During mild and moderate leaf dehydration, both leaf hydraulic conductance (Kleaf) and stomatal conductance decline but the potential causal linkage has been unclear. This linkage can be usefully investigated in model species Arabidopsis thaliana. Using a multi-tiered approach including high resolution in vivo imaging, physiological measurements and modelling, we aimed to characterize 1) the putative causal influence of Kleaf decline on stomatal and photosynthetic decline during dehydration and 2) the fundamentals of Arabidopsis leaf hydraulic design, and to determine 3) the dynamics of Kleaf with irradiance. For Arabidopsis Col-0, we constructed leaf hydraulic vulnerability curves, stomatal and photosynthetic responses to dehydration and pressure-volume curves, and determined key functional and anatomical traits including leaf mass per area (LMA) and cuticular conductance. Additionally, we partitioned the leaf hydraulic resistance, and using micro-computed tomography we imaged in-vivo water dynamics inside the xylem and quantified cell and tissue shrinkage as leaves dehydrated.
Results/Conclusions
We found high maximum values of Kleaf and gas exchange but strong declines during dehydration. Indeed, Arabidopsis leaf traits were similar to those associated with drought sensitivity, such as highest turgor loss point and low LMA and vein density, relative to diverse angiosperm species. However, the leaf veins of Arabidopsis were very resistant to embolism formation during dehydration, with no embolism observed even below turgor loss point, indicating that the strong declines observed in Kleaf were caused by changes in outside-xylem pathways. Using leaf-scale and whole plant mechanistic models, we show that declines in outside-xylem pathways would cause substantial stomatal closure and photosynthetic decline.