Effects of elevated atmospheric O3 concentrations on early and late leaf growth and elemental contents of Acer truncatum Bung under mild drought

To investigate the possible interactive effects of elevated atmospheric ozone (O₃) concentrations and periodic drought stress on physiology of Shantung maple (Acer truncatum Bung), an experiment was conducted from the growth season of 2012 to 2013 with open-top chambers (OTCs) in Changping district, a suburb of Beijing, China. Four treatments were administered with three replications in twelve OTCs which were NN (well watered + ambient air), NO (well watered + add 100 nl l^? 1 O₃ above ambient air), DN (drought stress + ambient air) and DO (drought stress + add 100 nl l^? 1 O₃ above ambient air). Leaf area (LA), leaf mass per area (LMA), individual leaf weight (ILW), carbon(C), nitrogen (N) and sulfur (S) contents in early and late leaves were measured at the end of the second year. The results showed: (1) Both elevated O₃ concentration and drought treatments significantly reduced early leaf LMA, LA, ILW, leaf N and S contents, with a reduction of 28.7, 45.7, 61.3, 39.6, 16.1% by O₃ stress and 12.5, 46.8, 53.5, 15.45 and 22% by drought stress, respectively, while only LMA of late leaf was reduced 12.1% by O₃ treatments and LA and ILW were significantly reduced 23.3% and 30% by drought treatments. (2) Significant interactions of elevated atmospheric O₃ concentration and mild drought were detected on LMA, LA, ILW, N and C contents in early leaves and LMA in late leaves. Except for LA, the decreases under interactive treatments were all less than independent O₃ effects. In conclusion, late leaf had less responses to elevated O₃ and drought stresses than early leaves which need to be considered separately. The interactive effects suggested drought had antagonistic effects with O₃ on growth indicators except for LA, indicating drought could mitigate the adverse efforts from O₃ effects.