PhotoAlthough fruits and vegetables are now sporting bar codes at some supermarkets, they don't come with built-in clocks. Or do they?

In a new study this week in Current Biology, researchers at Rice University and the University of California at Davis say that produce continues responding to the environment for days after being harvested.

"Vegetables and fruits don't die the moment they are harvested," said Rice biologist Janet Braam, the lead researcher. "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'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.

Circadian rhythm

The research is a follow-up to her team's 2012 study of the ways that plants use their internal circadian clocks to defend themselves from hungry insects. That study found that Arabidopsis thaliana -- an  organism that's similar to cabbage and is often used for plant studies -- begins ramping up production of insect-fighting chemicals a few hours before sunrise, the time that hungry insects begin to feed.

Some of those chemicals are known to be valuable to human health, so Braam's team decided to study whether they were affected by the time of day.

Braam's team began their research by attempting to "entrain" the clocks of cabbage in the same way they had Arabidopsis. Entrainment is akin to the process that international travelers go through as they recover from jet lag. After flying to the other side of the globe, travelers often have trouble sleeping until their internal circadian clock resets itself to the day-night cycle in their new locale.

Using controlled lighting in a sealed chamber, Rice graduate student and study lead author Danielle Goodspeed found she could entrain the circadian clocks of postharvest cabbage just as she had those of Arabidopsis in the 2012 study. Following the success with cabbage, Goodspeed and co-authors John Liu and Zhengji Sheng studied spinach, lettuce, zucchini, carrots, sweet potatoes and blueberries.

"We were able to entrain each of them, even the root vegetables," Goodspeed said. She and Braam said the findings suggest that storing fruits and vegetables in dark trucks, boxes and refrigerators may reduce their ability to keep daily 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."

"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.

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