"Vegetables and fruits don't die the moment they are harvested," said Rice biologist Janet Braam, the lead researcher on a new study this week in Current Biology. "They respond to their environment for days, and we found we could use light to coax them to make more cancer-fighting antioxidants at certain times of day." Braam is professor and chair of Rice's Department of Biochemistry and Cell Biology.
Braam's team simulated day-night cycles of light and dark to control the internal clocks of fruits and vegetables, including cabbage, carrots, squash and blueberries. The research is a follow-up to her team's award-winning 2012 study of the ways that plants use their internal circadian clocks to defend themselves from hungry insects. That study found that Arabidopsis thaliana -- a widely used model organism for plant studies -- begins ramping up production of insect-fighting chemicals a few hours before sunrise, the time that hungry insects begin to feed.
Braam said the idea for the new research came from a conversation with her teenage son.
"I was telling him about the earlier work on Arabidopsis and insect resistance, and he said, 'Well, I know what time of day I'll eat my vegetables!' Braam said. "That was my 'aha!' moment. He was thinking to avoid eating the vegetables when they would be accumulating the anti-insect chemicals, but I knew that some of those chemicals were known to be valuable metabolites for human health, so I decided to try and find out whether vegetables cycle those compounds based on circadian rhythms."
"We cannot yet say whether all-dark or all-light conditions shorten the shelf life of fruits and vegetables," Braam said. "What we have shown is that keeping the internal clock ticking is advantageous with respect to insect resistance and could also yield health benefits."
In the cabbage experiments, Braam, Goodspeed and Rice co-authors John Liu, Zhengji "Jim" Sheng and Wassim Chehab found they could manipulate cabbage leaves to increase their production of anti-insect metabolites at certain times of day. One of these, an antioxidant called glucoraphanin, or 4-MSO, is a known anti-cancer compound that has been previously studied in broccoli and other vegetables.
Braam's team has already begun follow-up research, which is supported by the Bill and Melinda Gates Foundation, into whether light and other stimuli, like touch, may be used to enhance pest resistance of food crops in developing countries.
"It's exciting to think that we may be able to boost the health benefits of our produce simply by changing the way we store it," Goodspeed said.
Does your salad know what time it is?
A time-lapse comparison of caterpillars eating postharvest lettuce: