THE INFLUENCE OF OZONE AND NUTRITION ON DELTA-C-13 IN BETULA-PENDULA

In the cellulose of stems and leaves, delta(13)C was investigated in a birch clone (Betula pendula), which was exposed throughout the growin g season to either <3 (control) or 90/40 nl O-3 1(-1) (day/night). Eac h regime was split into plants under high or low nutrient supply. delt a(13)C was increased (becoming less negative), in stems rather than le aves, by both high nutrition (+2 parts per thousand) and O-3 stress (1 parts per thousand). Whereas high nutrition raised the water-use eff iciency (WUE) while lowering the CO2 concentration in the inner leaf a ir space (c(i)), WUE decreased and c(i) increased under O-3 stress. Th erefore, only the nutritional effect on the carbon isotope fractionati on was reproduced by the model of Farquhar et al. (1982) which estimat es WUE by means of delta(13)C based on C-i, C-i was not biased by 'pat chiness' in respect to stomatal opening. The latter was verified by mi croscopical analysis and the complete water infiltration of the birch leaves through the stomata, independent of the diurnal course of the l eaf conductance for water vapour. Under low nutrient supply, the activ ity of phosphoenol pyruvate carboxylase (PEPC) was roughly doubled by ozone to about 1.3% of the total carboxylation capacity (by PEPC + rub isco), and was increased to 1.7% under high nutrition. The fractionati on model, extended to account for varying activities of the carboxylat ing enzymes, indicated that stimulated PEPC was the cause of elevated delta(13)C, although c(i) was increased under O-3 stress. The stimulat ion of PEPC and, as a consequence, elevated delta(13)C are discussed a s part of a whole-plant acclimation to O-3 stress.