Refixation of respiratory CO2 in the ears of C-3 cereals

The spatial arrangement of tissues within the ears of cereals, and gas exchange measurements on intact ears of barley and durum wheat suggest that respiratory CO2, associated with grain-filling processes, may be refixed close to its site of evolution. Apparent refixation of respiratory CO2 in intact ears was compared with that in flag leaves, by feeding both organs with C-14-labelled sucrose and trapping (CO2)-C-14 released by respiration. Apparent refixation in the ears was twice that measured in flag leaf blades of durum wheat genotype Durelle. In ears, the capacity of refixation of respiratory CO2 at 210 mmol mol(-1) O-2 ranged from 55% in barley genotype Roxana to 75% in barley genotype Hatif, and 60% in durum wheat genotype Bidi 17. A low O-2 concentration increased refixation of respiratory CO2 by up to 90%, 92% and 82%, respectively. The occurrence of CO2 refixation in the field, in a set of 12 barley genotypes grown under irrigated and rainfed Mediterranean field conditions, was consistent with observed carbon isotope ratios (delta(13)C) of current assimilates of the ear bracts, and delta(13)C of mature kernels whose ears had been darkened from 1 week after anthesis to maturity. Moreover, genotypic differences in delta(13)C and its divergence in light compared to darkened ears were observed, both under irrigated and under rainfed conditions. If these differences reflect differences in refixation and are genotypically stable, the capacity of refixation by ears of respiratory CO2 could then be considered in breeding programmes.