Impact of the Accumulation of QTLs for Heat Tolerance on Grain Appearance Quality of Rice and Differences Among Cultivars and Years Under Free-Air CO2 Enrichment (FACE).

Elevated CO₂ (E-[CO₂]) significantly reduces appearance quality of rice (Yang et al., 2007; Usui et al., 2014, 2016), by increasing chalky grains, particularly in the back or close to the basal parts of the caryopsiss (WBSG-type chalks). The effects of E-[CO₂] were smaller with cultivars that are tolerant to heat for better appearance quality (Usui et al., 2014), suggesting heat-tolerance can also be effective for reducing the negative effects of E-[CO₂].

Kobayashi et al. (2007, 2013) have identified two Quantitative Trait Loci (QTLs) that could reduce % of WBSG-type chalky grains under high temperatures; qWB6 and qWB3 derived from Hanaechizen (HE) and developed near isogenic lines (NILs) incorporating one or two QTLs in the susceptible Niigatawase (NW); qWB6 (NIL-6) and the other qWB6 and qWB3 (NIL-6+3). We hypothesized that accumulation of these QTLs could be effective under E-[CO₂], To examine their performance under E-[CO₂], we conducted Free-Air CO₂Enrichment (FACE) experiments at Tsukuba, Ibaraki, Japan for three years.

Percentage of undamaged grains (UDG) of the four genotypes ranged from 47 to 66 % under ambient-[CO₂] averaged over three years. UDG of all genotypes was decreased by E-[CO₂] by on average 16% point (P < 0.001), largely due to increase % of chalky grains. UDG differed among genotypes (P < 0.001) and years (P < 0.001), with the highest being observed in HE and the lowest in NW. When averaged over two [CO₂] treatments for three years, UDG of NIL-6 increased by 9 % point, and NIL-6+3 increased by 15 % point, compared to NW. This suggests that accumulation of heat-tolerant QTLs is effective to reduce the negative effects of E-[CO₂] on grain appearance quality of rice. However, year×[CO₂] ×genotype interaction for WBSG was significant suggesting the effects can vary under different environments, which needs further study.