A temperate rice, asked to live through 40°C

Rice, Oryza sativa L., is treated as a staple for more than half the world’s people, and in Asia as a base of survival and of food security. Heat tied to a changing climate is said to strike growth, yield, and quality, and to put a strain on production more widely. In a temperate country such as Korea, a summer with a daily mean of 30 to 40°C already leaves the crop exposed: at flowering and during grain filling, pollen can fail, stand establishment can weaken, and grain can form poorly.

The rice grown in Korea is mainly japonica, a thermal ecotype fitted to a temperate climate. It does best between 20 and 30°C, and above 35°C it comes under physiological stress. The thought experiment set here is harsher still: from July to September the daily mean stays close to 40°C, and the night minimum holds at 30°C. That heat would run through the vegetative phase, the reproductive phase, and grain filling.

In the vegetative phase, japonica photosynthesises most efficiently at 25 to 30°C. Near 40°C, enzymes in the photosynthetic machinery, Rubisco among them, lose activity, and chlorophyll breaks down faster. Tiller numbers could fall 20 to 30 percent, because heat tolerance is low. A night minimum of 30°C pushes root respiration too high, which can leave the roots short of oxygen and less able to take up nutrients. Indica, fitted to the tropics, can keep a more stable growth above 35°C; in japonica, protein synthesis and the metabolism of gibberellin and cytokinin are described as disturbed. A daily mean of 40°C is treated as past this ecotype’s thermal threshold, so energy may go into mere survival, and the shift into the reproductive stage may be delayed.

Flowering and filling, when the night does not cool

At flowering, temperatures above 35°C already interfere with pollen formation and with fertilisation. At 40°C, pollen survival can fall below 50 percent, the pollen tube is hindered, and sterility can reach 70 to 90 percent. That weakness is tied to japonica’s short-day character, and heat is said to weaken the signal that turns the plant toward reproduction. Spikelet development is suppressed, so the number of spikelets on a panicle may drop 20 to 40 percent. A night minimum above 30°C is described as especially hard on the reproductive organs: night respiration rises, carbohydrates are burned too quickly, and the energy shortfall makes reproductive failure worse.

During grain filling, heat lowers the activity of AGPase, the enzyme that helps make starch. The filling rate can fall 30 to 50 percent, grains come out small and incomplete, and the share of chalky grain can rise by more than half. Grain number on the panicle and thousand-grain weight may each fall 20 to 40 percent, and total yield may drop 40 to 60 percent. Quality suffers with the chalk: amylose falls, eating quality declines, and protein rises in relative terms, which is said to hurt processing quality. The comfortable temperature for filling in japonica is given as 20 to 25°C; 40°C is treated as enough to break the metabolic path that stores starch. Night heat at 30°C limits the carbohydrate supply that filling needs.

Put together, a July-to-September stretch at a daily mean of 40°C and a night minimum of 30°C is predicted to cut yield by 40 to 60 percent and to cut marketable quality by more than half, through the rise in chalky grain. The temperate ecotype, fitted to 20 to 30°C, is treated as past its limit; in some districts, growing rice at all could become impossible. Heat-tolerant japonica, or crosses with indica, earlier transplanting, stronger irrigation, and shade cloths are named as necessary. In a comparison of ecotypes, japonica’s weakness in extreme heat stands out, and a physiological collapse at a steady 40°C is treated as hard to avoid.

Korea’s harvest in the 2023 and 2024 releases, beside a rising world crop

The figures used here come from releases after 2023: FAO’s Cereal Supply and Demand Brief for 2023–2025 and the Statistical Yearbook 2023; USDA Rice Outlook updates in September and November 2024; Statistics Korea rice-production surveys published in November 2023 and November 2024; and MAFRA grain and feed updates for 2023–2024. The window at the centre is the decade 2014–2023, with 2024 estimates included. Rice is said to occupy about half of Korea’s cultivated land.

Korea’s 2023 crop is given as 3.70 million tons, down 1.6 percent from the year before. The 2024 estimate, from the Statistics Korea release in November 2024, is 3.585 million tons, down a further 3.2 percent. Harvested area is given as 0.708 million hectares in 2023 and 0.698 million in 2024, still shrinking. A government policy of ‘appropriate production’ — cutting area and shifting toward higher-value crops — is named as a main reason. Jeollanam-do was the largest producing province in 2023, at 0.737 million tons. From 2014 to 2023, harvested area fell by about 104,000 hectares, some 12.8 percent, at a compound annual rate of about −1.4 percent, and output fell by about 0.29 million tons, some 7.3 percent, at about −0.8 percent a year, from 3.99 million tons in 2014 to 3.70 million in 2023. Yield in 2023 is given as 523 kilograms per 10 ares, a slight rise in a favourable season, but the shrinkage of area still drove the total down. A brief recovery during the 2020 pandemic was followed by a continued fall. The further 3.2 percent drop estimated for 2024 is tied to heat and to damage in the rainy season, with more pests and disease. That may ease a surplus, given as 139,000 tons a year on average from 2013 to 2022, but it is also treated as a challenge to a self-sufficiency rate of about 90 percent. Those figures are left with the 2023 and 2024 releases.

The world crop, in the same window, is described as still rising, with Asia at the centre. For 2023/24, production is given as about 776 million tons in an FAO 2023 update, a record in that series; for 2024/25, FAO is cited at about 543 million tons in milled equivalent, or about 790 million tons in paddy. China is given as about 145 million tons in 2023, India about 135 million, together more than half the world total. Harvested area in 2023 is given as about 165 million hectares, fairly steady, with yield per hectare gradually improving. From 2014 to 2023, world area rose by about 1.5 million hectares, some 0.9 percent, at about 0.1 percent a year, and output by about 37.0 million tons, some 5.0 percent, at about 0.5 percent a year, from 739 million tons in 2014 to 776 million in 2023. A jump in 2017–2018 was followed by a brief dip in the 2020 pandemic. The 2024 outlook of 790 million tons, up about 1.8 percent, is tied to favourable weather in India and Southeast Asia. Korea, over the same decade, was shrinking at about 1 to 2 percent a year. The same Korean report, at fuller length, is on CampusDown.

Sources

1. Statistics Korea, rice-production surveys published in November 2023 and November 2024; MAFRA, Grain and Feed Update, 2023–2024.
2. FAO, Cereal Supply and Demand Brief, 2023–2025, and Statistical Yearbook 2023; USDA, Rice Outlook, updates in September and November 2024.

Last updated: 14 November 2026