Climate warming is reducing rice quality in East Asia, research reveals – Phys.org

Rice is a food staple for billions of people worldwide, with demand doubling over the past 50 years, and is predominantly grown in Asia before being exported globally. This crop is sensitive to weather conditions and, as such, understanding how yields are affected by climate change is of paramount importance to ensure sustainable food supplies into the future.

Beyond yield, the quality of rice may also be affected, determined by a combination of appearance, palatability, nutritional profile and milling properties.

Research published in Geophysical Research Letters, has highlighted a decline in rice quality in East Asia, coincident with changing temperatures.

Dr. Xianfeng Liu, of China’s Shaanxi Normal University, and colleagues used data collected over 35 years to explore patterns in rice quality from Japan and China. This is based upon head rice rate (HRR), which is a measure of the fraction of milled rice kernels that retain 75% of their length after milling, during which the husk and bran are removed.

The research team explored a number of climate variables via modeling to ascertain which had the greatest effect on HRR; these variables were nighttime temperature, daytime temperature, diurnal temperature, daily averaged temperature, hot days (above 30/35 °C), precipitation, precipitation frequency, soil moisture, solar radiation, cloud cover, relative humidity, daytime vapor pressure deficit, transpiration and carbon dioxide concentration.

Ultimately, the scientists determined that overnight temperatures are the main driver of reduced rice quality. In particular, as nighttime temperatures become warmer, a critical threshold for decline begins at 12 °C and 18 °C for Japan and China respectively. When flowering and grain growth occur in such conditions, rates of photosynthesis and starch accumulation in the grain decrease, leading to lower rice quality as more grains are susceptible to breakage.

Following this, daily solar radiation was the second most important factor contributing to rice quality changes (higher solar radiation leading to lower HRR), then daily precipitation (less precipitation corresponding to lower HRR) and lastly daytime vapor pressure deficit (beyond a threshold of 0.5–1 kPa, HRR declines).

The average rice quality across all sites and years for China had a HRR of ~62%, but reduced by 1.45% every decade. In Japan, the average HRR was slightly higher at ~66%, with a prominent rate of decline of 7.6% every decade.

In both countries, there is a strong gradient of diminished rice quality from northern to southern provinces and prefectures, likely linked to southern regions being closer to the equator and therefore experiencing higher nighttime temperatures.

Furthermore, Dr. Liu and the team noted that the models consistently showed the average climate data 40 days in advance of the harvest date was a good predictor of rice quality for that yield.

This research is important as projections under both moderate and high emissions scenarios suggest rice quality will continue to decline in the decades to come as climate change progresses.

In this study, HRR decreased by 0.5% and 1.5% for Japan and China respectively in projections between 2020 and 2100 for lower emissions scenarios. However, as modeled emissions increase, HRR is projected to decrease more prominently after 2050, potentially exceeding 5% in China by 2100.

The negative impact of climate change on rice quality in China is predicted to be harsher than for Japan, with spatial sensitivity (HRR decline of ~1.2% per degree of temperature rise) being higher than temporal sensitivity (~0.7% per degree of temperature rise). Thus, China’s southern provinces may be more limited to adapting to increasing nighttime temperatures, which is a significant challenge to overcome as these are the main areas of rice cultivation.

Given all of this, the likelihood of rice cultivars adapting in tandem with the rate of climate change may pose a hazard to sustainable food supplies, human nutrition and economic stability in years to come.

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