China's air pollution is putting the country's agriculture sector at risk, experts say.

Smog has blanketed Beijing for at least a week, and much of north and central China -- about one-seventh of the country -- was also covered in smog last weekend, state-run news agency Xinhua reports.

On Wednesday evening, PM2.5 levels -- the measurement of air pollution particles less than 2.5 micrometers in diameter, which pose health risks -- averaged 446 micrograms per cubic meter over a 24-hour period, readings from the U.S. Embassy in Beijing showed.

That's more than 18 times the World Health Organization's recommended level of 25 micrograms per cubic meter.

Scientists say China's smog blocks natural light, thus slowing down the photosynthesis process necessary for plants to thrive.

"Some government officials might worry that linking smog to agricultural production would prompt a panic," Meng Jihua, associate professor with the Chinese Academy of Sciences' Institute of Remote Sensing and Digital Earth, told the South China Morning Post. "But there is no denying the sunlight that reaches Chinese soil has been dramatically reduced in recent years."

He Dongxian, an associate professor with China Agricultural University's College of Water Resources and Civil Engineering, conducted experiments in Beijing over recent months which she says showed a drastic slowdown in the photosynthesis process.

As part of He's experiments, chili and tomato seeds, for example, took more than two months to sprout at a greenhouse farm in Beijing's Changping district, while the same outcome is typically achieved in 20 days under artificial light in a laboratory. Air pollutants adhere to greenhouse surfaces, reducing by half the amount of light available to the plants, she said.

Most seedlings at the farm were weak or sick, He told the Post. "They will be lucky to live at all."

If China's smog persists or intensifies, He warns, the country's food supply faces devastating consequences. "Now almost every farm is caught in a smog panic."

Agriculture accounts for 10 percent of China's gross domestic product.

"A large number of representatives of agricultural companies have suddenly showed up at academic meetings on photosynthesis in recent months and sought desperately for solutions," He said.

Gao Tanggui, a sales manager for the Beijing Shinong Seed Co., said the company's farms had been severely affected by smog. "The impact is serious. Everyone in the company, from farmers to salespeople, are deeply concerned," he told the Post, adding that its plants were sickly and growing more slowly than usual.

Counter-measures that agricultural companies are experimenting with include installing expensive and electricity-sapping artificial lighting, and many farms are boosting levels of plant hormones to stimulate growth, Gao said.