Abstract
BackgroundHeat-tolerant rice cultivars have been developed as a countermeasure to poor grain appearance quality under high temperatures. Recent studies showed that elevated CO2 concentrations (E-[CO2]) also reduce grain quality. To determine whether heat-tolerant cultivars also tolerate E-[CO2], we conducted a free-air CO2 enrichment (FACE) experiment with 12 rice cultivars differing in heat tolerance.ResultsThe percentage of undamaged grains of five standard cultivars (Akitakomachi, Kinuhikari, Koshihikari, Matsuribare, Nipponbare) averaged 61.7% in the ambient [CO2] (AMB) plot and 51.7% in the FACE plot, whereas that of heat-tolerant cultivars (Eminokizuna, Wa2398, Kanto 257, Toyama 80, Mineharuka, Kanto 259, Saikai 290) averaged 73.5% in AMB and 71.3% in FACE. This resulted in a significant [CO2] by cultivar interaction. The percentage of white-base or white-back grains increased from 8.4% in AMB to 17.1% in FACE in the sensitive cultivars, but from only 2.1% in AMB to only 4.4% in FACE in the heat-tolerant cultivars.ConclusionHeat-tolerant cultivars retained their grain appearance quality at E-[CO2] under present air temperatures. Further improvements in appearance quality under present conditions will be needed to achieve improvements under E-[CO2], because E-[CO2] will likely lower the threshold temperature for heat stress.
Highlights
Heat-tolerant rice cultivars have been developed as a countermeasure to poor grain appearance quality under high temperatures
Heading date ranged from 24 July to 7 August, depending on cultivar (Table 1), but the daily mean air temperature averaged over the 20 days after heading (T20DAH), an important determinant of grain quality, varied only in a limited range from 26.5 to 26.9°C, which exceeded the threshold for the occurrence of chalky grains (Morita 2008)
These results suggest that the current efforts in breeding for heat tolerance will be effective under projected climate change
Summary
Heat-tolerant rice cultivars have been developed as a countermeasure to poor grain appearance quality under high temperatures. High temperatures during grain filling often cause serious damage to the grain quality of rice (Oryza sativa L.) (Terashima et al 2001; Lanning et al 2011; Kondo et al 2012), reducing the proportion of first-grade rice (Hasegawa et al 2009) or milling quality (Lyman et al 2013). The occurrence of chalky grains under high temperatures is attributable mainly to the inhibition of starch accumulation (Morita 2008; Morita and Nakano 2011). Mean temperatures >26°C during the grain-filling period cause chalkiness in the grains of japonica cultivars (Morita 2008).
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