Methomyl-induced phytotoxicity in cotton (G̲o̲s̲s̲y̲p̲i̲u̲m̲ h̲i̲r̲s̲u̲t̲u̲m̲ L.)

Abstract

Understanding the causes of insecticide-induced stress to crop plants could help to increase yields. This study examined phytotoxicity of methomyl (S-methyl-N- [(methylcarbamoyl)oxy]-thioacetimidate) on cotton (Gossvpium hirsutum L.), with special reference to leaf spotting and photosynthetic activity. Three sets of experiments were performed. The objectives in the first set of experiments were 1) to establish the time courses of photosynthetic injury and leaf reddening and 2) to examine relationships between carbon exchange rate (CER), stomatal conductance, chlorophyll fluorescence, and ribulose-1,5-bisphosphate carboxylase (rubisco) in leaves of methomyl-treated, field-grown cotton. In methomyl-treated leaves, CERs, stomatal conductance, rubisco, and chlorophyll fluorescence all decreased after spraying, but recovered to levels near to controls. The CERs were more closely related to stomatal conductance than to chlorophyll fluorescence or rubisco. Ethylene production is often associated with plant stress. The objective of the second set of experiments was to determine if ethylene is involved in methomyl-induced photosynthetic injury and leaf reddening. Cotton plants grown in environmentally controlled growth chambers were treated with 0, 2, 4, or 8 mM Ag (an ethylene action inhibitor). Three hours later, plants were sprayed with methomyl or a control spray. Silver treatment alleviated methomyl-induced decreases in CER, usually by hastening recovery, but occasionally by nearly eliminating injury. Once again, CERs were more strongly related to stomatal conductance than to chlorophyll fluorescence. Leaf spotting in methomyl-treated leaves was alleviated, but not eliminated, by Ag treatment. Various physiological and cultural considerations may affect severity of phytotoxicity. Thus, the objective of the third set of experiments was to determine effects of spray number, leaf position, and methomyl rate on methomyl-induced photosynthetic injury and leaf spotting of field-grown cotton. Percent spotting was greater in younger leaves than in older leaves. Whole-plot reddening was greater with two sprays than with one spray and greater at 0.84 kg ha^-1 methomyl than at 0.62 kg ha^-1. The only major differences in chlorophyll fluorescence and pigment concentrations were between methomyl-treated and non-methomyl-treated plots.