Mucilage exudation facilitates root water uptake in dry soils

Abstract

As plant roots take up water and the soil dries, water depletion is expected to occur in the rhizosphere. However, recent experiments showed that the rhizosphere was wetter than the bulk soil during root water uptake. We hypothesise that the increased water content in the rhizosphere was caused by mucilage exuded by roots. It is probably that the higher water content in the rhizosphere results in higher hydraulic conductivity of the root–soil interface. In this case, mucilage exudation would favour the uptake of water in dry soils. To test this hypothesis, we covered a suction cup, referred to as an artificial root, with mucilage. We placed it in soil with a water content of 0.03 cm3 cm–3, and used the root pressure probe technique to measure the hydraulic conductivity of the root–soil continuum. The results were compared with measurements with roots not covered with mucilage. The root pressure relaxation curves were fitted with a model of root water uptake including rhizosphere dynamics. The results demonstrated that when mucilage is added to the root surface, it keeps the soil near the roots wet and hydraulically well conductive, facilitating the water flow from dry soils towards the root surface. Mucilage exudation seems to be an optimal plant trait that favours the capture of water when water is scarce.

Plant exudates improve the mechanical conditions for root penetration through compacted soils

“…Although exudation clearly represents a significant carbon cost to the plant, exudates are involved in engineering the rhizosphere by dispersion and gelling of soil (Naveed et al 2017;Barré and Hallett 2009;Tarchitzky and Chen 2002;Deng et al 2015), modulation of water and nutrient availabilities (Wang et al 2008;Ahmed et al 2014;Deng et al 2015), and attraction of rhizobacteria (Bais et al 2006). To our knowledge this is the first time that plant exudates have been demonstrated to ease soil compression and thus offer the potential for increased root elongation in soil.…”

“…Chia seed exudate has been widely used in other studies as a model root exudate (Ahmed et al 2014;Kroener et al 2014). It was extracted based on Naveed et al (2017) and Ahmed et al (2014) by mixing 100 g distilled water with 10 g chia seeds using a magnetic stirrer for 2 min at 50°C, followed by cooling to room temperature (20°C) and 4 h standing.…”

“…It was extracted based on Naveed et al (2017) and Ahmed et al (2014) by mixing 100 g distilled water with 10 g chia seeds using a magnetic stirrer for 2 min at 50°C, followed by cooling to room temperature (20°C) and 4 h standing. The exudate was separated from the seeds by repeatedly pushing the mixture through a 500 μm sieve under pressure using a syringe that was cut at the end.…”

“…One driver is enhanced pore structure, which Reszkowska et al (2011) found helped to recover hydraulic conductivity of rhizosphere soil in a degraded pasture field under wet conditions. Exudates can therefore decrease plant water stress by regulating water content dynamics and aiding capture of water in the rhizosphere Ahmed et al 2014).…”

Connecting the dots between computational tools to analyse soil-root water relations

“…1G). Their respective spatial domains are concentric around individual roots, and their properties differ substantially, so that the rhizosphere is often considered to critically affect plant water availability under water deficit [16,17] . The bulk soil and rhizosphere hydraulic properties may be described by their water retention and hydraulic conductivity curves.…”