Yield formation of CO₂-enriched inter-subspecific hybrid rice cultivar Liangyoupeijiu under fully open-air field condition in a warm sub-tropical climate

Hybrid rice (Oryza sativa L.) cultivars play an important role in rice production due to its heterosis, resistance to environmental stress and high yield potential. However, no attention has been given to its yield responses to rising atmospheric CO₂ concentration ([CO₂]). To address this need, we conducted a Free Air CO₂ Enrichment (FACE) experiment at Yangzhou, Jiangsu, China, in 2004-2006. A two-line inter-subspecific hybrid rice variety Liangyoupeijiu, recently bred in China, was grown at ambient or elevated (c. 570μmolmol⁻¹) [CO₂] under two levels of nitrogen (N) application (12.5 and 25gNm⁻²). Elevated [CO₂] slightly accelerated phenological development (1-2 days), and substantially enhanced grain yield (+30%). The magnitude of yield response to [CO₂] was independent of N fertilization, but greatly varied among years. On average, elevated [CO₂] increased panicle number per unit land area by 8%, due to an increase in maximum tiller number under FACE, while productive tiller ratio remained unaffected. Spikelet number per panicle showed an average increase of 10% due to elevated [CO₂], which was also supported by increased plant height and dry weight per stem. Meanwhile, Elevated [CO₂] caused a significant enhancement in both filled spikelet percentage (+5%) and individual grain mass (+4%). Compared with previous rice FACE studies, this hybrid cultivar appears to profit much more from elevated [CO₂] than inbred japonica cultivars (c. +13%), not only due to its stronger sink generation, but also enhanced capacity to utilize the carbon sources in a high [CO₂] environment. As sufficient intraspecific variation in yield response exists under field conditions, there is a pressing need to identify genotypes which would produce maximum grain yield under projected future [CO₂] levels.