Inside a building on the outskirts of Chengdu, the capital of Sichuan province, 16 robots tend to vegetables growing on a 10-meter-tall, 20-tier rack. Standing beside the world's tallest unmanned greenhouse rack, the urban farm's chief scientist, Yang Qichang, said 24,000 vegetables had been planted in an area of just 200 square meters.

Cultivation procedures at the plant factory at the Chinese Academy of Agricultural Sciences' Institute of Urban Agriculture are completely automated, including sowing, thinning, harvesting and packaging.

In an open field, a lettuce usually takes about 70 days to grow from seed to harvest, but at the urban farm, the growth period can be halved, resulting in 10 crops a year. Annual output is expected to reach 50 metric tons.

Nutrition has not suffered as a result of rapid growth and high productivity. The crude protein content of alfalfa grown by crop breeding accelerators has reached 30 percent, about 10 percentage points higher than that of normal species.