EFFECTS OF ELEVATED CO2 AND OR O-3 ON GROWTH, DEVELOPMENT AND PHYSIOLOGY OF WHEAT (TRITICUM-AESTIVUM L)/

Two cultivars of spring wheat (Triticum aestivum L. cvs. Alexandria an d Hanno) and three cultivars of winter wheat (cvs. Riband, Mercia and Haven) were grown at two concentrations of CO2 [ambient (355 mu mol mo l(-1)) and elevated (708 mu mol mol(-1))] under two O-3 regimes [clean air (< 5 nmol mol(-1) O-3) and polluted air (15 nmol mol(-1) O-3 at n ight rising to a midday maximum of 75 nmol mol(-1))] in a phytotron at the University of Newcastle-upon-Tyne. Between the two-leaf stage and anthesis, measurements of leaf gas-exchange, non-structural carbohydr ate content, visible O-3 damage, growth, dry matter partitioning, yiel d components and root development were made in order to examine respon ses to elevated CO2 and/or O-3. Growth at elevated CO2 resulted in a s ustained increase in the rate of CO2 assimilation, but after roughly 6 weeks' exposure there was evidence of a slight decline in the photosy nthetic rate (c.-15%) measured under growth conditions which was most pronounced in the winter cultivars. Enhanced rates of CO2 assimilation were accompanied by a decrease in stomatal conductance which improved the instantaneous water use efficiency of individual leaves. CO2 enri chment stimulated shoot and root growth to an equivalent extent, and i ncreased tillering and yield components, however, non-structural carbo hydrates still accumulated in source leaves. In contrast, long-term ex posure to O-3 resulted in a decreased CO2 assimilation rate (c.-13%), partial stomatal closure, and the accumulation of fructan and starch i n leaves in the light. These effects were manifested in decreased rate s of shoot and root growth, with root growth more severely affected th an shoot growth. In the combined treatment growth of O-3-treated plant s was enhanced by elevated CO2, but there was little evidence that CO2 enrichment afforded additional protection against O-3 damage. The red uction in growth induced by O-3 at elevated CO2 was similar to that in duced by O-3 at ambient CO2 despite additive effects of the individual gases on stomatal conductance that would be expected to reduce the O- 3 flux by 20%, and also CO2-induced increases in the provision of subs trates for detoxification and repair processes. These observations sug gest that CO2 enrichment may render plants more susceptible to O-3 dam age at the cellular level. Possible mechanisms are discussed.