Mixed Cultivation With Siberian Alder Worsens the Negative Impact of Salt Exposure on Growth and Major Physiological Processes of Black Locust Plants

Salinity is a major threat that can greatly affect the growth and development of plants. Mixed planting has been shown to effectively improve the salinity tolerance of tree species. However, it is still unknown whether admixing alder trees promotes the growth and development of N₂-fixing Robinia pseudoacacia on saline soil. To address this question, single-planted Robinia seedlings and Robinia seedlings mixed with Siberian alder (Alnus hirsuta Turcz. ex Rupr.) were subjected to salt exposure, and growth parameters, photosynthetic characteristics, cation contents and ratios, osmoregulation by compatible solutes, and antioxidant metabolism were analyzed. The results showed that salt exposure significantly reduced whole-plant biomass, net photosynthesis rate, total chlorophyll content, and nodule nitrogenase activity of Robinia at both single and mixed planting with Siberian alder. Salt exposure significantly increased the accumulation of Na⁺, malondialdehyde, and hydrogen peroxide in the leaves of both single and mixed planted Robinia. Under salt exposure, mixed planting significantly inhibited the growth and nitrogen fixation capacity of Robinia due to interspecific competition, indicated by reduced biomass accumulation and photosynthetic efficiency compared with single planting. In addition, mixed planting exacerbated Na⁺ toxicity in Robinia roots and enhanced oxidative stress in Robinia leaves and roots, indicated by elevated hydrogen peroxide contents. These results indicate that mixing Robinia with Siberian alder at the seedling stage not only inhibits the nitrogen fixation capacity of Robinia but also reduces its salinity tolerance. They provide an important theoretical reference for the establishment of mixed Robinia forests at the seedling stage, particularly in salinized soils.