Irrigating with cooler water does not reverse high temperature impact on grain yield and quality in hybrid rice

High temperatures cause significant yield losses in major rice-growing areas of the world, posing a stiff challenge in meeting the demand for rice from a steadily increasing population. Given that global warming is expected to continue with more frequent heat episodes, adaptation of field-scale crop management practices for preventing heat-induced yield and grain quality losses is vital. Water management is considered to be one of the important practices to prevent yield losses under high air temperature conditions.

The global mean air temperature has increased by 0.6–0.8 °C during the last century and is predicted to rise by 1.6, 2.4, and 4.4 °C between 2021–2040, 2041–2060, and 2081–2100, respectively. High temperatures cause significant yield losses in major rice-growing areas of the world, posing a stiff challenge in meeting the demand for rice from a steadily increasing population.

In China, frequent and extreme high temperatures during flowering and grain-filling stages resulted in significant yield loss from 8.9% in the 1970s to 17.9% in the early 21st century, particularly in the Yangtze River Valley. Further, it is predicted that total rice production in China would decrease by 5.3%–11.4% under 1.5 °C and 2.0 °C warming scenarios.

Given that global warming is expected to continue with more frequent heat episodes, adaptation of field-scale crop management practices for preventing heat-induced yield and grain quality losses is highly necessary.

More than 57% of the world’s rice is grown under irrigated conditions with a standing water depth of 0–10 cm during the entire crop growth period. Thus, growth and developmental processes for irrigated rice are strongly influenced by water management. Water management impacts the microclimate including soil and canopy temperature, nutrient uptake, phenology, and morphology of a rice plant, ultimately affecting rice grain yield and quality.

Water management is also considered to be one of the important practices to prevent yield losses under high air temperature conditions. For example, deep-flood irrigation or continuous irrigation with running water, decreased soil and water temperature, increased the available assimilate supply per grain, and improved grain quality by reducing the proportion of chalky (milky-white) grains.