Effects of elevated ozone on leaf δ13C and leaf conductance of plant species grown in semi-natural grassland with or without irrigation

Stable carbon isotope ratios (δ13C) and leaf conductance (gs) were measured (2002, 2003) in Holcus lanatus L., Plantago lanceolata L. Ranunculus friesianus (Jord.), and Trifolium pratense L. at two levels of ozone (O3) with or without irrigation. In non-irrigated control plots, R. friesianus showed the least negative δ13C, and the smallest response to the treatments. Irrigation caused more negative δ13C, especially in H. lanatus. Irrespective of irrigation, O3 increased δ13C in relationship to a decrease in gs in P. lanceolata and T. pratense. The strongest effect of O3 on δ13C occurred in the absence of irrigation, suggesting that under field conditions lack of moisture in the top soil does not always lead to protection from O3 uptake. It is concluded that in species such as T. pratense plants can maintain stomatal O3 uptake during dry periods when roots can reach deeper soil layers where water is not limiting.