4.1 Assessment of field warming and the impact on rice quality formation
Since 2009, we have continuously carried out warming experiments in rice fields based on the open-field free air temperature enhancement system (FATE), and the effects of increased temperature on rice growth and development were guardedly studied. The ultimate goal of our study is to find key regulatory factors or metabolic pathways involved in rice growth process and apply appropriate field cultivation measures to reasonably respond to the challenge of climate warming on rice. The FATE device is suspended above the field and uses 12 sets of ceramic infrared heaters to perform uniform heating in an area of 7.1m2. In the fully activated state, the daytime canopy temperature of rice can be increased by 2.4℃ on the basis of natural temperature, and the rice canopy night temperature can be increased by 5.4℃. This warming range is within the prediction of possible temperature increase by 1.4-5.8 °C at the end of the 21st century by IPCC (IPCC, AR5, 2014). Furthermore, the increase in night temperature is significantly greater than that during the day and that is consistent with the asymmetric trend of climate warming (Pachauri et al., 2014). Compared with closed or semi-closed warming scenario, the warming method and effect adopted in this project could closely simulate climate warming characteristics and that provides a more reliable platform for us to conduct related experiments in the actual field.
Rice quality is a complex characteristic, including appearance, milling, nutrition, cooking and eating quality. Our field evidence shows that the overall temperature increase has a relatively negative effect on rice quality, including significantly increased chalky rate, chalky area and chalkiness. Meanwhile, the milling quality indicators milled rice rate and head rice rate were decreased significantly, and that would exceedingly reduce the market recognition of rice. Our previous researches have been devoted to exploring the mechanism of the influence of increased temperature on rice quality and to our knowledge, the changes of external temperature inevitably affect the morphological composition and structure of the grain storage material, and that further induce the changes of related quality traits (Dou et al., 2017; Dou et al., 2018; Tang et al., 2018; Tang et al., 2019). Among these attributes, eating and cooking quality (ECQ) is one of the most important indicators, especially from the consumer’s perspective. The eating quality refers to the sensory perception of consumers on rice, and is related to the gloss, flavor, and viscosity of rice. Although the physical and chemical properties of starch in rice endosperm can be used as an indirect indicator of ECQ, it is still a difficult task to assess ECQ through these traits. At the same time, the increase of glutelin content in rice grain is particularly obvious under the condition of increased temperature, which leads to a change in the overall balance of grain storage materials and has a negative impact on the taste and appearance quality of the rice.