Interactive effects of drought and salt stresses on the growth and physiological characteristics of Thinopyrum ponticum

Saline soils are mainly distributed in arid and semi-arid regions where both soil salt content and drought affect plant growth at the same time. Therefore, soil pot experiments were conducted to study the effects of soil salt content, drought, and their interactions on the growth and physiological characteristics of Thyropyrum ponticum seedlings to understand the adaptability of T. ponticum seedlings in saline soil areas and to provide a theoretical basis for the population establishment of T. ponticum in such areas. Twelve treatments, including four soil salinity levels (0, 4, 8, and 12 g∙kg⁻¹ NaCl) and three soil moisture levels (75%–85%, 55%–65%, and 35%–45% of field capacity) were used. Shoot and root dry weights, leaf chlorophyll contents (SPAD values), leaf photosynthetic parameters, leaf antioxidant enzymes activities, and Na⁺ and K⁺ contents in shoots and roots were measured. The results showed that the growth of T. ponticum was significantly inhibited under drought stress or salt stress, whereas root/shoot ratios increased under the interactions of soil salt and drought stress, and the plants could still survive under 35%–45% field capacity with 12 g∙kg⁻¹ NaCl. Drought and salt stress significantly reduced leaf chlorophyll content, net photosynthetic rate, stomatal conductance, and transpiration rate, whereas these parameters were increased under high salinity levels under moderate drought stress (55%–65% of field capacity) compared with those under normal irrigation. Moderate drought or salt stress increased the activities of superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT), but these activities were decreased under 12 g∙kg⁻¹ NaCl. The leaf malondialdehyde (MDA) content increased under extreme drought or salt stress. In terms of ion accumulation, Na⁺ and K⁺ contents, and K⁺/Na⁺ ratio were higher in shoots than in roots. Roots and shoots Na⁺ contents were increased with decreasing soil water content or increasing soil salinity, whereas K⁺ contents decreased but remained relatively high. Regarding salt-drought interactions, drought treatment could reduce salt stress in plants by promoting the accumulation of Na⁺ in roots. Under drought stress, root Na⁺ content was further increased by increasing soil salinity, while root K⁺/Na⁺ ratio remained stable and shoot K⁺/Na⁺ ratio was significantly decreased. The above results indicate that T. ponticum can survive under high drought and salt conditions, possibly due to its strong root system, higher root/shoot ratio, relatively complete antioxidant enzyme system, Na⁺ accumulation, and stable K⁺/Na⁺ ratio in roots. Although T. ponticum can survive under high drought and salt conditions, the high production of biomass still requires lower soil salinity and moderate drought conditions because the shoot biomass significantly decreases with the increase in soil salinity or decrease in soil moisture.