One category of green tea stands out among Japanese green tea lovers: shade teas. For a few weeks before the harvest, the tea bushes are covered with a tarpaulin that limits the passage of light. The goal is to push the plant to enter a "survival" mode, which allows an accumulation of molecules that will give a strong flavor to the buds and young tea leaves. One of the visual characteristics of these leaves is that they take on a "fir green" color, more marked than conventional tea harvests.
The green color of tea leaves comes mainly from chlorophyll, a pigment that allows to absorb part of the sunlight to convert it into chemical energy that will be used by the plant for its autonomous development: this is photosynthesis. Research on shaded teas has shown that the plants synthesize more chlorophyll than non-shaded tea plants, which explains why shaded leaves are much greener.
There is however a catch, chlorophyll is synthesized ... thanks to sunlight! How does the plant synthesize this pigment with little sunlight? Just like humans, plants secrete hormones, molecules that are able to modify the functioning of cells. Several modifications in the shaded plant cells take place: the structures of the plant cell allowing the synthesis of chlorophyll (chloroplasts) grow and present a stronger metabolic activity. Briefly they will synthesize more of the main plant hormones influencing the structure of the chloroplasts, the chlorophyll synthesis pathways and the molecules detecting sunlight. It's basically the entire expression of the plant's genetic profile that will change just with shading to keep the plant growing!
However, this represents a form of stress for the plant. If it favors the synthesis of chlorophyll to optimize the reception of solar energy, it will no longer correctly produce certain molecules necessary for its self-development. Like muscle cramps after exercise, plant cells are no longer as well armed to deal with oxidative chemical stress. This raises two major questions: is it dangerous for the growth of the plant after harvest? Is it a long-term danger for the health of the plant?
The first answer is positive: researchers have shown that if the cells of shaded leaves show damage linked to oxidative stress, putting them back in the light for 2 weeks allowed the leaves to regenerate well.
Secondly, and more worryingly, a recent study by Yamashita et al. showed that repeated shading could induce over-stress. After three successive shades on Yabukita plants, the authors observed a decrease in the quality of chlorophyll synthesis. For 6 years, shaded Japanese tea fields were monitored by the same team. It seems that the capacity of the leaves to photosynthesize is reduced and that the synthesis of certain essential nutrients such as starch is lower. In addition, the heat generated by the tarps seems to have further damaged the leaves because the leaf pores that allow transpiration to regulate internal heat (yes, like humans) no longer open as efficiently.
In the long term, won't overhang affect the quality of future shaded teas? Hard to say, but for today, you know the secret to the greenness of these teas!
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