Ozone and Low Concentrations of Nitric Oxide have Similar Effects on Carbon Isotope Discrimination and Gas Exchange in Leaves of Wheat (Triticum aestivum L.)

Field-grown spring wheat (Triticum aestivum L. cv. Albis) was exposed in open-top chambers to three levels of ozone (O3) with or without addition of 8 ppb nitric oxide (NO), resulting in six different mixtures of ozone, NO, and their gas-phase reaction product nitrogen dioxide (NO2). The aim was to study the effect of the different mixtures on leaf gas exchange and carbon isotope discrimination in flag leaves. From emergence to the first appearance of senescence symptoms, leaf conductance to water vapour (gs) and net CO2 exchange (PN) was analysed together with stable carbon isotope ratios of the plant material (δ13Cp). Relative to the control with low ozone and no NO addition, mixtures with either additional ozone or NO added to charcoal-filtered air caused an increase in gs. but no change in PN , resulting in increased sub-stomatal CO2 concentration (ci). In both cases δ13Cp was not significantly changed. This indicated the occurrence of similar effects of ozone and NO on enzymatic reactions of carbon fixation. When applied in combination with medium or high ozone levels, the effect of NO addition was smaller than in combination with low ozone. These results suggest non-specific responses to reactive oxygen-species generated by both ozone and NO. It is concluded that the effects of NO in the low ppb-range are mainly due to shifts in gasphase chemistry at the leaf-atmosphere interface, and that they are negligible in pollution climates dominated by ozone.