Developmental and morphological responses of rice to elevated atmospheric CO_2 concentration

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Growth enhancement of rice by elevated CO_2 can be mediated by the developmental and morphological traits. Much has been studied for the photosynthetic responses but understandings on the effects of elevated CO_2 on morphogenesis of rice are still limited. We therefore attempted to determine the effects of elevated CO_2 (200μmol/mol above ambient) on development and morphology or rice varieties differing in maturity and plant types. Naturally sun-lit growth chambers and free air CO_2 enrichment (FACE) facilities were used for the experiments. Elevated CO_2 advanced heading dates of medium maturing varieties but did not foster development of two early varieties. A contrasting response of lamina length to elevated CO_2 was also observed among the genotypes ; a significant positive effect was found in early varieties but a negative one in medium varieties. The genotypic difference in CO_2 responses of leaf appearance and lamina length occurred mainly in the upper three to four leaves, which developed after panicle initiation. The results suggest that the genotype x CO_2 interaction on the developmental time can influence leaf number and lamina size which have a direct relevance to the rice productivity. We also tested whether genotypic difference in panicle size influences the growth responses to elevated CO_2 using a mutant with limited spikelet number. A parent and its mutant showed a similar growth enhancement due to elevated CO_2 despite the mutant having only 50-60 % of spikelet number per panicle, suggesting that limited spikelet per se does not limit growth enhancement. In all experiments, genotypic variation in photosynthetic responses to elevated CO_2 was mainly due to the difference in leaf greenness or nitrogen content. The results suggest that genotypic variation in growth responses to elevated CO_2 can mostly ascribed to the traits associated with development and nitrogen nutrition.