrogel control of flow through stems, or 3) physiological feedbacks between water potential and stomatal conductance during plant function.  These data indicate that while leaf and stem hydraulic systems are important components of whole-plant water movement, maximum capacity for water flow in stems and leaves in not necessarily coordinated.