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 sugges t that respiratory CO2, associated with grain-filling processes, may b e refixed close to its site of evolution. Apparent refixation of respi ratory CO2 in intact ears was compared with that in flag leaves, by fe eding both organs with C-14-labelled sucrose and trapping (CO2)-C-14 r eleased 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 concentratio n increased refixation of respiratory CO2 by up to 90%, 92% and 82%, r espectively. The occurrence of CO2 refixation in the field, in a set o f 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 o f mature kernels whose ears had been darkened from 1 week after anthes is to maturity. Moreover, genotypic differences in delta(13)C and its divergence in light compared to darkened ears were observed, both unde r 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 b reeding programmes.