[Effects of Combined Pollution of Cd and Microplastics on Winter Wheat Based on the PLS Model: Phytotoxicity and Soil Properties].

Abstract

To explore the effects of heavy metals, microplastics, and their combined action on the growth, physiological ecology, and soil physicochemical properties of winter wheat （Triticum aestivum L.）, we sought to identify the major controlling factors and thus to provide a theoretical basis for revealing the physiological ecology response mechanism and ecological restoration of contaminated soil. Soil culture treatment experiments were conducted to study the effects of the heavy metal cadmium （Cd） （0 mg·kg^-1 and 5 mg·kg^-1） and polypropylene microplastics （PP-MPs） with different particle sizes （10 μm and 500 μm） and mass concentration （0, 0.5%, 1.0%, and 5.0%） on winter wheat growth, photosynthetic physiology, antioxidant enzyme activity, leaf anatomy, canopy temperature, soil nutrients, and soil enzyme activity. Moreover, a partial least squares （PLS） model was used to quantify the relationship between physical and chemical indicators and winter wheat growth status and to identify the major controlling factors. The results showed that the plant height, leaf area, and total biomass of winter wheat decreased by 10.3%-59.9%, 5.8%-94.2%, and 20.0%-84.0%, respectively, under the pollution condition of small particle size PP-MPs alone and combined with Cd. In addition, photosynthetic characteristics, such as photosynthetic efficiency and chlorophyll content of wheat leaves were significantly inhibited under the conditions of both pollutants. With the increase of pollution stress, the canopy temperature of the wheat population increased, and the leaf thickness decreased. Compared with that in CK, the superoxide dismutase （SOD）, peroxidase （POD）, and catalase （CAT） of leaves increased by 13.4%-99.0%, 45.5%-122.7%, and 2.8%-89.2%, respectively, and the interaction between them was extremely significant （P<0.01）. In addition, Cd and PP-MPs also slightly increased the contents of soil organic matter, alkali-hydrolyzed nitrogen, available phosphorus, and available potassium and significantly improved the activities of soil urease, acid phosphatase, and dehydrogenase. In summary, the combined Cd-PP-MPs pollution had synergistic inhibition effects on the above indicators, and the inhibiting effects of 10 μm PP-MPs were significantly stronger than those of 500 μm. PLS results showed that soil acid phosphatase was the key control factor affecting the growth and development indices of winter wheat under the combined pollution stress of Cd and 10 μm PP-MPs, and soil available phosphorus was the key influencing factor of 500 μm large-particle size PP-MPs. The results provide reference for evaluating the ecological effects of heavy metal Cd and MPs combined pollution in the soil-plant system.