INFLUENCE OF OZONE ON THE STABLE CARBON ISOTOPE COMPOSITION, DELTA-C-13, OF LEAVES AND GRAIN OF SPRING WHEAT (TRITICUM-AESTIVUM L)

The relative composition of stable carbon isotopes, delta-C-13, 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 O3 levels during the growing season. The aim of the study was to establish exposure-response relationships for the radiation-weighted seasonal mean O3 concentration and delta-C-13 (relative deviation of the C-13/C-12 ratio) values of the two plant parts. Samples were collected at harvest in 1986, 1987, and 1988. With increasing O3 concentration, delta-C-13 values increased (became less negative) proportionally. Year to year delta-C-13 differences at equivalent O3 concentrations were small. The shift in delta-C-13 caused by O3 was more pronounced in grain than in leaves. According to models of C-13 discrimination in C3 plants, these results indicate increasing limitation of photosynthesis by CO2 diffusion relative to limitation by carboxylation with increasing O3 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 O3, indicating a dominating effect of O3 on CO2 carboxylation.