UNHEALTHILY pale lettuce can make even the best salads look wan and unsavoury. But a couple of days under weak ultraviolet light before you pick it can make the lettuce more palatable.

Tests at a US Department of Agriculture (USDA) lab in Maryland show that an LED radiating just a few milliwatts in the UV-B band stimulates red-leaf lettuce to make the antioxidants that give it its characteristic colouring.

Growing crops in greenhouses, done to supply fresh greens all year round, blocks the UV in sunlight that normally prompts lettuce and other crops to make antioxidants outdoors.

Fluorescent lamps could …