Plants are not passive things. Oh, they’re not running and fighting and directly acting on the local environment with any sort of mammalian consciousness or intent, but they do employ defenses against hungry animals, insects, intrusive plant life, and disease – just like we do. We differ greatly in a few major areas, of course. Plants make themselves (or their seeds) unpalatable, indigestible, and downright poisonous through the lectins, gluten, and other antinutrients we enjoy railing against; to defend themselves and their offspring (fuzzy “seeds”), animals bare teeth and claws, run incredibly fast, climb trees, burrow into the ground, or wield semiautomatic rifles. But plants’ and animals’ respective modes of management of “internal” threats, like disease or infection, are more similar than not: we all manufacture antioxidants. With animals, the immune system, which defends from pernicious invading forces and helps determine the inflammatory response to harmful stimuli, is well known by all, but there are also the endogenous antioxidants that animals produce to deal with oxidative stress and free radicals. Humans, for example, have the potent arsenal of glutathione, superoxide dismutase, alpha lipoic acid, catalase, and CoQ10. Vitamin C is another common, endogenous animal antioxidant, just not in bats, guinea pigs, tarsiers, monkeys, humans and other apes.
One funny thing about plant phytochemicals is that they’re not intended for us. They exist to protect the plant. The fact that they seem to be pretty good for us is secondary (but I’ll take it!). Another funny thing about phytochemicals is that they are not static stores. They fluctuate, both up and down. Just as our bodies produce hormones and other endogenous chemicals as they’re needed in the amounts they’re needed, so to do plants increase or decrease levels of phytochemicals in response to stimuli. Take ascorbate, or vitamin C – photosynthetic plants use it for protection from oxidative stress during photosynthesis (using the sun’s energy to create organic compounds, like sugars, out of carbon dioxide, or “cookin’ up some plant food”). More sunlight creates the possibility for more photosynthesis, which creates the need for more ascorbate; more exposure to light, then, results in higher concentrations of ascorbate. This makes sense and is widely accepted by the scientific community. The same general rule applies to other phytochemicals.
Last year, a USDA research plant physiologist by the name of Dr. Gene Lester conducted a fascinating study on the phytochemical metabolism of two types of spinach. He wondered if, since spinach growing in the field relies so heavily on visible sunlight exposure for producing the phytochemicals that make humans want to eat it, perhaps exposing the spinach to light post-harvest continued to affect its phytochemical composition. And so he found out.
Using freshly-harvested, conventionally-grown Texas spinach, Lester’s team measured the various light-sensitive bioactive compounds in the leaves: vitamin C (ascorbate), vitamin K1 (phylloquinone), vitamin B9 (folate), vitamin E (alpha-tocopherol), and the carotenoids (lutein, beta-carotene, zeaxanthin, violaxanthin). The leaves were then packaged into standard clear plastic containers, which were either placed on a shelf with exposure to full retail display lighting or placed in double-thick brown paper bags with zero exposure to light. This went on for nine days. Bioactive compound levels were tested throughout the duration of the study; here’s what changed (or didn’t):
- Vitamin C – Exposed spinach had significantly more ascorbate than covered spinach. At day three, ascorbate levels in exposed spinach were greater than original levels at harvest, and by day nine, exposed spinach had retained original ascorbate levels.
- Vitamin K1 – Vitamin K1 is required for electron transport during photosynthesis, and it increased in both covered and exposed spinach leaves. Exposed leaves, however, saw generally greater gains.
- Vitamin B9 – Vitamin B9 levels were preserved in exposed spinach leaves, while they degraded in covered leaves. Previous studies have shown spinach folate levels to generally decrease with storage, but this study shows that light exposure is the deciding factor – not simply “storage.”
- Carotenoids – Carotenoids, as a general rule, increased across the board in both exposed and covered leaves, with exposed spinach seeing more on average. Lutein and zeaxanthin were highest in exposed leaves, while beta-carotene and violaxanthin were higher in covered leaves.
- Vitamin E – Tocopherols were “generally higher” in exposed leaves than covered leaves.
Lester also noted differences between bottom-canopy leaves (older, larger), mid-canopy leaves (younger), and top-canopy leaves (youngest, “baby”). The smaller and younger the leaf, the greater the phytochemical content. Taken together, then, it seems sensible to consider both leaf age/size and exposure to light. If you can’t get freshly harvested spinach, for example, try choosing baby spinach that’s been sitting on top of the pile or stack in the store, exposed to constant supermarket lighting.
I shared a few emails with Dr. Lester, who plainly stated that similar studies would probably be “not necessary” for other green vegetables, since the key variable is photosynthesis. If a plant uses photosynthesis to accumulate bioactive compounds, it’s safe to assume that exposing that plant to continuous light post-harvest will also increase the vitamin and antioxidant content. So – kale, chard, lettuce, broccoli, etc., should all operate under the same principles.
He also speculated that berries should accumulate beneficial compounds and grow richer in color post-harvest when exposed to light, but that it must be UVB as opposed to basic store lighting. I did a bit of digging and found that exposing blueberries to UVC radiation (the type that’s blocked by the ozone and never actually reaches us or our berries) after harvesting had mixed results. It reduced decay and increased or maintained the anthocyanin content, but it wasn’t clear whether the antioxidant effect was due to reduced decay or some other effect of the radiation, or whether it was due to the natural ripening of the berry that would have happened on or off the plant anyway. It’s inconclusive at best.
I did come across a study (PDF) discussing the anthocyanin production of blue corn in response to UVB exposure, in which it’s determined that UVB light, in concert with phytochromes (photoreceptor pigments plants use to detect light), does increase anthocyanin content. This was blue corn still attached to the stalk, not just sitting there after harvest, so it’s impossible to extrapolate it to blueberries sitting in a basket. I guess the question is this: are the phytochromes located on the berry itself and do they remain active after harvest? This study on harvested cranberries, in which light exposure increased anthocyanin content, would indicate that they are and that they do, especially if the same holds true for other berries. Why wouldn’t it?
In light of last week’s sweet potato post, I came across another interesting study on how wounding purple sweet potatoes – literally cutting them with a knife while they were growing – increased antioxidant content, specifically the valuable anthocyanins. They also tried fooling around with light exposure, temperature changes, as well as ethylene and methyl jasmonate treatments, but nothing had any significant effect. Getting in there with a blade did the trick. This makes perfect sense, intuitively; if a plant receives trauma, it’s a good chance something is trying to eat, invade, or infect it. Antioxidants like anthocyanins are there to protect the plant from these exact situations, and so they produce more. And then we eat them anyway. Good deal.
Fascinating stuff, huh? If you take anything away from this at all, it’s that maybe a little corporal punishment is good for your misbehaving plants and that storing berries on the counter or in the sun is probably better than in the fridge (unless your refrigerator light stays on with the door closed). It’s also good to know that we, as consumers of produce, have at least a modicum of agency when it comes to the nutritional density of our food. If we can’t decide what the food producers do to the food while it’s in the ground, and we can’t make it to the farmers’ market for freshly harvested produce from people we trust, and we can’t grow our own food, maybe we can take a few tiny steps to imbue our mass-produced food with more vitamins and phytochemicals. Every little bit helps. And, since self-experimentation is the backbone of this community, put some of these findings into practice and let me know what happens. I don’t expect a full phytochemical composition breakdown, but some subjective experiences are welcome.