Cabbage circadian clocks tick even after picking

Cabbages with jet lag are less nutritious and more vulnerable to insect pests.
Fruits and vegetables have an internal clock that can be reset by a daily cycle of light and dark, but storing produce in darkened refrigerators could disrupt this natural rhythm, researchers report June 20 in Current Biology.

Plants, even after being cropped from the stalk, are much more responsive to their external environment than we give them credit for, says Janet Braam, a plant biologist at Rice University. “When we harvest them they’re still metabolizing,” she says. “They’re still alive.”
Braam normally studies circadian rhythms in plants that are growing, but an offhand comment by her son inspired her to turn to the grocery store for new research subjects.

She and her colleagues had previously found that the plant Arabidopsis thaliana schedules production of insect-repelling chemical defenses to match caterpillar feeding peaks. These defenses include compounds called glucosinolates, which are thought to have anticancer and antimicrobial properties in addition to their caterpillar-discouraging ones.

When Braam told her son about these experiments, he joked that now he knew the best time to eat his vegetables. She realized that cabbages — which also produce glucosinolates — might have similar daily cycles even after being picked, packed and shipped.

“So we went to the grocery store, bought some cabbage and put them under dark/light cycles that were either in phase or out of phase with our insects, and then asked whether the insects could tell the difference,” says Braam.

Like Arabidopsis, the cabbage leaves had daily glucosinolate cycles if the vegetables were exposed to alternating 12-hour periods of light and dark. Caterpillars on a cycle offset by 12 hours to the cabbages’ (so the cabbages’ dawn was the caterpillars’ dusk) ate about 20 times more than did caterpillars on a schedule synchronized to their food. Caterpillars also ate twice as much cabbage if the vegetable had been kept either in constant light or constant darkness.

It’s not just cabbages that adjust daily rhythm to better fend off caterpillars; the team found similar results for spinach, zucchini, sweet potatoes, carrots and blueberries. These fruits and vegetables don’t produce glucosinolates, so they must make some other kind of defenses on a daily cycle, says Braam.

The researchers suggest that we might improve the health benefits and pest resistance of fruits and vegetables by storing them under lighting conditions that mimic day and night. But Cathie Martin, a plant biologist at the John Innes Centre in England, is skeptical. She says most postharvest vegetable losses are from fungal infections, not the insects that eat vegetables in the field. And cabbages are sometimes cold-stored for months in the dark before being sold. Cabbages lose the clock-regulated pest resistance about a week after harvesting, the new study shows.

“But maybe I’ll be proven completely wrong,” says Martin. “Maybe one day we’ll all have little LEDs in the fridge.”