Why Plants Don’t Get Sunburnt

For some reason, it’s a widespread belief that watering plants in full sun damages plants. Do droplets really act as tiny magnifying glasses, as some people claim? Rather than digging into professional research immediately to reveal the truth, how would we go out and find out for ourselves?

sherlockIf I go ahead and use a sprinkler on a plant in the midday sun to see what happens, it would be a good start. But I can’t take short cuts such as merely watering a single plant’s leaves. That won’t produce anything conclusive. If I report a change, someone skeptical of the magnifying-glass- hypothesis will justifiably suggest that perhaps some of the leaves were already damaged. If, on the other hand, no changes are observed, someone with the opposite viewpoint will wonder if no droplets persisted on the leaves, or if they were at the necessary angle to the sun.

It would also be better to water a variety of plants. After all, if I observe no burning upon watering only tomato plants, I could conclude that tomatoes are not vulnerable, but everyone will be left wondering if other types of plants are. And in general, whatever variables exist should be controlled by changing only one at a time. The experiments have to be repeated a number of times to be statistically meaningful.

Thinking about it alone won’t settle the question. For example, suppose I imagine that each drop resting on the leaf does act as a magnifying glass. That would imply that the concentrated spot of sunlight potentially burning the leaf would gradually heat up as it does when a convex lens is placed at the right distance above a newspaper. But in the leaf’s case, water is sitting on the hot spot. Isn’t water efficient at removing heat? And wouldn’t the transferred heat cause the little bit of water to evaporate away? Probably not—because if you use a magnifying glass to concentrate sunlight on a wet hand, as I did, you will still feel a burning sensation. The water doesn’t remove the heat quickly enough.

But what if the typical water droplet is not at the right distance from the surface of the leaf to concentrate sunlight? If the drops are too close as one would guess, there would be no magnifying glass-effect and no damage done. In fact, upon closer observation of droplets on a citrus plant early in the morning, I notice no focusing of light rays on the leaf’s surface. The bright, tiny points I see are reflection off the surface of droplets, no matter how I angle the leaf towards the sun.P1140287d

Actual researchers used computer modelling to check if droplets were at the right distance and then did tests on real leaves. Only some tropical plants with hairy leaves held the droplets at the required distance to focus sunlight, but in the field, the droplets evaporated before sunlight caused any damage. From the point of evolution, the conclusion is not startling. After a rainfall, given that the sun often appears before droplets have had a chance to dry off, a fair amount of damage would have ensued since the dawn of foliage, with or without gardeners’ advice.

But if there’s a grain of truth in every myth, the one involved here is that strong sunlight could potentially damage a plant but by an alternate mechanism. In humans, the ultraviolet portion of sunlight affects DNA, potentially leading  to the excess production prostaglandins and cytokines in just a few hours. These compounds stretch blood vessels, leading to redness and pain. If the DNA is damaged enough, the cells die, leading to peeling. In many dark-skinned humans this rarely happens because their skin contains high concentrations of pigments known as eumelanins. They absorb UV and through proton transfer, quickly convert UV to heat. Similarly plants could potentially be damaged by singlet oxygen, a more reactive form of reactive oxygen gas which can form from exposure to intense sunlight, but plants usually convert the latter to harmless infrared.

Zeaxanthin is the particular xanthophyll that’s formed from violaxanthin in bright conditions and helps protect plants from intense sunlight. This yellow pigment is also responsible for the colour of corn and is mixed with carotenes in paprika.

Fluorescence spectroscopy has helped unravel the protective mechanism in both plants and humans.. The melanin-like shield in plants is the light-harvesting complex (LHC). It’s a group of proteins that embed the chlorophylls and accessory pigments needed for photosynthesis. When sunlight becomes intense, increasing photosynthesis leads to further acidification. The pH-drop in turn leads to altered conformations of LHC proteins. This changes their position relative to xanthophylls pigments, drawing them closer after they too had undergone changes in highly bright conditions.   Xanthophylls, a type of carotenoid, are known for their ability to absorb photons of frequencies that chlorophylls are blind to.  But here xanthophylls have a second role. From the approaching LHC protein, xanthophylls pick up the potentially damaging energy and dissipate it by vibrating their long tail-like structure.

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