Plant hydration dynamics: measurement and uptake pathways

Abstract

Transpiration accounts for most terrestrial water fluxes, and agriculture uses most of the water managed by humans. Transpiration is tightly regulated by plants, so climate models and irrigation water use efficiency could be improved by understanding how plants regulate water status. In this work, I address questions that are relevant to our understanding of plant hydration dynamics: (1) Can foliar water uptake (FWU) restore leaf hydraulic conductance (Kleaf) lost due to dehydration? (2) Is embolism refilling involved in FWU-induced hydraulic recovery? (3) Can plant water status be measured by uniaxial compression of the leaf lamina? Many plants are able to access atmospheric water through FWU; however, the physiological consequences of FWU are unclear. While FWU represents a small water flux, it may play a role in restoring hydraulic conductivity lost during dehydration. My results showed that FWU can restore Kleaf in Avicennia marina lost during dehydration. While hydraulic recovery retraced the same path observed during dehydration-dependent loss of Kleaf, a reduced ability for FWU impaired Kleaf recovery under severe dehydration. Most of the resistance to FWU was located in the leaf surface. I conclude that FWU may play a role in the maintenance of shoot hydraulic function during changing water status.

Plants living in saline environments experience constant xylem tension. Under these conditions, it is unclear how embolism refilling can take place. Using micro-CT imaging, I imaged Avicennia marina twigs in a dehydrated state and 4-48 h after wetting the twig surface. Emboli were present in the stem and leaves in the dehydrated state. Stem emboli were likely caused by cutting, while leaf emboli were likely caused by dehydration. Emboli in stems and leaves refilled with water after wetting, taking up to 48 h in the process, which is slower than the documented FWU rehydration kinetics. Possibly, refilling was facilitated by a vascular constriction at the stem-petiole junction and/or by loading of inorganic solutes into xylem vessels. My results substantiate that FWU is an important source of water for this widespread mangrove species; however, differences between field and experimental conditions currently preclude extrapolating these results to natural settings.

Turgor is an essential indicator of plant water status; however, turgor measurements are not routine. Turgor can be measured by localised compression of cells or tissues, but an accessible method to perform these measurements is lacking. I hypothesized that leaf turgor pressure can be monitored by uniaxially compressing the leaf lamina and by measuring the stress under a constrained thickness ('stress relaxation', SR); and that leaf water content can be monitoring by measuring the thickness of leaves compressed under a constant force ('constant stress', CS). Using a c. US$300 leaf squeeze-flow rheometer, I showed that uniaxial compression provides accurate measurement of plant water status with high temporal resolution at low cost. Experimental results and a simple hydrostatic equilibrium model indicate that the stationary bulk modulus during compression is largely determined by the bulk osmotic pressure. Leaf squeeze-flow rheometry is presented as a novel, automatable and potentially standard method to quantify plant water status. Show more