Study on Tobacco Phytotoxicity Thresholds Caused by Residues of Two Herbicides Used in Paddy Fields

In order to guide the rational use of 2,4-D butyl ester and fluroxypyr in paddy fields in tobacco-rice rotation areas, the residues and digestion dynamics of the two herbicides in soil and tobacco, tobacco plant phytotoxicity symptoms and endogenous hormone content changes were studied by indoor pot experiment and field experiment. The results showed that:(1) The residues of two herbicides could lead to dwarfing of tobacco plants, serious contortion of stems, curling and yellowing of tobacco leaves. (2) The residues of two herbicides significantly decreased the contents of IAA and GA₃, and significantly increased the content of ABA in tobacco (p<0.05). (3) With the increase of application dose and treatment time, the inhibition effect on agronomic traits of tobacco plants was enhanced. Taking the inhibition rate of 10%-15% for plant height or leaf area as the standard, the critical thresholds of 2,4-D butyl ester and fluroxypyr in soil were 1.022 mg/kg and 0.053 mg/kg, respectively, and the critical thresholds in leaves were 0.047 8 mg/kg and 0.027 6 mg/kg, respectively. (4) Combined with the critical threshold, the safety intervals of 2,4-D butyl ester and fluroxypyr in the field were estimated to be 53 days and 160 days, respectively.