An apple at 1 p.m. may pack a more nutritious punch than an apple at 9 p.m., according to a new study that looks at the effects of fruits' and vegetables' internal clocks on their nutritional value.

"Vegetables and fruits don't die the moment they are harvested," said Rice biologist Janet Braam, the lead researcher and professor and chair of Rice University's Department of Biochemistry and Cell 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."

The study is a follow-up to her team's award-winning 2012 report that examined the ways plants employ their internal clock to defend against hungry insects. In doing so, they discovered that Arabidopsis thaliana, a widely-used model organism for plant studies, begins ramping up production of its insect-fighting chemicals a few hours before sunrise, right around the time bugs begin to feed.

This time, Braam said, the idea for the study came in talking 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!'" she 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."

Because Arabidopsis and cabbage are related, Braam and her team started by attempting to "entrain" the clocks of cabbage in the same manner they had Arabidopsis, a process similar to the one that international travelers go through when recovering from jet lag.

Sure enough, 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 shed had its relative in the 2012 study.

Based on this success, the team applied the same technique to spinach, lettuce, carrots, sweet potatoes and blueberries.

"We were able to entrain each of them, even the root vegetables," Goodspeed said.

Based on this finding, both she and Braam argue that storing fruits and vegetables in dark trucks, boxes and refrigerators likely make it difficult for them to maintain their 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."

Specifically, at least in the case of the cabbage experiments, the researchers found that they could manipulate cabbage leaves to increase the production of the antioxidant called glucoraphanin, or 4-MSO.

Based on their success, Braam's team has already begun follow-up research supported by the Bill and Melinda Gates Foundation into whether light and other stimuli, such as 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.