Soil texture shapes plant adaptation to edaphic stress

The role of root mucilage in facilitating water uptake during soil drying has been studied for decades. Recently, we demonstrated that mucilage slows the dissipation of water potential in the rhizosphere of actively transpiring plants. While these findings provide new insights into how mucilage maintains the hydraulic continuity between soil and roots under drying conditions, the interaction between mucilage and soil texture remains underexplored.

We used two cowpea genotypes with contrasting mucilage production, grown in two distinct soil textures (coarse and fine), and measured physiological and morphological parameters during and after a dry-down experiment. We hypothesized that mucilage would have a greater role in coarse-textured soils due to its ability to form polysaccharide networks within larger soil pores, enhancing hydraulic connectivity during drying.

Although shoot biomass did not differ between genotypes and soil textures, root morphological analysis revealed significant adaptations to soil texture. The low-mucilage genotype developed a root system twice as long in sand compared to loam, while the high-mucilage genotype showed only a slight increase in root length in sand. Normalized transpiration rates and leaf water potential were similar between genotypes in loam. However, in sand, the high-mucilage genotype maintained relatively lower leaf water potentials (≤ -1.0 MPa), while the low-mucilage genotype closed its stomata at less negative leaf water potentials (≤ -0.6 MPa). These results underscore the critical role of soil texture in shaping plant drought responses and highlight the importance of mucilage in enhancing water uptake in coarse soils.

The ability of mucilage to maintain hydraulic continuity during soil drying is particularly beneficial in coarse-textured soils, where larger pores cause steep decline in water potential in the rhizosphere. The contrasting strategies observed in the two cowpea genotypes—root system elongation versus mucilage-driven water retention—highlight the diverse adaptations plants employ to cope with edaphic stress.