Effects of 2,4-dichlorophenoxyacetic acid in artificial acid rain on Brassica napus and Sinapis alba.

The effect of 2,4-dichlorophenoxyacetic (2,4-D) acid in artificial rain at pH 3.3 was examined for young plants of rape (Brassica napus L.) and white mustard (Sinapis alba L.). Plants were treated in a closed simulation chamber system with varying CO2 supply (198-418 ppm). Single or daily showers were applied for 20-30 min at varying 2,4-D concentrations (0, 0.03, 0.1, 0.5, and 2.5 mg liter-1). The plants were sampled 5 days after the last treatment. Epinasty was observed for Brassica at 0.5 mg liter-1 and for Sinapis at 0.1 mg liter-1 2,4-D. Leaf to stem dry weight ratios increased with increasing CO2 concentrations in the air and decreased following single treatments with 0.5 or 2.5 mg liter-1 2,4-D in the rain. Dose-effect relationships are proposed for both plant species. Leaf dry matter weights per area unit decreased with increasing CO2 concentrations, but were not significantly affected by the 2,4-D treatments.