Abstract
Replacing ultra-pure water solution with ion solution closer to the composition of natural xylem sap increases stem hydraulic conductance by up to 58%, likely due to changes in electroviscosity in the pit membrane pores. This effect has been proposed to contribute to the control of plant hydraulic and stomatal conductance and potentially to influence on carbon balance during dehydration. However, this effect has never been directly tested for leaf xylem, which constitutes a major bottleneck in the whole plant. We tested for an ion-mediated increase in the hydraulic conductance of the leaf xylem (Kx) for seven species diverse in phylogeny and drought tolerance. Across species, no significant changes in Kx were observed between 0 and 15 mM KCl. We further tested for an effect of ion solution during measurements of Kx vulnerability to dehydration in Quercus agrifolia and found no significant impact. These results for leaf xylem contrast with the often strong ion effect reported for stems, and we suggest several hypotheses to account for the difference, relating to the structure of xylem conduits across vein orders, and the ultrastructure of leaf xylem pores. A negligible ion response in leaves would weaken xylem sap ion-mediated control of plant hydraulic conductance, facilitating modeling of whole plant hydraulic behavior and its influence on productivity.
Highlights
In the last fifty years, lab techniques to measure hydraulic conductance of plant organs have revolutionized the understanding of water relations and allied fields (Sack et al, 2016)
Testing the ion effect on maximum leaf xylem hydraulic conductance We tested for an ion enhancement of leaf xylem hydraulic conductance by measuring Kx using the vacuum pump method, as described in previous studies (Scoffoni and Sack, 2015; Scoffoni et al, 2017b) in which the flow rate of water through the petiole and vein xylem is measured at different vacuum levels, and Kx is calculated as the slope of flow rate against vacuum level (Figure 1)
Our results demonstrated that overall across seven species, there was no statistical effect of xylem sap ion concentration on Kx at full hydration (Repeated measures ANOVA)
Summary
In the last fifty years, lab techniques to measure hydraulic conductance of plant organs have revolutionized the understanding of water relations and allied fields (Sack et al, 2016). The ion-mediated response, discovered in stems (Zimmermann, 1978; van Ieperen et al, 2000; Zwieniecki et al, 2001) held particular promise as an important modulator of plant hydraulic conductance over the course of a day, and for maintaining photosynthesis during drought (Nardini et al, 2011). If such a response were to influence the whole plant hydraulic system, it would need to be considered in models of plant water transport and its influence on productivity
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