Elevated CO2-induced changes in cytokinin and nitrogen metabolism are associated with different responses in the panicle architecture of two contrasting rice genotypes

The yield response of Yangdao6 (YD6, Oryza sativa L. ssp. indica) to an enriched CO2 concentration was at least 30% higher, an approximately twofold increase, compared with that of Wuyunjing23 (WYJ23, Oryza sativa L. ssp. japonica). The differences in grain yield response levels between these two contrasting rice genotypes were mainly attributed to the disparities in the number of spikelets per panicle in response to elevated CO2. The number of spikelets per panicle was significantly increased by ~ 11.3% for YD6, but that of WYJ23 was decreased by 6.0% under elevated CO2 conditions. The increase in the differentiated spikelet number and decrease in the percentage of degenerated spikelet number on secondary branches were the key factors accounting for this phenomenon. To clarify the underlying physiological mechanisms corresponding to the above phenotype, the cytokinin content and nitrogen assimilation enzymes were assayed in the panicles and roots of YD6 and WYJ23 under ambient and elevated CO2 conditions (+ 200 µmol mol− 1). The results showed that locally synthesized cytokinins in panicles under elevated CO2 increased the differentiated spikelet number on secondary branches for YD6. Elevated CO2 also caused the enhanced or stabilized catalytic activity of most nitrogen assimilation enzymes in the panicles of YD6 but caused only a slight increase, or even a significant decrease, in the panicles of WYJ23. The increased activity of nitrogen assimilation enzymes in the panicles may reduce the degenerated spikelet number of YD6. These findings have significant implications for the utilization of germplasm resources in ensuring food security under future elevated CO2 conditions.