Genotypic Variation of Yield Enhancement of Rice Cultivars with Different Maturity Group of Exposure to Free-Air CO2 Enrichment (FACE).

Adequate N status in plants is a critical factor for ensuring their ability to respond positively to elevated atmospheric CO₂ concentrations ([CO₂]).  A cultivar with higher N concentration might thus respond better to elevated [CO₂].  To test this hypothesis, we examined four rice cultivars with different times to maturity in a 2-year free-air CO₂ enrichment (FACE) experiment.  Plant N concentration at the heading stage differed significantly between cultivars but without any significant [CO₂] ´ cultivar interaction; cultivars that matured earlier had higher N concentrations.  Grain yield was significantly increased by elevated [CO₂] (by up to 23%), but the magnitude differed among the cultivars due to a significant [CO₂] ´ cultivar interaction; two cultivars (one with early and one with late maturity) responded more strongly to elevated [CO₂] than those with intermediate maturity dates, resulting from increased spikelet density.  Biomass and N uptake at the heading stage were closely correlated with grain yield and spikelet density.  Our 2-year field trial rejected the N-response hypothesis and showed that the magnitude of the growth enhancement before heading rather than plant N concentration is a useful criterion for selecting cultivars capable of adapting to future elevated levels of [CO₂].