Leaf photosynthesis, stomatal resistance, and growth of wine grapes (Vitis vinifera L.) after exposure to simulated chlorsulfuron drift

Abstract

The lower Yakima Valley of Washington is a highly diversified irrigated agricultural region bordered by dryland wheat fields. Residues of herbicides sprayed in wheat can be atmospherically transported to susceptible nontarget crops in the valley. Nontarget crops may be exposed repeatedly to low levels of herbicide residues. The effects on grapes exposed to phenoxyacetate herbicide drift has been well documented, but the effects of comparatively newer wheat herbicides like sulfonylureas (SUs) are less known. Potential effects of repeated exposures of grapes to an SU herbicide were assessed in a simulated drift study. Grape vines of the cultivar “Lemberger”; were sprayed up to three times at a weekly interval with 1/100 (0.01X) of a field application rate of chlorsulfuron, which is a postemergence wheat herbicide. Thirty‐five days after the first application, photosynthesis and stomatal resistance of randomly tagged, fully expanded leaves were measured. Total leaf area and chlorotic leaf area were also measured on the same leaves. The percentage of chlorotic area was calculated to determine the correlation between chlorotic area and photosynthetic activity of the leaf. Foliar injury was indexed on a numerical scale and scored in three intervals over 120 days of growth. The severity of leaf injury increased after multiple exposures to chlorsulfuron, but symptoms had dissipated by 120 days after application. Leaf photosynthesis was reduced and stomatal resistance was increased linearly with an increase in the percentage of chlorotic leaf area. Three consecutive applications of chlorsulfuron resulted in 50% chlorotic leaf area that was associated with 25% reduction in the net leaf photosynthesis and a 59% increase in the stomatal resistance. The reduction in photosynthesis was probably caused in part by an increase in stomatal resistance and in part by a large increase in the chlorotic leaf area. Shoot length and pruning weight were not affected by the number of exposures. However, leaf area was significantly reduced following three weekly exposures. The results suggested that significant reduction in photosynthetic activity only occurred when general leaf chlorosis exceeded 50%. Isolated diffuse chlorotic spots have previously been reported as characteristic of nonpoint atmospheric deposition of SU herbicide residues. However, our results showed that such symptoms should not adversely affect plant photosynthesis.