Plant hydration dynamics: measurement and uptake pathways

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

Secondary leaves of an outbreak-adapted tree species are both more resource acquisitive and more herbivore resistant than primary leaves

The magnitude and frequency of insect outbreaks are predicted to increase in forests, but how trees cope with severe outbreak defoliation is not yet fully understood. Winter deciduous trees often produce a secondary leaf flush in response to defoliation (i.e., compensatory leaf regrowth or refoliation), which promotes fast replenishment of carbon (C) storage and eventually tree survival. However, secondary leaf flushes may imply a high susceptibility to insect herbivory, especially in the event of an ongoing outbreak. We hypothesized that in winter deciduous species adapted to outbreak-driven defoliations, secondary leaves are both more C acquisitive and more herbivore resistant than primary leaves. During an outbreak by Ormiscodes amphimone F. affecting Nothofagus pumilio (Poepp. & Endl.) Krasser forests, we (i) quantified the defoliation and subsequent refoliation by analyzing the seasonal dynamics of the normalized difference vegetation index (NDVI) and (ii) compared the physiological traits and herbivore resistance of primary and secondary leaves. Comparisons of the NDVI of the primary and second leaf flushes relative to the NDVI of the defoliated forest indicated 31% refoliation, which is close to the leaf regrowth reported by a previous study in juvenile N. pumilio trees subjected to experimental defoliation. Primary leaves had higher leaf mass per area, size, carbon:nitrogen ratio and soluble sugar concentration than secondary leaves, along with lower nitrogen and starch concentrations, and similar total polyphenol and phosphorus concentrations. In both a choice and a non-choice bioassay, the leaf consumption rates by O. amphimone larvae were significantly higher (>50%) for primary than for secondary leaves, indicating higher herbivore resistance in the latter. Our study shows that secondary leaf flushes in outbreak-adapted tree species can be both C acquisitive and herbivore resistant, and suggests that these two features mediate the positive effects of the compensatory leaf regrowth on the tree C balance and forest resilience.This study was supported by the Chilean National Foundation of Science and Technology (Fondecyt) with the Grant No. 1160330 to F.I.P. and Post-doctoral Grant No. 3170829 to A.H., and the Chilean National Commission of Scientific and Technological Research (CONICYT) with the I+D Grant No. R17F10005