Synergistic effects of acid rain and (±)-2-(2,4-Dichlorophenoxy)-Propanoic acid (2,4-DP) on growth, mineral content and stress-induced ethylene in lettuce

Lettuce grown hydroponically in growth chambers was exposed daily for 10 days to simulated acid rain at pH 3·3, 4·1 and 5·6 with and without 1·5 mg litre⁻¹ 2,4-DP. Increasing acidity reduced contents of Mn, Fe, Mg and Co, increased visible damage, but did not affect yield. Growth was stimulated by 2,4-DP at pH 4·1 and 5·6 but inhibited at pH 3·3. Contents of Zn and Co, as well as visible damage, were affected in a similar synergistic way. Contents of stress-ethylene 8 and 18 h after exposure indicated that the mode of action for synergism may be due to the pH-pK_a-dependent cuticular penetration.