CHANGE IN STOMATAL-RESISTANCE AND WATER-USE IN SUBTERRANEAN CLOVER (TRIFOLIUM-SUBTERRANEUM L) IN RESPONSE TO BROADLEAF HERBICIDES

The effect of 5 broadleaf herbicides on the water use and stomatal res istance of 2 cultivars of subterranean clover (Trifolium subterraneum L.) was examined in a glasshouse study. The herbicide treatments 2,4-D B, 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 an d 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 he rbicide 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 M CPA and 2,4-DB the least reduction (16-22%). Stomatal resistance incre ased substantially within 2 days of application of each of the herbici des. The magnitude of the change differed with herbicide and increased with herbicide rate. The effect of the herbicides on stomatal resista nce 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 MCPA+terbutryn and MCPA+diuron and bromoxynil caused the largest increase in stomatal resistance and 2,4-DB the least. Stom atal resistance was found to be highly negatively correlated with dail y water use by the clover plants at 2 days (r = -0.84, P < 0.01) and 3 0 days (r = -0.88, P < 0.01) after herbicide application. All of the h erbicides reduced the LAI 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 culti var, leaf stage, and spraying time accounted for a relatively small pr oportion of the variance. The findings support the hypothesis that som e broadleaf herbicides can result in a water-saving effect in subterra nean clover swards through increasing stomatal resistance and decreasi ng the LAI, thereby potentially reducing moisture stress during seed s et.