Physiological Responses of Populus cathayana and Salix babylonica to Combined Stress of Diesel Fuel and Sr2+ Stress in Soil

Diesel spills and nuclides pollution cause global ecosystem and human health problems. The remediation of contaminated soil using woody plants has received considerable attention. Differences in plant species and sex can lead to differences in tolerance to various stressors. We aimed to investigate the response of male and female seedlings of <i>Populus cathayana</i> and <i>Salix babylonica</i> to diesel and Sr²⁺ stress and to compare the enrichment characteristics of Sr²⁺ in trees. Male and female seedlings of <i>P. cathayana</i> and <i>S. babylonica</i> were treated with diesel fuel and 0, 10 (low), and 100 (high) mg Kg^-1 of Sr²⁺. Results showed that <i>P. cathayana</i> and <i>S. babylonica</i> had good enrichment characteristics and tolerance. <i>S. babylonica</i> had a more robust tolerance and ability to remediate contaminated soil than <i>P. cathayana</i>. The defense mechanisms of both female seedlings in response to stress were similar, while males showed different defense strategies. Male trees had higher Sr²⁺ enrichment capacity, antioxidant enzymes, soil enzyme activity, and soluble matter content, indicating that males had higher tolerance capacity than females. Under diesel stress alone, the reduced photosynthetic rate of male seedlings of <i>P. cathayana</i> was mainly limited by stomatal factors, and their photosynthetic system was more tolerant to diesel. POD and APX activities, as well as alkaline phosphatase and urease activities in the soil, were significantly higher in <i>S. babylonica</i> seedlings than in <i>P. cathayana</i>, indicating that <i>S. babylonica</i> seedlings were more resistant to diesel pollution. At low concentrations of the Sr²⁺ complex, diesel and Sr²⁺ showed antagonistic effects in reducing the damage caused by stress. As the Sr²⁺ concentration increased, damage to the plants manifested primarily through synergistic enhancement. The results of this study provide a scientific basis for the remediation of diesel fuel and nuclides contaminated soils using woody plants.