Root exudates reduce electrical conductivity and water potential of rhizospheres and facilitate non-halophytes to survive in dry land saline soils.

Abstract

High amount of salts in dry-land soils (high ECe) reduces water potential which causes plasmolysis in agricultural crops and result in food grain yield reduction. Salt tolerant non-halophytic trees and shrubs, however, survive the highly saline soils as they extrude Na+ from their cell cytoplasm by an active Na+/H+ antiport and/ or compartmentalize it into vacuoles and adjust osmotic pressure of the cytoplasm and also uptake water from that part of rhizosphere and soil depth, which contain a relatively lower amount of salts. But, how they change electrical conductivity and water potential in their rhizospheres is not yet known. Aims of this study were to know root distribution in different soil depths; changes in salt concentrations in the soil depths across seasons; and to understand how glucose exuded from roots changes pHs, ECe, salt concentrations and water potential in rhizospheric soils of the trees and shrubs in highly dry-land saline soils. Distribution of fine roots of two salt-tolerant non-halophytic trees (i.e. Acacia nilotica, Tamaryx aphylla) and a shrub (i.e. Prosopis juliflora) species in different soil depths (0–15, 15– 30, 30–45, 45–60, 60–75, 75–90 and 90–105 cm) and changes in salt concentrations across the soil depths between two seasons (i.e. rainy: rainfall 400 mm, temp. 10 °C; summer: rainfall 0 mm, temp. 46 °C) were examined. To test the effect of glucose on ECe, pHs and salt concentrations, an ex-situ experiment was conducted where soils from the rhizospheres of the species and open plots were treated with three doses of dextrose (0 g + 1 g soils, 0.25 g + 1 g soils, 0.50 g + 1 g soils). The study revealed that fine roots were found in all depths of soils under the trees and shrub species. Maximum (32 to 60%) salts were located in to 0–15 cm during the summer season, which declined to 20 to 40% during rainy season. During raining season reverse was observed, which created vertical heterogeneity in salt concentrations across the seasons. In the ex-situ experiment, the dextrose (0.25 g + 1 g-1 and 0.50 g + 1 g−1 rhizospheric soils) was found to form gluconic acid, which lowered pHs, proportionately greater in the soils treated with higher amount of the dextrose. In the presence of gluconic acid / dextrose, Cl- probably formed OCl4 - , and Na+ and K+ formed their gluconates with some amount of gluconic acid; these changes lowered ECe, which dropped water potential of the rhizospheric soils and created horizontal heterogeneity in salt concentrations. These horizontal and vertical heterogeneities likely facilitated water uptake to the non-halophytic tree-shrub species. The decline in ECe and pHs due to addition of dextrose sheds a new light on how glucose, exuded through roots, helps the non-halophytic trees to survive in the highly dryland saline soils; these declines are a major target for development of techniques for reclamation of salt affected soils.