How do extreme wet events affect rice quality in a changing climate?

• We examined how multiple climate change drivers affect the grain quality of rice.
• In addition to warming, elevated [CO2] deteriorated grain quality by increasing the number of immature chalky grains (ICGs).
• Furthermore, warming and elevated [CO2] synergistically increased ICGs.
• Extreme wet events (EWEs) further exacerbated the quality deterioration by elevated [CO2].
• Elevated [CO2] reduced grain protein content (PC), whereas EWEs increased the PC.

Despite the growing body of evidence that ongoing warming modifies the cereal quality of several crop species, little is known about how it interacts with other climate change drivers. Here, we examined how elevated [CO2], warming, and extreme wet events (EWEs) would affect the grain quality of a temperate rice (Oryza sativa L.). In 2009 and 2010, plants were grown at two levels of [CO2] (c. 390 and 650 μl l−1) and three levels of warming (ambient, +1.2 °C and +2.2/2.4 °C) over the seasons in six independent temperature gradient field chambers (three each for ambient and elevated [CO2]). From 20 days after heading, plants were exposed to EWE with a simulated rainfall of 30 mm a day and cloud cover by shading (c. 18% of full sunlight) for 10 consecutive days. There were also control plots, which were non-treated (NT) in terms of EWE. We found that warming and EWE, both independently and in their interaction, lead to a significant reduction in the percentages of sound grains, whereas elevated [CO2] does not seem to do so. Not only warming and EWE, but also elevated [CO2] resulted in a significant increase of immature chalky grains (ICGs), which mostly consisted of milky-white chalky grains, degrading the grain quality. In addition, warming interacted positively with elevated [CO2] on ICGs, whereas the warming sensitivity of IGCs was moderated in EWEs: e.g., on average, the % of ICGs in ambient vs. elevated [CO2] were 8.5% vs. 9.4%, 10.7% vs. 18.1% and 17.8% vs. 29.8% at ambient (24.7 °C), +1.2 °C and +2.4 °C, respectively, whereas they in NT vs. EWE were 1.2% vs. 16.6%, 7.7% vs. 21.1% and 19.4% vs. 28.2%. Our results tightly confirmed that elevated [CO2] leads to a significant reduction in rice grain protein content (PC). While warming had little effect on the PC, EWE had a significant positive effect, and the EWE-induced increase in PC was smaller in elevated (+3.3%) than in ambient [CO2] (+10.9%). Given the rice quality predictor of ICGs and PC, our findings imply that expected climate change and weather extremes have a great potential to degrade the quality of rice, in terms of its milling, cooking, eating, and nutritional quality, as well as market value.