Effects of Saline–Alkali Composite Stress on the Growth and Soil Fixation Capacity of Four Herbaceous Plants

Abstract

Plants play a crucial role in soil fixation and enhancement of slope stability, and saline–alkaline stress is one of the main restrictions inhibiting plant growth and development. At present, there is a lack of research on the effects of saline–alkaline composite stress on the mechanical properties of the root system and the erosion resistance of the root–soil complex. In this study, three gradients of saline–alkaline composite stress treatments and a control of saline-free treatment was set up for Oenothera biennis, Perilla frutescens, Echinops sphaerocephalus, and Lychnis fulgens. The plant salt damage rate, osmotic index, antioxidant enzyme activity and plant root morphological indicators were measured. The biomechanical characteristics were determined by stretching tests, the resistance of the plant was measured by a whole-plant vertical uprooting test, and the anti-erosion capacity of the root soil composite was measured by scrubbing test. The results showed that, at 200 mM, the salt damage index and salt damage rate of the four plants, in descending order, were as follows: E. sphaerocephalus < L. fulgens < O. biennis < P. frutescens. Among them, SOD of Perilla frutescens did not play an obvious protective role, and the substantial changes in CAT and POD, as well as the content of soluble sugars, soluble proteins, and proline, showed its sensitivity to saline and alkaline stresses. Root growth was also significantly suppressed in all four plants, the 100- and 200-mM concentrations of saline solution significantly reduced the average tensile strength of O. biennis and P. frutescens, while the saline–alkali solution of 200 mM significantly reduced the elongation of E. sphaerocephalus and L. fulgens, and significantly elevated the soil detachment rate of the root–soil composite for E. sphaerocephalus. Additionally, all three concentrations of saline treatments significantly reduced the pullout resistance of all 4 plants. There was a negative power rate relationship between tensile resistance and root diameter in four plant species, while the relationship between tensile strength and root diameter showed a negative power law only for L. fulgens treated with 0–50 mM saline solution. There was no significant correlation between elongation and root diameter in the four plants. P. frutescens had the greatest tensile resistance and strength, as well as the lowest rate of elongation, while L. fulgens possessed the greatest pullout resistance, and both had comparable resistance to erosion of the root–soil complex. Therefore, compared to the other three plants, L. fulgens is more suitable for soil reinforcement applications on saline slopes.