Resolving local, transpiration-induced gradients in water stress to elucidate hydraulic properties of mesophyll tissue in leaves
The local equilibrium between liquid water in the soft tissues within leaves and the unsaturated water vapor in the atmosphere generates stress that pulls liquid up through the plant’s vasculature and out of the soil. Macroscopic studies on leaves have shown that these hydration stresses influence the plants’ responses to drought and the balance between water loss and the carbon dioxide uptake. To date, the tools used to understand the driving forces (gradients in chemical potential of water) for the water transport either require tampering with the natural physiological state or they provide averaged measurements at leaf-scale which obscures our understanding of the cellular-scale water dynamics. In this talk, I will present the development of an in-situ hydrogel nanosensor (AquaDust) that serves as a reporter of the stress (chemical potential) of water within leaves. The gel matrix responds to changes in chemical potential by swelling; the distance between covalently linked dyes change with reconfiguration of the polymer, leading to changes in the emission spectrum via Fluorescence Resonance Energy Transfer (FRET). I will show that this spectral response is well-described by Flory-Rehner model for volume change of the gel coupled to a model of FRET interaction between the dyes. I will proceed to demonstrate the use of AquaDust to perform unprecedented measurements of transpiration-induced gradients of water stress across the leaf thickness, elucidating the local, material properties of the mesophyll tissue. I will conclude with a discussion of opportunities for further studies enabled by AquaDust in both biological and physical systems.