June 20, 2013
You probably don’t feel much remorse when you bite into a raw carrot.
You might feel differently if you considered the fact that it’s still living the moment you put it into your mouth.
Of course, carrots—like all fruits and vegetables—don’t have consciousness or a central nervous system, so they can’t feel pain when we harvest, cook or eat them. But many species survive and continue metabolic activity even after they’re picked, and contrary to what you may believe, they’re often still alive when you take them home from the grocery store and stick them in the fridge.
The most recent evidence of this surprising phenomenon? A new paper, published today in Current Biology by researchers from Rice University and UC Davis, found that a range of harvested fruits and vegetables—including cabbage, lettuce, spinach, zucchini, sweet potatoes, carrots and blueberries—behave differently on a cellular level depending on their exposure to light or darkness. In other words, these fresh produce have an internal “body clock,” or circadian rhythm, just like we do.
Previously, Rice biologist and lead author Danielle Goodspeed had found that some plants depend on light cycles and their internal circadian rhythm to fend off predatory insects, at least while still in the ground. In experiments, she had noticed that thale cress plants used reliable daily exposure to sunlight as a basis for anticipating the arrival of insects during the day, and were able to build up reserves of defensive chemicals beforehand, during the night.
In this new study, she and others sought to determine whether already harvested samples of plant species that we commonly eat demonstrate the same kind of circadian behavior. They started by looking at cabbage, a close relative of thale cress, subjecting samples to similar experiments employed to arrive at the previous finding.
The team bought cabbage at the grocery store and took small leaf samples, and also acquired cabbage loopers, small moth larvae that like to feed on cabbage. The larvae were kept on a routine 24-hour light cycle: 12 hours of light alternating with 12 hours of darkness.
For three days, half of the cabbage samples were put on this same cycle, to “train” their circadian rhythms, but the other half were put on an entirely opposite cycle. As a result, plants in this second group would “think” it was night when the larvae behaved as if it were actually daytime, and vice-versa. If the harvested cabbage tissue depended on light exposure in the same way as the planted thale cress, then it’d build up defense chemicals at exactly the wrong time of day, and would likely suffer for it if the pests were given a chance to feed.
When the researchers let cabbage loopers loose on their favorite food, that’s exactly what happened. Cabbage leaves in the out-of-sync group showed significantly less resistance than the other samples, suffering more tissue damage and losing weight more quickly. The cabbage loopers feeding on these leaves also grew more quickly than those feeding on the first group. When the team directly measured levels of one specific class of chemicals involved in metabolic defense activity in the samples, they found that they did indeed cycle along with what the plants had been “trained” to anticipate as daytime.
The researchers put harvested lettuce, spinach, zucchini, sweet potatoes, carrots and blueberries through the same sort of experiment and arrived at the same results. All the plant samples “trained” to anticipate day at the correct time suffered less damage from the larvae than those with circadian rhythms that had been set incorrectly. It’s unclear why the root vegetables—carrots and sweet potatoes—would demonstrate a circadian rhythm (after all, they grow under the ground), but it’s possible that the entire plant simply uses the light cycle to orient its metabolic activity, and the pattern affects the roots as well as the leaves.
In a sense, the produce used in the experiment got jet lagged—their circadian rhythms told them it was nighttime, so they didn’t need to produce the defensive chemicals, when in fact it was day. It’s not so different from flying, say, to India, and your body telling you it’s time to sleep when you arrive, when in truth it’s 11 a.m. local time. Except, of course, that your jet lag doesn’t make you more prone to being consumed alive by insects.
Our burgeoning understanding of the circadian rhythms and metabolic activity of plants could eventually make an impact on another animal species that consumes fruits and vegetables: Homo sapiens.
The reason, the researchers say, is that some of the same chemicals involved in defense against insects appear to also act as anti-cancer agents. In trials, cabbage samples kept entirely in the dark (like, say, the vegetables in your refrigerator) suffered greater tissue loss than those with the circadian rhythm that aligned with the larvae, indicating they had lower overall levels of anti-pest (and anti-cancer) chemicals. So designing harvest, transport and storage systems with a focus on light exposure could be the next step in maximizing the nutrition we get when we eat fruits and vegetables.
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