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Enhancing crop traits that increase grain yield under elevated CO2 concentrations is an important option for increasing the future productivity of rice. Here, we compared the growth and yield of five varieties with different genetic background under Free-Air CO2 Enrichment (FACE) conditions to identify traits responsible for varietal differences in yield increase under elevated CO2. Three high-yielding and two standard rice varieties grown under FACE conditions commonly had (1) shorter growth periods, (2) higher dry matter production, (3) higher numbers of spikelets (sink capacity) and panicles; and (4) higher yield than those grown under ambient CO2. Yield enhancement by elevated CO2 (FACE/Ambient), however, differed significantly among varieties, ranging from 1.10 to 1.25. The greater response of the sink capacity, defined as the product of spikelet number and single grain mass, was the main factor involved in yield increase. Three high-yielding varieties (Momiroman, Takanari, and Hokuriku 193) had greater sink capacity than two standard varieties and the sink capacity of these varieties significantly increased under FACE condition. However, yield enhancement in elevated CO2 was lower in Hokuriku 193 than in Momiroman and Takanari. In Hokuriku 193, sink production was relatively low while dry matter production was similar to the others. Therefore, larger increase in sink production efficiency per unit of dry matter production under FACE was found to be a particularly important varietal trait, suggesting that efforts to develop varieties suited to the predicted elevated CO2 condition should focus on this and the related traits. [ABSTRACT FROM AUTHOR]

A new rice (Oryza sativa L.) cultivar, 'Oonari', was developed to reduce the shattering habit of the high-yielding cultivar 'Takanari'. To evaluate the effects of environment on its yield and related traits, Oonari and Takanari were grown in multiple environmental conditions at three locations in Japan. The whole and filled brown rice yields of Oonari were around 1000 g m−2, similar to those of Takanari. But the panicle number m−2 of Oonari was slightly but significantly higher than that of Takanari and the spikelet number per panicle was slightly but significantly lower. With respect to the effect of environment on yield of Oonari, cumulative radiation during 40 days after heading was positively related to whole brown rice yield of Oonari. Yield tended to be higher under increased atmospheric CO2 concentration than under ambient. Oonari was confirmed to exhibit high yield in various environmental conditions. [ABSTRACT FROM AUTHOR]

Atmospheric CO2 concentrations ([CO2]) have increased by more than 100 μmol mol−1 over the last century and are projected to rise further. Breeding cultivars with a greater response to elevated [CO2] (E-[CO2]) can be an effective adaptation to global climate change. We wondered whether E-[CO2]-responsive cultivars have been unintentionally selected through empirical breeding as [CO2] has increased. If so, modern cultivars may respond better to E-[CO2] than old ones. We conducted free-air CO2 enrichment (FACE) experiments in 2 years to examine whether rice cultivars bred in different eras differ in response to E-[CO2] and to determine any associated traits. We tested five Japanese cultivars: Aikoku (released in 1882), Norin 8 (1934), Koshihikari (1956), Akihikari (1976) and Akidawara (2009). The yields of Aikoku and Norin 8 increased by 19.3% and 30.3%, respectively, under E-[CO2], while those of Koshihikari and Akihikari increased by 15.9% and 3.4%, respectively. However, that of Akidawara, the newest cultivar, also increased by 19.0%. Norin 8's strong response to E-[CO2] was associated with increases in both spikelet density and percentage of ripened grains, both of which were closely related to nitrogen uptake. These results suggest that breeding has not necessarily improved cultivars' response to E-[CO2], and that selection for traits such as sink capacity and nitrogen uptake can be effective to improve rice productivity under E-[CO2]. [ABSTRACT FROM AUTHOR]

An effective strategy for increasing crop production is increasing the rate of photosynthesis. In this study, we conducted gas exchange and chlorophyll fluorescence measurements for a high-yielding rice cultivar, Takanari, to identify the leaf physiological properties that contribute to high capacity for photosynthesis of the uppermost leaves before (panicle initiation stage) and after heading (grain-filling stage) in the Tsukuba free-air CO2 enrichment (FACE) facility. The higher photosynthesis rate of Takanari compared with that of the commonly cultivated cultivar, Koshihikari, was mainly attributed to the greater stomatal conductance for CO2 (gsc) at the panicle initiation stage and to the greater mesophyll conductance (gm) at the grain-filling stage in both current and elevated atmospheric CO2 concentrations [CO2]. Takanari had a higher level of leaf nitrogen content (Nl) compared with Koshihikari at the grain-filling stage, which led to greater gm and maximum carboxylation rate (Vc,max), but Nl alone did not explain the variations of gm within the variety. A clear correlation was found between Vc,max and Nl. Calculating Vc,max taking gm into consideration removed the artifact of Vc,max25 in relation to Nl that was observed when gm was assumed to be infinite. Our results emphasize the need to separate the roles of Vc,max and gm to accurately understand the ecophysiological processes that control leaf photosynthesis in Takanari. [ABSTRACT FROM AUTHOR]

Rice straw can be used as a feed for livestock. It accumulates soluble oxalate, which is harmful to livestock as it can cause mineral deficiency. It is therefore necessary to reduce the oxalate content in rice leaves so that the usage of rice straw for a livestock feed can be increased. Several oxalate synthesis pathways have been reported in various plants. However, the oxalate synthesis pathway in rice has not yet been elucidated. In the present study, we found that the oxalate content of the leaves of two rice cultivars, Koshihikari and Takanari, was different. The differences in the oxalate contents in the leaves of two cultivars were also maintained under the elevated CO2 concentration and/or temperature conditions. These findings suggest that oxalate accumulation has a genetic basis and does not depend on CO2 and temperature conditions. Abbreviations: A-[CO2]: ambient CO2 concentration; CE-QQQ-MS: capillary electrophoresis-triple quadrupole-mass spectrometry; E-[CO2]: elevated CO2 concentration; E-T: elevated soil/water temperature; FACE: free-air CO2 enrichment; TCA: tricarboxylic acid; N-T: normal soil/water temperature [ABSTRACT FROM AUTHOR]

Flower opening time (FOT) is important for reproductive success in higher plants. Rice plants exhibit phenotypic plasticity in FOT in response to environmental factors such as temperature. However, the effect of the concentration of atmospheric CO2 ([CO2]) on FOT has not yet been reported. Elevated [CO2] (E-[CO2]) increases the temperature of panicles, which may in turn advance FOT. We investigated the effect of E-[CO2] on FOT in rice using a free-air CO2 enrichment facility where we increased [CO2] by about 200 μmol mol−1 above the ambient level (A-[CO2]). By photographing panicles at 10-min intervals, we determined 10%FOT and 50%FOT (the time of the day in Japan Standard Time when 10% and 50% of the flowers had opened, respectively). E-[CO2] advanced 10%FOT and 50%FOT by 4 and 5 min at the 10% and 5% levels of significance, respectively. Daily mean air temperature (Ta), solar radiation, and vapor-pressure deficit were negatively correlated with 50%FOT. Regression line slopes for Ta versus 10%FOT and 50%FOT were slightly steeper for A-[CO2] than those for E-[CO2]. Our results suggest that the most probable reason why E-[CO2] advanced FOT is an increase in panicle temperature arising from a reduction in leaf stomatal conductance. Abbreviations: 10%FOT: the time of the day in Japan Standard Time when 10% of the flowers had opened; 50%FOT: the time of the day in Japan Standard Time when 50% of the flowers had opened; A-[CO2]: ambient CO2; [CO2]: the concentration of atmospheric carbon dioxide; E-[CO2]: elevated [CO2]; FACE: free-air CO2 enrichment; FOT: flower opening time; JST: Japan Standard Time; RH: relative humidity; Rs: solar radiation; Ta: air temperature; Tp: panicle temperature; VPD: vapor-pressure deficit [ABSTRACT FROM AUTHOR]

Global climate changes may cause heat-induced sterility in rice, threatening the global production of this important crop. Although little is currently known about the combined effects of the concentration of atmospheric CO2 ([CO2]) and temperature on heat-induced sterility, elevated [CO2] (E-[CO2]) will likely increase the panicle temperature and thereby exacerbate heat-induced sterility, but this was not tested in open fields. Therefore, we investigated the effect of E-[CO2] on heat-induced sterility and sterility-related traits in rice by increasing E-[CO2] by approximately 200 μmol mol−1 above ambient levels using a free-air CO2 enrichment (FACE) facility for six growing seasons with variable growing season temperatures. The percentage fertility was not significantly correlated with the air temperature (Ta) between 09:00 and 12:00 on each flowering day, but it did significantly vary among the years, with 2011 experiencing cool temperatures resulting in chilling-induced mild sterility. When data from 2011 were removed, there was a significant negative correlation between fertility and Ta between 09:00 and 12:00 on each flowering day under E-[CO2], whereas no such effect was seen under ambient [CO2]. E-[CO2] also significantly reduced the number of pollen grains deposited on the stigma by 10%, but it slightly increased the anther length by 1.3%, indicating that it had both negative and positive effects on heat-induced sterility. These findings suggest that E-[CO2] affects many traits related to heat-induced sterility and may sometimes exacerbate sterility by reducing pollen grain deposition. Abbreviations: A-[CO2]: ambient CO2; [CO2]: the concentration of carbon dioxide; E-[CO2]: elevated [CO2]; FACE: free-air CO2 enrichment; Ta: air temperature [ABSTRACT FROM AUTHOR]