How the tea leaf protects itself from too much UV irradiation

One of the burning questions people proposed to me in my survey is: why are mountain-grown teas so often more flavorful than teas grown in the valley?

A tea garden on high altitude slopes in Munnar, Kerala, India, with dramatic clouds in the background.

Photo by Bala Karthikeya Pavan Guda on Unsplash.

Of course there are multiple factors involved in the flavor of mountain-grown teas, including increased drainage of the soils, increased fog, which increases ambient humidity, and an increased chance of experiencing the stress of a cold snap. 

One of the most fascinating set of effects of high altitude is the result of higher levels of UV irradiation from the sun. 

UV light causes the leaf chlorplasts to form carotenes and their derivatives, carotenoids. These chemicals serve multiple functions, among which are:

• Capture of UV light energy to transfer of that energy to chlorophyll for photosynthesis, or to dissipate it as heat;
• Capture of excess hydrogen and reactive oxygen;
• Serve as precursors for a number of useful compounds for the leaf cell, and for aroma compounds in tea.

Capture of UV light energy:

UV light from the sun comes in three different wavelength ranges:

• UV-A, or near UV (315–400 nm)
• UV-B, or middle UV (280–315 nm)
• UV-C, or far UV (180–280 nm)

UV-C is completely absorbed by the atmosphere, while some UV-B and most UV-A gets through. The amount of UV-B that gets through depends on altitude: the higher the altitude the thinner the atmosphere, so plants grown at higher elevations are exposed to more UV-B light. 

Exposure to UV light leads the plant to produce carotenoids. These compounds and their precursors, called carotenes, absorb light at the blue and UV end of the spectrum and reflect light in the orange-red range—this is why carrots are orange to our eyes. By contrast, chlorophyll a, the major form of chlorophyll in tea leaves, absorbs and uses light energy primarily in the orange-red end of the spectrum, and reflects green light, making leaves appear green. (Note that chlorophyll b, which is present in smaller amounts, serves as an accessory energy collector for chlorophyll a. It absorbs light in the blue end of the spectrum, but not efficiently at wavelengths below 400nm; in other words it doesn't absorb much UV light.)

By contrast, carotenoids can capture UV-A light energy relatively efficiently, and to some extent can capture UV-B light successfully as well. Then they transfer the light energy to chlorophyll, or they simply dissipate the energy as heat. Incidentally (or perhaps not so incidentally!), in low light conditions (shade, rain, fog), the leaf makes carotenoids to help the chlorophyll capture more energy.

Capture of excess hydrogen and reactive oxygen:

Carotenoids provide a system for dealing with two problems the leaf encounters: the excess hydrogen (H) produced by photosynthesis; and especially the reactive oxygen (O) that comes from photosynthesis and from UV light damage. 

First, the excess hydrogen ions: more hydrogen ions are produced in photosynthesis than are needed for making the leaf’s building blocks, such as sugars, fatty acids, and amino acids. Some of this hydrogen goes to forming a compound called NADPH. 

Second, both photosynthesis and UVB light produce reactive oxygen, that is, oxygen atoms and oxygen-containing structures that react with other chemical compounds, oxidize them, and destroy them. For example, a reactive oxygen atom can attach itself to a polyunsaturated fatty acid in the cell membrane, in a process called lipid peroxidation. It then can build a bridge attaching the fatty acid to adjacent molecules, so the whole membrane breaks up. Other peroxidation reactions lead to breakup of proteins, RNA, and DNA. 

The carotenoids quickly (in a matter of nanoseconds!) take up the reactive oxygen produced. They hand the reactive oxygen over to enzymes that combine it with the hydrogen from NADPH to form water (H2O). In this way the threat posed by reactive oxygen is reduced, while at the same time, NADP is released ready to take up another hydrogen. Having handed over the oxygen, the carotenoids are ready to take up more reactive oxygen, and the cycle can begin all over again.

Aroma compounds from carotenes and carotenoids

Some of the most delicious aromas in teas are made from carotenes and carotenoids, especially aromas with a floral quality, such as rose, lavender, and violet. 

Tea rose 'Mrs Dudley Cross'. Tea roses got their name because many have an aroma reminiscent of black tea, not surprising considering they share many aroma chemicals!
Photo by Solicitr from Wikipedia.

Carotenoids can also be transformed into safranal—safranal is a color and aroma chemical found in saffron—that gives oolongs a subtle but fascinating flavor. 

Beta-damascenone, a carotene derivative, together with the chemical phenyl acetaldehyde found in tea, can give teas the smell and flavor of honey. 

Last in this list, but by far not the only aroma compound in tea derived from carotenoids, is methyl salicylate. It is derived from abscisic acid, which is formed from the carotenoid zeaxanthin when the leaf is stressed, for example with a cold snap. The leaf transforms abscisic acid into methyl salicylate, with its sweet smell of wintergreen, so delicious and desirable in high altitude Darjeelings.

The buildup of carotenes and carotenoids in tea plant leaves grown at high altitudes gives us one reason for their more delicious teas. There are many more reasons, as you can imagine. I’ll be writing about them all in my upcoming book, “The Science and Pleasures of Tea.” Will keep you posted!