Influence of Ozone on the Stable Carbon Isotope Composition, δ13C, of Leaves and Grain of Spring Wheat (Triticum aestivum L.)

The relative composition of stable carbon isotopes, δ 13 C, was determined in flag leaves and grain of spring wheat ( Triticum aestivum L. cv Albis) grown in open-top field fumigation chambers and exposed to different O 3 levels during the growing season. The aim of the study was to establish exposure-response relationships for the radiation-weighted seasonal mean O 3 concentration and δ 13 C (relative deviation of the 13 C/ 12 C ratio) values of the two plant parts. Samples were collected at harvest in 1986, 1987, and 1988. With increasing O 3 concentration, δ 13 C values increased (became less negative) proportionally. Year to year δ 13 C differences at equivalent O 3 concentrations were small. The shift in δ 13 C caused by O 3 was more pronounced in grain than in leaves. According to models of 13 C discrimination in C 3 plants, these results indicate increasing limitation of photosynthesis by CO 2 diffusion relative to limitation by carboxylation with increasing O 3 exposure. This conclusion is not in agreement with results from gas exchange analysis. Water use efficiency in green flag leaves tended to decrease with increasing O 3 , indicating a dominating effect of O 3 on CO 2 carboxylation.