Change in stomatal resistance and water use in subterranean clover (Trifolium subterraneum L.) in response to broadleaf herbicides

Abstract

The effect of 5 broadleaf herbicides on the water use and stomatal resistance of 2 cultivars of subterranean clover (Trifolium subterraneum L.) was examined in a glasshouse study. The herbicide treatments 2,4-DB, MCPA, bromoxynil, MCPA+terbutryn, and MCPA+diuron were applied at 6 rates at 2 times (14 May, 14 June) to plants at 2 leaf stages (3-4 and 8-10 leaves). Each of the herbicides reduced water use by the clover within 24 h, the size of the reduction increasing with the rate of herbicide applied. The herbicide treatments MCPA+terbutryn, MCPA+diuron, and bromoxynil caused the largest reductions (44-52%) in total water use over the 30-day period when applied at the recommended rate, and MCPA and 2,4-DB the least reduction (16-22%). Stomatal resistance increased substantially within 2 days of application of each of the herbicides. The magnitude of the change differed with herbicide and increased with herbicide rate. The effect of the herbicides on stomatal resistance declined 10-20 days after herbicide application in all treatments except 2,4-DB, but stomatal resistance of all herbicide-treated plants was still higher than the control 30 days after herbicide application. The herbicides LICPA+terbutryn and LlCPA+diuron and bromoxynil caused the largest increase in stomatal resistance and 2,4-DB the least. Stomatal resistance was found to be highly negatively correlated with daily water use by the clover plants at 2 days (r = -0.84, P < 0.01) and 30 days (r = -0.88, P < 0.01) after herbicide application. All of the herbicides reduced the LA1 of the plants, the effect increasing as the herbicide rate increased. Herbicide and herbicide rate had the largest effect on both water use and stomatal resistance; the effect of cultivar, leaf stage, and spraying time accounted for a relatively small proportion of the variance. The findings support the hypothesis that some broadleaf herbicides can result in a water-saving effect in subterranean clover swards through increasing stomatal resistance and decreasing the LAI, thereby potentially reducing moisture stress during seed set.