Declining hydraulic efficiency as transpiring leaves desiccate: two types of response*

Abstract

The conductance of transpiring leaves to liquid water (K leaf ) was measured across a range of steady-state leaf water potentials ( Y leaf ). Manipulating the transpiration rate in excised leaves enabled us to vary Y leaf in the range -0.1 MPa to less than -1.5 MPa while using a flowmeter to monitor the transpiration stream. Employing this technique to measure how desiccation affects K leaf in 19 species, including lycophytes, ferns, gymnosperms and angiosperms, we found two characteristic responses. Three of the six angiosperm species sampled maintained a steady maximum K leaf while Y leaf remained above -1.2 MPa, although desiccation of leaves beyond this point resulted in a rapid decline in K leaf . In all other species measured, declining Y leaf led to a proportional decrease in K leaf , such that midday Y leaf of unstressed plants in the field was sufficient to depress K leaf by an average of 37%. It was found that maximum K leaf was strongly correlated with maximum CO 2 assimilation rate, while K leaf = 0 occurred at a Y leaf slightly less negative than at leaf turgor loss. A strong linear correlation across species between Y leaf at turgor loss and Y leaf at K leaf = 0 raises the possibility that declining K leaf was related to declining cell turgor in the leaf prior to the onset of vein cavitation. The vulnerability of leaves rehydrating after desiccation was compared with vulnerability of leaves during steady-state evaporation, and differences between methods suggest that in many cases vein cavitation occurs only as K leaf approaches zero.

Weak coordination among petiole, leaf, vein, and gas‐exchange traits across Australian angiosperm species and its possible implications

“…2013). Yet, it is equally important to understand why coordination among these traits is often weak or absent across species, as reported here and elsewhere (angiosperms in Brodribb and Holbrook 2006; Maherali et al. 2006; angiosperms in Brodribb et al.…”

“…2007; Brodribb and Jordan 2008), but also within a single species (Brodribb and Jordan 2011). In contrast, other studies have found poor coordination between these traits (angiosperms in Brodribb and Holbrook 2006; Maherali et al. 2006; angiosperms in Brodribb et al.…”

The Role of Plasma Membrane Aquaporins in Regulating the Bundle Sheath-Mesophyll Continuum and Leaf Hydraulics

“…Moreover, this control of BS AQP activity and thereby K leaf can explain the dynamic K leaf response to changing ambient conditions, including stress, or via changes in the turgor of the BS (e.g. in response to light or temperature conditions; Brodribb and Holbrook, 2006;Sack and Holbrook, 2006;Shatil Cohen et al, 2011). The fact that BS hydraulics control the permeability of the mesophyll to water under initial stress conditions suggests the existence of a tight hydraulic connection between the BS and the mesophyll as well as feed-forward regulation.…”

Viewing leaf structure and evolution from a hydraulic perspective

“…Xylem must, therefore, branch and taper (Coomes et al 2008) within the leaf lamina, a process that leads to the xylem itself contributing a significant resistance to the hydraulic transport pathway in the leaf. The relative contributions of xylem and mesophyll tissue to the total hydraulic resistance of the leaf can vary across species and can change with microhabitat, since light, desiccation and temperature differentially affect xylem and mesophyll tissues (Sack et al 2002;Brodribb and Holbrook 2006;Scoffoni et al 2008). A possible involvement of aquaporins in modulating the extra-xylary hydraulic resistance of leaves has also been demonstrated (Cochard et al 2007;Kaldenhoff et al 2008;Heinen et al 2009).…”