Is that apple in your hand still alive? For researchers at Rice University, it's less of an existential debate, and more of a scientific quandary whose answer may change the way we store our fruits and vegetables. In a study published in Current Biology, researchers demonstrate that harvested food crops still respond to daylight, which affects their ability to produce natural defenses to insect pests.

When the sun rises in the morning, the cells in our bodies run through a coordinated sequence of genetic and physiological events to maximize our alertness and metabolism. Known as a circadian rhythm, this internal clock is precisely tuned to light-dark cycles.

Given sunlight is so vital for their survival, it's not too surprising that plants are similarly equipped with circadian clocks. One function of these plant clocks is to regulate natural defenses against insect herbivores that feast on their leaves during the day. When the sun rises, plants will start producing compounds that the pests detest. The pests are less active night, so at sunset, plants have evolved to stop making these natural pesticides in order to conserve energy

But Dr. Janet Braam, a biochemist and cell biologist at Rice, wondered if fruits and veggies keep this clock even after being pulled from the ground or plucked from a tree. Braam and her team suspected they might because plants are often modular, meaning their different segments — roots, leaves, branches, fruit — often behave autonomously.

Their hunches were right. Cabbage retained its internal clock even after being harvested. When stored under a 12-hour light/dark cycle, cabbage continued to pace its levels of glucosinolate, a compound that protects it from caterpillars.

Cabbage loopers — a breed of worm/moth that feasts on the veggie — was less inclined, by over 20-fold, to eat cabbages that were stored under a light-dark cycles. It didn't matter if the cabbage was grown in a lab or purchased from a market, light-dark cycles protected them from pests. This trait lasted for about four to six days after harvest.

"Vegetables and fruits, even after harvest, can respond to light signals and consequently change their biology in ways that may affect health value and insect resistance," said Braam.

Leaf Movements Are Controlled By Plant Circadian Clocks Too (video source)

Natural pesticides weren't the only compounds affected by this storage practice. 4-methylsulfinylbutyl (4MSO), a glucosinolate with anticancer and antimicrobial propersties, also witnessed a boost in production with this storage practice.

This phenomena occurred even if the harvested cabbage was stored in a refrigerator, just as long as it was kept under a 12-hour light/dark cycle.

Lettuce, spinach, zucchini, sweet potatoes, carrots, and blueberries had similar responses to light-dark storage.

"Perhaps we should be storing our vegetables and fruits under light-dark cycles and timing when to cook and eat them to enhance their health value," continued Braam. "It may be of interest to harvest crops and freeze or otherwise preserve them at specific times of day, when nutrients and valuable phytochemicals are at their peak."

Source: Goodspeed D, Liu JD, Chehab EW, et al. Postharvest Circadian Entrainment Enhances Crop Pest Resistance and Phytochemical Cycling. Current Biology. 2013.