Shining a light on circadian rhythms

by Ari LeVaux

We take it for granted that the refrigerator light goes off when we shut the door. But perhaps fridges of the future will be different, as new research suggests exposing fruits and vegetables to light during storage could make them more nutritious.

Plant parts, like leaves and roots, keep living after having been separated from the plants they grew on. For days, weeks, even months after being harvested, the component cells of these plant parts can carry on with their metabolic functions as usual. Most any fruit or vegetable that hasn’t rotted to the point of inedibility can be considered living. This is one of the selling points of a raw food diet, as living foods contain enzymes and other molecules that are destroyed by cooking. A recent paper indicates that cyclic exposure to light and dark causes fruits and vegetables to regulate their cellular metabolism such that the amounts of certain compounds are increased, some of which are beneficial to humans.

The paper’s researchers, based at Rice University and the University of California at Davis, have published previously on the phenomenon of “entrainment” in plants. Entrainment refers to the programming of an organism’s circadian rhythms into certain routines. An example of entrainment in humans would be jetlag, which lingers until the traveler reprogams his or her biological clock to match a new time zone. This reprogramming is guided by changes in daylight patterns. Plants function in much the same way.

An earlier paper published by this group had examined how exposure to light influences the ability of Arabidopsis, a plant in the cabbage family, to ward off insect damage. They found that the plant ramps up its levels of natural insecticide chemicals shortly before dawn, when insects begin feeding. In a controlled experiment, the researchers were able to use light/dark cycles to dramatically slow the rate at which caterpillars devoured a leaf compared to a control.

One category of insect-discouraging chemical, glucosinolates, are known for anti-cancer properties in humans, and there might be others, said Janet Braam of Rice University, the team’s leader.

“We have found that two plant hormones that are critical for plant defense are controlled by the circadian clock, that is, their levels vary depending upon time of day. We hypothesize that these hormones regulate the levels of metabolites important for defense. Glucosinolates are one group of these chemical compounds. So far, we have only measured glucosinolates in cabbage and our research plant, Arabidopsis,” Braam explained via email.

Her team’s new paper, published this June in Current Biology, examined the effect of simulated day and night cycles on the internal clocks of blueberries, spinach, lettuce, carrots and squash. All of these exhibited entrainment in response to light – even carrots, which spend their lives where the sun doesn’t shine.

“It is surprising that carrots responded to the light/dark cycles because they are typically underground and not, therefore, exposed to light,” she said. However, the carrot cells retain structures that are related to chloroplasts (plant cellular structures where photosynthesis takes place) and perhaps in response to light, these structures remain light responsive. In other words, perhaps the cellular machinery responsible for photosynthesis plays a role in entrainment as well.

This research could have implications not only for how food is stored, but how it is prepared, and when. The jury is still out, however, over whether micromanaging your produce’s internal clocks will yield enough health benefit to justify the trouble. “More work needs to be done to determine whether the clock controls enough important metabolites that would have a meaningful effect on human health,” Braam said.

She does store some of her fruits and vegetables in the open, to maintain their circadian rhythms. But so far that’s the extent to which Braam has put her research into practice. She hasn’t jury-rigged her fridge yet.

But maybe, someday, the refrigerators of the future will have crisper lights to mimic outdoor daylight. There’s currently a buzz about the high glucosinolate levels in broccoli, and if their levels could be increased in other vegetables, there could be a market for that.

It might very well be worth our while to remind our produce what time it is. And even if the refrigerator light is destined to remain off when the door is closed, this research might have opened other doors, into our understanding of how post-harvest treatment of plants might affect their nutritional values, and perhaps their shelf lives too. It also adds more nuance to the idea of living foods, and begs questions like what other environmental cues they are responsive to. As future research shines more light on the inner workings of harvested produce, there may be more to chew on in the produce department.