Of course, they are alive not in the sense that you can see them heavily breathing begging you not to eat them. After all, plants don’t have a central nervous system and therefore can’t feel pain, but in the sense that they still maintain some aspects of metabolism, such as respiration.
Respiration is the process of breaking glucose to produce energy to use for cellular processes such as ripening. You probably could observe this phenomena yourself – tomatoes continue to redden and bananas turn yellow even after being picked. In short, after being picked a fruit continues the ripening process – skin changes in colour, flesh softens and sweetens and attractive aroma develops.
However, one could argue that fruits are seed-bearing structures and are designed to be detached from a parent and still continue to ripen to promote seed dispersal by animals. They will be right and the argument is a good one. Does it mean that metabolism of non-seed bearing structure cease to exist after harvest?
Non-seed bearing structures are all other plant parts – roots, leaves and stems – that do not contain seeds and are given a common culinary term “a vegetable”
Of course not and, in fact, research team led by Goodspeed in 2013 showed that vegetables after harvest are more alive than one would think. It was observed that not only do the vegetables respond to light and temperature, they also continue to produce phytonutrients after harvest.
Researchers put the cabbage discs for three days at room temperature (22°C) either under simulated natural day settings, that is 12-hour day and 12-hour night cycles (known as entrainment; day-night cabbage), in constant dark (constant night cabbage) or in constant light (constant light cabbage).
Entrainment occurs when plant’s endogenous (located inside) rhythmic processes become adjusted to the surrounding environment by external cues, such as light and temperature.
Then all discs were exposed to a predator, cabbage looper larvae, kept under the same conditions as day-night cabbage. They found that the larvae fed day-night cabbage gained two times less weight than the larvae fed either constant day cabbage or constant night cabbage. This correlated with levels of glucosinolate, anti-herbivore metabolite, meaning that the day-night cabbage had elevated levels of glucosinolate during the simulated day, the usual time of larvae arrival. This showed that the cabbage responded to light since it has matched its glucosinolate level appropriately according to the time of simulated day and could therefore protect itself against the larvae, just like it did when attached to a parent plant.
The experiment was repeated under 4°C, and although the glucosinolate level was reduced, the results were analogous.
Finally, the researchers replicated the experiment with lettuce, spinach and carrot and arrived at the similar results.
Besides showing that vegetables respond to light and temperature after harvest, the study also indicated that phytonutrients are still being produced at considerable levels under simulated day and night conditions.
This study has an important implication in storage of harvested produce. Since certain beneficial phytonutrients could be targeted if vegetables are stored at warmer temperature under simulated day and night conditions, they can be consumed accordingly when the levels of those nutrients are the highest.
Goodspeed, D.G., Liu, D.L., Chehab, E.W., Sheng, Z., Francisco, M., Kliebenstein, D.J., Braam, J. (2013). Postharvest Circadian Entrainment Enhances Crop Pest Resistance and Phytochemical Cycling. Current Biology, 23 (13), 1235–1241.