Elevated CO2 cannot compensate for japonica grain yield losses under increasing air temperature because of the decrease in spikelet density
Introduction
Rice (Oryza sativa L.) is an important staple food throughout the world. The productivity of rice needs to be increased by nearly 30% by 2050 to satisfy the demands of the growing global population (Alexandratos and Bruinsma, 2012). Crop production will be significantly affected by changes in the growth environment that are projected to occur in the future, including carbon dioxide (CO2) and temperature changes. Therefore, it is crucial to evaluate the response of rice growth to these two factors of climate change to assess food security.
Numerous studies have shown that elevated CO2 generally increases rice yield mainly by improving the leaf photosynthesis rate and the number of panicles per unit area (Clough et al., 1981; Kim et al., 2003a,b; Yang et al., 2006; Hasegawa et al., 2013; Chen et al., 2014). In general, global temperature increases have negative effects on rice growth (Wang et al., 2016; Wu et al., 2016). When high temperature occurs during temperature-sensitive stages of rice growth (panicle initiation and anthesis), the number of spikelets per panicle and the number of panicles per unit area significantly decrease, but the percentage of infertile spikelets increases (Madan et al., 2012; Wu et al., 2016; Xiong et al., 2017). The results of a crop growth model indicated that crop yield will increase as the global temperature slightly increases but will significantly decrease when the temperature change exceeds 0.8 °C (De Vries, 1993). Researchers found similar results for crops grown under combination of elevated CO2 and temperature in chamber and greenhouse experiments (Frank and Bauer, 1996; Rowland-Bamford et al., 1996; Matsui et al., 1997; Vu et al., 1997; Ziska et al., 1997; Thomas et al., 2003; Roy et al., 2012; Pereira-Flores et al., 2016). When using a crop growth model to evaluate the effects of elevated CO2 and high temperature, researchers predicted that elevated CO2 may alleviate the negative effects of increased temperature if the increase in temperature is no more than 2 °C (Krishnan et al., 2007). Long et al. (2006) reported that an increase in crop yields at a free-air CO2 enrichment (FACE) site was much lower than the increase predicted by a crop growth model and believed that elevated CO2 would not offset the negative effects of high global temperatures. However, few studies have focused on the combined effects of elevated CO2 and temperature on rice growth at field sites. Japonica rice is broadly cultivated in Asia because of its outstanding grain qualities, such as its taste and cooking characteristics. Wang et al. (2015) summarized the effects of elevated CO2 on rice with a meta-analysis and indicated that the average rice yield increased by 20%. However, our previous studies showed that the response of japonica rice yield to elevated CO2 at a field site was nearly 1/2 that of indica rice (Yang et al., 2006; Liu et al., 2008; Zhu et al., 2014). We inferred that japonica rice may be severely affected by increased air temperature under elevated CO2 conditions in the future. Therefore, it is necessary to evaluate the response of japonica rice to the combined influence of elevated CO2 and increased air temperature.
A previous field experiment revealed that the filled grain number per unit area was the key factor accounting for the effects of elevated CO2 and temperature on rice (Cai et al., 2015). An increase in grain number per unit area is always associated with rice yield improvements (Sheehy et al., 2001). The filled grain number per unit area consists of panicles per unit area and filled spikelets per panicle. Cai et al. (2015) showed that increased temperature had a minor effect on the panicles per unit area regardless of whether the CO2 was elevated. In contrast, Wang et al. (2016) indicated that high temperature alone or combined with elevated CO2 significantly decreased the number of panicles per plant. Therefore, the effect of elevated CO2 and temperature on the number of panicles per unit area is not clear. Although Cai et al. (2015) primarily evaluated the combined effect of elevated CO2 and temperature on the number of spikelets per panicle, they did not explore the response of the spikelet architecture. The number of surviving spikelets is related to nitrogen (N) accumulation during panicle initiation (Yao et al., 2000; Yang et al., 2006; Wang et al., 2012). Spikelet degeneration is related to the availability of photoassimilates nearly two weeks before anthesis (Kobayasi et al., 2001). Therefore, the changes in N uptake and dry matter related to spikelet architecture under elevated CO2 and increased temperature conditions need to be explored to further understand the corresponding mechanisms.
In this study, we hypothesized that elevated CO2 would compensate for the japonica rice yield losses caused by rising air temperature. The first objective of this study was to evaluate whether elevated CO2 would offset the negative effects caused by increased air temperature. For this purpose, we detected the yield and yield components of japonica rice under these two conditions of climate change over two growth seasons. The second objective was to explore the response of panicle formation to temperature and CO2 changes. Leaf photosynthesis, N uptake, and spikelet architecture of rice from the jointing to the heading stage were assessed in 2016 to answer this question. Air temperature (Ta) is difficult to control under open-field conditions, and researchers have widely used infrared heaters to explore the effect of increased temperature on crop growth and physiology (Nijs et al., 1996, 1997; Cai et al., 2015; Ding et al., 2016; Wang et al., 2017). In this study, we directedly increased the air temperature to evaluate the combined effect of temperature and CO2 on rice growth under field conditions.
Section snippets
Free-air CO2 enrichment (FACE) facility
The FACE facility was established in Zongchun village (119°42′0′′E, 32°35′5′′N), Yangzhou city, Jiangsu Province. Detailed descriptions of the design and operation of the FACE facility can be found elsewhere (Yang et al., 2006). In brief, three uniform rectangular fields were selected at the FACE experimental site. There was a distance of 90 m between the centre of the CO2 treatment plot and the ambient plot to avoid CO2 contamination. Each octagonal FACE plot was encircled by tubes that
Growth duration
Elevated CO2 alone shortened the pre-heading phase by 2 days, and increased air temperature alone notably shortened the pre-heading phase by 2.5 days on average, as measured in calendar days (Table 1). The combination of elevated CO2 and temperature further exacerbated this phenological tendency by shortening the pre-heading phase by 4 days (Table 1).
Grain yield and yield components
Significant interactions of the grain yield and 1000 grain weight between years were observed in this study (Table 2). There were significant
Discussion
Researchers have used closed- or open-top chambers (Ziska et al., 1997; Cheng et al., 2009) and FACE systems (Cai et al., 2015) to investigate the independent or combined effects of elevated CO2 and increased global surface temperature on rice yield. However, the yield responses to elevated CO2 reported from FACE experiments were half of or less than those reported from enclosure experiments (Long et al., 2006). Our study showed that CO2 elevated by 200 ppm significantly increased rice yield by
Conclusions
The findings obtained in this study have important implications for adaptive strategies for future rice production systems. This study showed that the decrease in japonica rice yield induced by increased air temperature could not be alleviated by elevated CO2. Spikelet density was the factor that determined the rice yield and was most affected by the interaction between elevated CO2 and increased temperature. The dry matter per unit area before heading was the crucial factor that accounted for
Acknowledgements
This work was supported by the National Natural Science Foundation of China (Grant No. 31261140364, 31370457). The authors express their thanks to Shah Fahad for comments on the manuscript. The FACE system instrument was supplied by the National Institute of Agro-Environmental Sciences and the Agricultural Research Center of Tohoku Region (Japan).
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