Root mucilage enhances plant water use under combined soil and atmospheric drought

Abstract

Background and aims: Plants have evolved various root adaptive traits to enhance their ability to access soil water under stress conditions. Although root mucilage has been suggested to facilitate root water uptake in drying soils, its impact under combined edaphic and atmospheric stress remains unknown. We hypothesized that mucilage decreases the saturated soil hydraulic conductivity and consequently, a genotype with high mucilage production will exhibit lower maximum soil-plant hydraulic conductance and restrict transpiration at relatively low vapor pressure deficit (VPD). On the other hand, in drying soil, mucilage attenuate the gradients in matric potential at the root-soil interface and hence facilitate root water uptake, especially at high VPD.

Methods: We compared two cowpea genotypes with contrasting mucilage production rates and subjected them to three consecutively increasing levels of VPD (1.04 kPa, 1.8 kPa, and 2.8 kPa) while the soil was left to dry out. We measured the transpiration rate, soil and leaf water potentials and estimated canopy and plant hydraulic conductance during soil drying.

Key result: Under wet soil conditions, the high mucilage genotype restricted transpiration rate at lower VPD (1.46 kPa) compared to the low mucilage genotype (1.58 kPa). Similarly, the initial slope of transpiration rate in response to VPD (the maximum conductance) was significantly lower in the high mucilage genotype compared to low mucilage genotype. During soil drying, the transpiration rate declines earlier in the low mucilage genotype compared to the high mucilage genotype, supporting the hypothesis that mucilage helps maintain the hydraulic continuity between roots and soil at lower water potentials in high mucilage genotype.

Conclusions: Root mucilage is a promising trait that reduces water use under wet soil conditions, hereby conserving soil moisture for critical phases (e.g. flowering and grain filling), both on a daily basis (increasing VPD) and seasonal timescale (soil drying).

Keywords: Cowpea; Soil drying; Soil hydraulic conductivity; Vapor pressure deficit.