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 (g_c) 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 g_c 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 g_c 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 g_c response during and following drought.

Root hydraulic conductance shows a similar high sensitivity to soil water deficit in herbaceous and woody roots and may control canopy diffusive conductance during drought and following recovery.