Rice production under climate change

Members of the National Agriculture and Food Research Organization, Japan, have been conducting ‘Free-Air CO₂ Enrichment’ experiments to ascertain the impact that climate change is having on rice production.

Rice directly feeds more people than any other crop and is relied on as a staple food source by half of the world’s population. But while its adaptability allows it to grow in a wide range of conditions, from cold to tropical climates and low to high altitude areas, projected climate change is becoming a major threat to the stable production of high-quality products.

Atmospheric CO₂ concentrations are rising at an unprecedented rate, along with other greenhouse gases, and we are seeing increasing temperatures and differing precipitation amounts and patterns, as well as more frequent occurrences of extreme weather.

But, while many of these changes are projected to have negative effects, an increase in CO₂ has a positive effect on rice crops – enhancing photosynthesis, biomass, and thereby grain yield – through a phenomenon known as ‘CO₂ fertilisation’.

Early studies used enclosure chambers to determine the crop’s response to increases in CO₂ but there was a growing need to test CO₂ fertilisation in the open field and in 1998, a technology known as ‘Free-Air CO₂ Enrichment’ (FACE) was applied to a paddy field in Japan (see Figs 1 and 2) by members of the National Agriculture and Food Research Organization, a Japanese research facility headquartered in Tsukuba Science City, Ibaraki.

Since then, they have conducted a series of FACE experiments on paddy fields in Shizukuishi in northern Japan, and Tsukuba in central Japan, to find out how rice crops grown in cold and warm climates respond to high CO₂ in order to design crop cultivar and management protocols for the future.

Do yield responses to elevated CO₂ differ between warm and cool FACE sites?

FACE experiments have been carried out, using a common cultivar, at the two sites over 13 seasons and have shown that yield increased by 11% on average in response to elevated CO₂ (+200ppm above the current level) but the effect varied considerably (0-21%) from year to year.²

The experiments have shown a correlation between decreasing grain yield enhancements and rising temperatures, with yield enhancements falling by 2.4% for every degree that the temperature goes up (see Fig 3). This suggests that the positive effect of elevated CO₂ on yield may be partially offset in future warmer climates.

Do yield responses to elevated CO₂ differ between rice producers?

Yield enhancements due to elevated CO₂ differed largely between cultivars ranging from 3 to 36%, suggesting that great opportunities exist for improving rice productivity in the future and higher yielding cultivars with large panicles or large grains showed a greater CO₂ response.^3-6

In rice, some genes that confer greater grain numbers have been identified^{7, 8} and the research has shown that introducing these genetic regions into conventional cultivars can improve grain yields in elevated CO₂.⁹ This strategy can be used to develop high-yielding varieties under elevated CO₂.

Does elevated atmospheric CO₂ degrade grain quality in rice?

Grain quality is becoming increasingly important in the rice market, and serious concerns are emerging about quality losses due to heat stresses under global warming.^{10, 11} The amount of chalky immature grains increases with rising temperatures during the grain growth stage and this decreases marketability and milling quality. The FACE experiments have shown that elevated CO₂ also significantly degrades the grain appearance quality by increasing the percentage of chalky grains¹², which is particularly pronounced in hot years.¹³

Simultaneous increases in CO₂ and air temperature will negatively impact high-quality rice production, while elevated CO₂ also decreases zinc, iron, and protein content in the grains.^{14, 15} It is important to note that considerable variation exists in the quality degradation across different rice cultivars,^{14, 15} which suggests that there is scope for breeding rice cultivars whose micronutrient levels or appearance quality are less vulnerable to increasing CO₂.

Do CO₂ and varieties matter in micro-, local-, and global climate systems?

The expected adverse effects of global warming on crop production are alarming, but the size of the impact is not solely dependent on the air temperature. Climatic and physiological factors that affect tissue or sensing organ temperatures can induce heat stress.

Elevated CO₂ increases daytime canopy temperatures by as much as 1°C through its influence on stomatal conductance,¹⁶ which could, in turn, exacerbate heat stress under hot conditions. Canopy cooling by transpiration could potentially alleviate heat stress, but this needs to happen at the expense of greater water use.

Two cultivars with contrasting transpiration capacity have been studied during the experiments. Takanari, a high-yielding indica cultivar, was identified as a candidate for high productivity under elevated CO₂ (see Fig 4) and for comparison, researchers chose a japonica cultivar Koshihikari, the main rice cultivar grown in Japan for over 50 years.

At a single-leaf level, Takanari had 30 to 40% higher stomatal conductance (a measure of stomatal opening), but at the canopy level, the difference in evaporation reduced to 4-5%, which could be cancelled by decreased evapotranspiration by elevated CO₂. Despite the small difference in evapotranspiration between these cultivars, the model simulations showed that ‘Takanari’ clearly decreased canopy and air temperatures within the planetary boundary layer compared to ‘Koshihikari.’ This indicates that lowland rice varieties characterised by their high stomatal conductance can play a key role in enhancing productivity and moderating heat-induced damage in response to climate change without significantly increasing crop water use.¹⁷

Additionally, Takanari’s canopy architecture with erect leaves may also contribute to lower canopy temperature, which could also have an impact on micro and local climates.¹⁸

Compared to other major cereals, rice is peculiar as it is mostly grown under submerged conditions with irrigated and rain-fed lowland areas accounting for more than 80% of the rice-growing regions.¹

Rice paddy accounts for about 11% of global methane (CH₄) emissions¹⁹. FACE experiments combined with soil warming by 2°C indicated an increase in CH₄ emission by as much as 80%, suggesting that climate change promotes CH₄ emissions from paddy fields, which in turn accelerates global warming.²⁰

There is a strong need to quantify these effects to mitigate greenhouse gas emission in paddy fields effectively and because much of the CH₄ emitted to the atmosphere is through plants²¹, understanding plant responses and carbon flow under climate change is the key to estimate and control CH₄ emission from paddy fields. FACE was also used for a large open field carbon tracer experiment to quantify the source of carbon for CH₄ emission from the paddy field and found that about half of CH₄ released during the growing period originated from the current season photosynthate.²² This finding highlights the need to understand carbon flow in the soil-plant-atmospheric continuum quantitatively.

References

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Toshihiro Hasegawa
Group Leader
Agricultural Meteorology Group
Tohoku Agricultural Research Center
National Agricultural and Food Research Organization (NARO)
thase@affrc.go.jp

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