Use of plant growth regulators to reduce 2-methyl-4-chlorophenoxy acetic acid-Na (MPCA-Na) damage in cotton

Background: 2-methyl-4-chlorophenoxy acetic acid-Na (MPCA-Na) is a phenoxy carboxylic acid selective hormone herbicide that is widely used in crop fields. However, drift of MPCA-Na during application is highly damaging to cotton plants. This study was carried out from 2019 to 2020 to determine the effects of different concentrations of MPCA-Na on physiological metabolism and cotton yield at the seedling, budding, flowering and boll stages. Moreover, we evaluated the different combinations of brassinosteroids, gibberellin, phthalanilic acid and seaweed fertilizer, which used to ameliorate herbicide damage. Results: MPCA-Na exposure caused a decrease in the chlorophyll content, and an increase in the soluble protein content, MDA content and protective enzyme activity. MPCA-Na also caused significant reductions in plant height, boll number and the single boll weight at the seedling and budding stages, but had little effect on plant height and the single boll weight at flowering and boll stages. Under the maximum recommended dose of MPCA-Na (130 g/L), the number of cotton bolls at seedling and bud stages decreased by 75.33% and 79.50%, respectively, and the single boll weight decreased by 46.42% and 36.31%, respectively. However, the number of cotton bolls and single boll weight at flowering and boll stages decreased by 48.15% and 5.38%, respectively. Application of plant growth regulators decreased the MDA content, and increasing chlorophyll, soluble protein content and protective enzyme activity, and reducing the degree of MCPA-Na damage. Positive effects were also observed in terms of cotton yield. Conclusions: The results suggest that certain plant growth regulators could be used to alleviate MPCA-Na damage and maintain cotton yield. When the cotton exposed to MCPA-Na at the seedling stage should be treated with phthalanilic acid + seaweed fertilizer, while plants exposed at the budding stage should be treated with brassinosteroids + seaweed fertilizer, and those exposed at the flowering and boll stages should be treated with gibberellin + seaweed.