Effects of Combined Stress of Polyethylene and Sulfamethazine on Seed Germination, Seedling Growth, and Physiological Characteristics of Soybean

Abstract

The combined pollution of antibiotics adsorption by microplastics has become inevitable in soil ecosystems; moreover, the plant biological effects under combined stress remain unclear. This study used soybean variety Jindou 21 as the material and conducted seed germination test and soil-potted seedling experiment to study the effects of different single and combined treatments of polyethylene (PE) and sulfamethazine (SMZ) on seed germination, seedling growth, photosynthetic parameters, chlorophyll fluorescence parameters, and nitrogen metabolism. The results showed that single PE treatment at low levels promoted soybean seed germination and seedling growth physiology; however, inhibited them at a high level. A lower-level PE treatment[10 mg·L-1 (or mg·kg-1)] could promote soybean seed germination, seedling growth, photosynthesis, and nitrogen metabolism, whereas a higher level PE treatment[100 mg·L-1 and 200 mg·L-1 (or mg·kg-1)] had significant inhibition. The single SMZ treatment had different degrees of inhibition on soybean seed germination and seedling growth physiology, and the inhibition degree increased with the increase in SMZ treatment level. Under the different levels of combined treatments of PE and SMZ, adding the lower level PE treatment could alleviate the inhibition of the single SMZ treatment on soybean, with 10 mg·L-1(or mg·kg-1) PE+1 mg·L-1(or mg·kg-1) SMZ treatment having the best comprehensive mitigation effect, which could increase soybean seed germination potential, germination rate, germination index, vigor index, plant height, root length, shoot and root fresh weight, Pn, Gs, Tr, chlorophyll contents, Fv/Fm, ΦPSⅡ, ETR, qP, and key enzyme activities for nitrogen metabolism such as NR and decrease the average germination time, Ci, NPQ, and NO3--N and NH4+-N contents compared with those in the single SMZ treatment. Adding the higher level PE treatment enhanced the inhibition of SMZ on soybean, and the inhibition degree increased with the increase in SMZ treatment level, in which 200 mg·L-1(or mg·kg-1) PE+50 mg·L-1(or mg·kg-1) SMZ treatment yielded the greatest inhibition. In summary, the lower level PE treatment could alleviate the inhibition of SMZ on soybean seeds and seedlings to a certain extent; however, the higher level PE treatment could produce a synergistic effect with SMZ, thus aggravating the toxic effect of the single stress treatment.