C and N metabolism in barley leaves and peduncles modulates responsiveness to changing CO(2)

Balancing of leaf carbohydrates is a key process for maximising crop performance in elevated CO(2) environments. With the aim of testing the role of the carbon sink–source relationship under different CO(2) conditions, we performed two experiments with two barley genotypes (Harrington and RCSL-89) exposed to changing CO(2). In Experiment 1, the genotypes were exposed to 400 and 700 ppm CO(2). Elevated CO(2) induced photosynthetic acclimation in Harrington that was linked with the depletion of Rubisco protein. In contrast, a higher peduncle carbohydrate-storage capacity in RSCL-89 was associated with a better balance of leaf carbohydrates that could help to maximize the photosynthetic capacity under elevated CO(2). In Experiment 2, plants that were grown at 400 ppm or 700 ppm CO(2) for 5 weeks were switched to 700 ppm or 400 ppm CO(2), respectively. Raising CO(2) to 700 ppm increased photosynthetic rates with a reduction in leaf carbohydrate content and an improvement in N assimilation. The increase in nitrate content was associated with up-regulation of genes of protein transcripts of photosynthesis and N assimilation that favoured plant performance under elevated CO(2). Finally, decreasing the CO(2) from 700 ppm to 400 ppm revealed that both stomatal closure and inhibited expression of light-harvesting proteins negatively affected photosynthetic performance and plant growth.