Resistance to Water Transport in Soil and Young Wheat Plants

Herb and conifer roots show similar high sensitivity to water deficit

Root systems play a major role in supplying the canopy with water, enabling photosynthesis and growth. Yet, much of the dynamic response of root hydraulics and its influence on gas exchange during soil drying and recovery remains uncertain. We examined the decline and recovery of the whole root hydraulic conductance (Kr) and canopy diffusive conductance (gc) during exposure to moderate water stress in two species with contrasting root systems: Tanacetum cinerariifolium (herbaceous Asteraceae) and Callitris rhomboidea (woody conifer). Optical dendrometers were used to record stem water potential at high temporal resolution and enabled non-invasive measurements of Kr calculated from the rapid relaxation kinetics of water potential in hydrating roots. We observed parallel declines in Kr and gc to <20% of unstressed levels during the early stages of water stress in both species. The recovery of Kr after rewatering differed between species. T. cinerariifolium recovered quickly, with 60% of Kr recovered within 2 h, while C. rhomboidea was much slower to return to its original Kr. Recovery of gc followed a similar trend to Kr in both species, with C. rhomboidea slower to recover. Our findings suggest that the pronounced sensitivity of Kr to drought is a common feature among different plant species, but recovery may vary depending on root type and water stress severity. Kr dynamics are proposed to modulate gc response during and following drought.

Developmental regulation of water uptake in wheat

The discovery of aquaporins has provided a new basis for studying and interpreting water relations in plants. However, slow progress has been made in elucidating the functional facets of the aquaporin-mediated water pathway in whole plant systems. While increasing experimental evidence suggests that these proteins are directly involved in mediating water homeostasis at varying environmental conditions, only a few attempts have been made to understand their contribution to overall water transport at different developmental stages. By using a chemical inhibitor (HgCl(2)) of aquaporins function, here we present in planta evidence for both diurnal and developmental regulation of aquaporin activity in wheat. We demonstrate that the greatest sensitivity of water flux to pharmacological blockage occurs at the stage of ear emergence and does not coincide with the phenological stage at which the greatest plant water uptake occurs (milky ripeness). The relationship transpiration flux (Q) vs. soil-leaf water potential difference (DeltaPsi(soil-leaves)) revealed a gradual decrease of plant resistance to water flux from tillering to milky ripeness, both in HgCl(2)-treated and untreated control plants. However, the mercury-inhibition of water flux began to gradually increase at ear emergence, suggesting that a larger portion of water moves through aquaporins from this developmental stage on. Although the intercept of the DeltaPsi(soil-leaves)/Q regression line, i.e. the DeltaPsi required to initiate the water flux through the soil-plant-air continuum, was generally not affected by mercury treatment, a significant mercury effect on the intercept was observed at the stage of ear formation. These findings may have important implications for predicting which strategy plants utilize to optimize water use during their life cycle.