Mechanical Failure of Fine Root Cortical Cells Initiates Plant Hydraulic Decline during Drought (original) (raw)
Related papers
Plant Physiology, 2016
Root systems perform the crucial task of absorbing water from the soil to meet the demands of a transpiring canopy. Roots are thought to operate like electrical fuses, which break when carrying an excessive load under conditions of drought stress. Yet the exact site and sequence of this dysfunction in roots remain elusive. Using in vivo x-ray computed microtomography, we found that drought-induced mechanical failure (i.e. lacunae formation) in fine root cortical cells is the initial and primary driver of reduced fine root hydraulic conductivity (Lp r) under mild to moderate drought stress. Cortical lacunae started forming under mild drought stress (20.6 MPa C stem), coincided with a dramatic reduction in Lp r , and preceded root shrinkage or significant xylem embolism. Only under increased drought stress was embolism formation observed in the root xylem, and it appeared first in the fine roots (50% loss of hydraulic conductivity [P 50 ] reached at 21.8 MPa) and then in older, coarse roots (P 50 = 23.5 MPa). These results suggest that cortical cells in fine roots function like hydraulic fuses that decouple plants from drying soil, thus preserving the hydraulic integrity of the plant's vascular system under early stages of drought stress. Cortical lacunae formation led to permanent structural damage of the root cortex and nonrecoverable Lp r , pointing to a role in fine root mortality and turnover under drought stress.
Journal of Experimental Botany, 2014
Several root anatomical phenes affect water acquisition from drying soil, and may therefore have utility in breeding more drought-tolerant crops. Anatomical phenes that reduce the metabolic cost of the root cortex ('cortical burden') improve soil exploration and therefore water acquisition from drying soil. The best evidence for this is for root cortical aerenchyma; cortical cell file number and cortical senescence may also be useful in this context. Variation in the number and diameter of xylem vessels strongly affects axial water conductance. Reduced axial conductance may be useful in conserving soil water so that a crop may complete its life cycle under terminal drought. Variation in the suberization and lignification of the endodermis and exodermis affects radial water conductance, and may therefore be important in reducing water loss from mature roots into dry soil. Rhizosheaths may protect the water status of young root tissue. Root hairs and larger diameter root tips improve root penetration of hard, drying soil. Many of these phenes show substantial genotypic variation. The utility of these phenes for water acquisition has only rarely been validated, and may have strong interactions with the spatiotemporal dynamics of soil water availability, and with root architecture and other aspects of the root phenotype. This complexity calls for structural-functional plant modelling and 3D imaging methods. Root anatomical phenes represent a promising yet underexplored and untapped source of crop breeding targets.
Root architecture governs plasticity in response to drought
Aims Root characteristics are important for predicting plant and ecosystem responses to resource scarcity. Simple, categorical traits for roots could be broadly applied to ecosystem function and restoration experiments , but they need to be evaluated for their role and behaviour under various stresses, including water limitation. We hypothesised that more complex root archi-tectures allow more plastic responses to limited water than do tap roots. Methods We carried out two greenhouse experiments: one with a range of grassland plant species; the other with only species of Asteraceae to test the responsiveness of root architectural classes to location of limited water in the soil column. Using trait screening techniques and X-ray tomography, we measured the plasticity of the roots in response to water location. Results Plasticity of root biomass was lowest in tap rooted species, while fibrous and rhizomatous roots allocated biomass preferentially to where the soil was wettest. X-ray tomography indicated that root morphology was least plastic in rhizomatous species. Conclusions Our results provide a starting point to effective categorisation of plants in terms of rooting architecture that could aid in understanding drought tolerance of grassland species. They also demonstrate the utility of X-ray tomography in root analyses.
Soil textures rather than root hairs dominate water uptake and soil–plant hydraulics under drought
Plant Physiology, 2021
Although the role of root hairs (RHs) in nutrient uptake is well documented, their role in water uptake and drought tolerance remains controversial. Maize (Zea mays) wild-type and its hair-defective mutant (Mut; roothairless 3) were grown in two contrasting soil textures (sand and loam). We used a root pressure chamber to measure the relation between transpiration rate (E) and leaf xylem water potential (w leaf_x) during soil drying. Our hypotheses were: (1) RHs extend root-soil contact and reduce the w leaf_x decline at high E in dry soils; (2) the impact of RHs is more pronounced in sand; and (3) Muts partly compensate for lacking RHs by producing longer and/or thicker roots. The w leaf_x (E) relation was linear in wet conditions and became nonlinear as the soils dried. This nonlinearity occurred more abruptly and at less negative matric potentials in sand (ca.-10 kPa) than in loam (ca.-100 kPa). At more negative soil matric potentials, soil hydraulic conductance became smaller than root hydraulic conductance in both soils. Both genotypes exhibited 1.7 times longer roots in loam, but 1.6 times thicker roots in sand. No differences were observed in the w leaf_x (E) relation and active root length between the two genotypes. In maize, RHs had a minor contribution to soil-plant hydraulics in both soils and their putative role in water uptake was smaller than that reported for barley (Hordeum vulgare). These results suggest that the role of RHs cannot be easily generalized across species and soil textures affect the response of root hydraulics to soil drying.
Tree Physiology, 2004
The spatial heterogeneity of water uptake by fine roots under field conditions was analyzed in situ with miniature sap flow gauges in a mature beech-oak-spruce mixed stand. Sap flow rate (J), sap flow density (J d), and root surfacearea-specific flow rate (uptake rate, J s) were measured for eight to 10 small-diameter roots (3-4 mm) per species in the organic layer (superficial roots) and in the mineral soil (30-80 cm, deep roots) during four months in summer 1999. We calculated J s by relating J to the surface area of the section of the fine root system distal to the position of the gauge on the root. When measured synchronously, roots of the three species did not differ significantly in mean J s , although oak roots tended to have lower rates. However, J d decreased in the sequence spruce > beech > oak in most measurement periods. Microscopic investigation revealed differences in fine root anatomy that may partly explain the species differences in J d and J s. Oak fine roots had a thicker periderm than beech and spruce roots of similar diameter and spruce roots had fewer fine branch rootlets than the other species. Synchronously recorded J d and J s of nearby roots of the same tree species showed large differences in flow with coefficients of variation from 25 to 150% that could not be explained by patchy distribution of soil water. We hypothesize that the main cause of the large spatial heterogeneity in root water uptake is associated with differences between individual roots in morphology and ultrastructure of the root cortex that affect root radial and root-soil interface conductivities. The high intraspecific variation in J s may mask species differences in root water uptake. Superficial roots of all species typically had about five times higher J d than deep roots of the same species. However, J s values were similar for superficial and deep roots in beech and spruce because small diameter roots of both species were more branched in the organic layer than in mineral soil. In oak, deep roots had lower J s (maximum of 100 g m-2 day-1) than superficial roots (about 1000 g m-2 day-1). We conclude that temperate tree species in mixed stands have different water uptake capacities. Water flow in the rhizosphere of forests appears to be a highly heterogeneous process that is influenced by both tree species and differences in uptake rates of individual roots within a species.
Editorial: Root functional traits: From fine root to community-level variation
Frontiers in Plant Science
Root hydraulic redistribution Water relations are key to understanding the ecology of terrestrial plant communities, and one determining component of water balance is the process of hydraulic redistribution (HR; Caldwell et al., 1998). The term refers to the passive movement of water through plant roots from moist to dry soil layers following a water potential gradient (Prieto et al., 2012). Many factors, such as plant transpiration (Howard et al., 2009), root architecture (Scholz et al., 2008) and soil conditions, e.g. soil humidity and texture (Prieto et al., 2010) affect the direction and magnitude of HR that has been found to be highly variable across species and Frontiers in Plant Science frontiersin.org 01
Response of tree roots to drought condition: A review
International Journal of Chemical Studies, 2019
Plant root systems are key components of forest ecosystem. They are responsible for water and nutrient uptake, provide physical stabilization, store nutrients and carbohydrates, and provide carbon and nutrients to the soil through the process of fine-root turnover. Larger roots contribute to the tree’s stability and act as a food storage organ during periods of dormancy. However, to take up water and nutrients efficiently (and increase structural support), it is necessary for root systems to establish a near interface with the surrounding soil. This is achieved by the production of many fine roots and root hairs that significantly increase the surface area at the root and soil interface. Roots may act as sensors for any stress conditions and send signals to shoots above ground. During the drought plants respond physiologically and structurally to prevent excessive water loss according to species – specific water use strategies.