Sun Hongrong, who operates a family farm in the suburban Shanghai district of Songjiang, said high temperatures have been a headache since July.

Although his rice-planting area doubled this year to about 13 ha, Sun estimated that output will shrink 15 to 20 percent this year. Harvesting is due to start in about 20 days.

"For good-quality rice, the best temperature for pollination is about 32 degrees Celsius. But the reality in Shanghai is a temperature above 38 degrees Celsius for more than a week," said Sun.

The heat has also pushed up his costs for irrigation and labor.

Rice is different from corn or wheat, in that several countries such as the United States, Canada and Argentina are big producers of the latter grains. The world's other major rice-growing nations are mainly in Asia, such as Laos, Vietnam, Thailand, India and Indonesia.

"The problem that China must contend with is that these countries have limited land and production capacity in comparison with China.

"China might confront a severe situation if the rice output in these Asian countries was cut by natural disasters" or other developments, said Zheng Dawei, a professor at the China Agricultural University in Beijing.

China is the world's biggest rice importer. The Food and Agriculture Organization of the United Nations has forecast that the country's rice imports will reach 3 million tons, about 8 percent of the total global rice trade, in the 2013-14 crop year.

The country already imported about 1.32 million tons of rice in the first half of 2013, up about 12 percent from a year earlier.

China's National Meteorological Center has said that the heat wave will last only until the middle of August. But the drought may persist until the end of the month, because rain forecast for the final 10 days of the month may prove insufficient.

To tackle the drought, the government has allocated 1.65 billion yuan ($269.5 million) for the exploration of ground water sources, artificial rainmaking, building new water conservation facilities and other relief efforts.

"The impact of the drought on rice output in South China may be limited because the northeastern regions, rather than the south, have become the top contributor to the nation's grain harvest in recent years," said Li Guoxiang, a rural development researcher at the Chinese Academy of Social Sciences.

"However, if the floods in the northeast continue, it will have a worse impact on this year's rice output," Li said.

The adverse weather conditions have triggered concerns that the grain market will be the target of domestic and international speculators.

Li said domestic rice prices won't show a broad-based rise, even if China fails to achieve a second harvest this year, because the global supply-demand equation is balanced. China can adjust by resorting to the international grain markets.

"Grain stocks in China are at a high level, which also weakens the possibility of capital speculation on rice," Li said.