The impact of shading on tea quality and yield

Tea trees are often affected by adverse environmental factors such as high light and high temperature during their growth process, which in turn affects the quality and yield of tea leaves. Shade, as a commonly used agronomic measure in tea garden management, can improve the quality of tea leaves in terms of color, aroma, and taste by improving the tea garden microenvironment, especially the light environment. It has a wide range of application prospects. This article reviews the research progress on the effects of shading measures on the quality and yield of tea, such as color, aroma, and taste, in order to provide reference for further research and application of shading in tea gardens in the future.

Numerous studies have found that shading not only enhances the frost resistance of tea trees, but also significantly improves the color, aroma, and taste of summer and autumn tea. However, some studies have found that long-term and severe shading can cause a decrease in tea production, manifested as a decrease in fresh and dry weight of new shoots, as well as the quality of 100 buds, and thinning of leaves. Therefore, controlling the shading period and degree is particularly important.. There are two forms of tea tree shading: ecological shading and canopy shading (Figure 1). Ecological shading refers to planting shading trees in tea gardens or intercropping with economic trees and tall and suitable tree species, including forest tea intercropping, fruit tea intercropping, and rubber tea intercropping; Covering shade mainly includes plastic greenhouse covering and shading net covering, using a wide range of materials such as straw, wheat straw, grass curtains, various tree branches, non-woven fabrics (such as canvas), various greenhouse films, shading nets, etc.

1.  The influence of shading on the color of tea leaves

Numerous studies have found that shading can cause chlorophyll accumulation in tea leaves, manifested in an increase in chlorophyll a and chlorophyll b content and a significant decrease in chlorophyll a/b values. The chlorophyll a/b ratio is one of the indicators for measuring plant tolerance to shade. The lower the ratio, the stronger the light harvesting ability, which is beneficial for tea trees to absorb scattered light from the environment. This compound change is the result of tea trees adapting to shaded environments. In addition, shading can also regulate chloroplast development. Liu et al. found that after 14 days of shading, the number of chloroplasts in tea plants increased, their morphology became rounder, the number of thylakoids increased, and their stacking became tighter, resulting in an increase in intracellular starch granules. In addition to plant photosynthetic pigments, theaflavins (TFs) in tea also contribute to the color and quality of tea. Their color is orange or orange red, and it is generally believed to be a product of the oxidation and polymerization of tea polyphenols, which are dimers of catechins converted by polyphenol oxidase (PPO).

The impact of shading on tea color is multifaceted, not only by affecting the metabolic levels of photosynthetic pigments such as chlorophyll and carotenoids, but also by influencing the metabolic pathways of phenolic compounds to affect color quality. In recent years, an increasing number of researchers have focused on the scientific issues behind the color enhancement of tea leaves under shade. Liu et al. found through a combination of transcriptome and metabolomics analysis that receptor proteins and transcription factors in the light signaling pathway may participate in the process of chlorophyll accumulation in tea plants under shading conditions through plant hormones. The research results provide a preliminary basis for further understanding the regulatory mechanism of shading on tea color formation.

2.  The influence of shading on tea aroma

As early as the last century, Shen Shengrong and others used gas chromatography technology to discover that the aroma components in tea leaves were significantly increased to varying degrees after shading, including linalool, nerolidol, vanillol, geraniol, jasmonic acid and its methyl ester, as well as thymol and other substances. Yang et al. used metabolomics techniques to discover that shading can significantly increase the content of VPBs, such as benzaldehyde, benzyl alcohol, and 2-phenylethanol, which contribute to the floral aroma of tea leaves; The study further found that the reason for the increase of these substances under shading conditions is due to the increase in the content of their common upstream precursor L-phenylalanine. In addition, other studies have also found that shading can improve the aroma quality of tea by increasing the types and contents of aroma substances and their precursors.

Shading not only affects light intensity, but also changes the quality of light in the environment. Recent studies have found that changes in light quality can significantly affect the aroma composition of tea. Fu et al. found that blue light (470 nm) and red light (660 nm) can significantly increase the production of most volatile substances in pre harvest tea, including VFADs, VPBs, and VTs, mainly by inducing the expression of related synthetic genes. However, these two light quality treatments have little effect on post harvest tea. Although different from traditional shading, this discovery also provides new ideas and methods for improving tea quality in practical production.

3.  The influence of shading on the taste of tea

Research has found that shading can improve the composition of tea leaves. After shading, the content of tea polyphenols in tea leaves decreases, and the total amount of catechins also shows a decreasing trend with increasing shading degree and prolonged shading time. After analyzing the content of catechins in 8 types of tea leaves after shading, it was found that ester catechins such as EGCG and ECG showed an increasing trend, while most non ester catechins such as EGC and EC showed varying degrees of decrease.

Shading can not only reduce the bitterness of tea by lowering its polyphenols, but also enhance the freshness and sweetness of tea by increasing the content of free amino acids, enriching the taste and texture of tea. Theanine is the main amino acid component in tea leaves. After shading, the content of theanine significantly increases, and the precursor glutamate in the theanine metabolism pathway also accumulates significantly. However, the expression level of the key synthesis gene, theanine synthase gene (CsTS), does not change significantly with prolonged shading time, and the specific regulatory mechanism is still unclear and needs further research. And other free amino acids such as threonine, histidine, asparagine, aspartic acid, arginine, phenylalanine, etc. have been increased to varying degrees. The increase in free amino acid content can significantly reduce the phenol ammonia ratio. The phenol ammonia ratio is the main factor for evaluating the taste of green tea, and the phenol ammonia ratio of fresh leaves after shading is lower, making them more suitable for making green tea. In addition, shading can also increase the content of caffeine and soluble sugars in fresh leaves, thereby improving the taste quality of tea soup. Among them, the effect of shading on caffeine may be greatly affected by species, and in some varieties, the changes in caffeine are not significant, and there are even reports of a decrease in caffeine content after shading.

During the hot summer season, the application of shading measures in tea garden management can greatly enhance the taste and quality of tea leaves, with direct economic benefits. However, the practical application of shading in production is often limited by labor costs and has not been fully promoted. In addition, the effect of shading measures on improving the quality of spring tea is very limited. Studies have found that shading in spring causes a decrease in the amino acid content of tea leaves, while the level of caffeine content remains basically unchanged or even decreases. This may be related to the climate of the production area and the characteristics of spring tea (low phenol ammonia ratio). Factors such as terrain, climate, season, shade tolerance of cultivated varieties, sprouting of new shoots, and production needs in different production areas have different requirements for the degree and method of shading of tea trees, which further limits the application and promotion of shading measures. Therefore, analyzing the specific mechanism of shading on tea tree quality improvement has profound significance for guiding practical production. However, the elucidation of the mechanism is often limited by the imperfect genetic transformation system of tea plants, and there are still a large number of research gaps to this day.

4.  The impact of shading on tea yield

There is currently no unified conclusion regarding the impact of shading on tea tree yield, which is constrained by factors such as origin, climate, variety, degree and method of shading. There are currently two views: moderate shading improves the microclimate of tea gardens, optimizes growth conditions, and promotes tea tree growth; The effect of shading on improving quality gradually increases with the degree of shading, and tea quality is better under severe shading, but the yield decreases. Among them, a relatively consistent conclusion is that shading will increase the moisture content of new shoots, causing a decrease in tea dry matter content and affecting yield.

In addition to directly affecting the density and quality of new shoots, shading can also indirectly affect yield by altering leaf structure and affecting shoot morphology. Research has found that shading can cause thinning of the thickness of leaves, their epidermal layer, stratum corneum, and palisade tissue; The new shoots become shorter; Increase in leaf area; The number of layers in the fence organization decreases and the arrangement becomes loose; Increased interstitial spaces in sponge tissue; The ratio of fence tissue to sponge tissue decreases. The changes in this morphological structure are also consistent with the phenotypic changes of plants in shaded environments. After shading, tea leaves not only undergo morphological changes, but also become softer and more tender in texture. Tenderness retention refers to the ability of tea leaves to maintain their tenderness, and it is one of the important measurement criteria in the process of tea variety breeding. Research has found that the decrease in crude fiber content, lignin content, and water content in tea caused by shading may be the direct cause affecting the tenderness of tea. High leaf tenderness may affect the defense ability of tea leaves. However, after analyzing the composition of the cuticle layer on the leaf epidermis, it was found that shading does not affect cuticle synthesis, which reflects the high adaptability of tea trees to low light intensity.

Although the improvement of tea quality by shading is greatly influenced by the environment and tea tree varieties, the overall effect still shows a trend of better quality improvement with the increase of shading degree. Specifically, shading significantly increases the levels of chlorophyll and free amino acids in tea, reduces the content of tea polyphenols, and improves the color, aroma, and taste quality of tea. In addition, there is controversy over the impact of shading on tea tree yield. Some articles have reported that shading increases new shoot yield, but overall, with increasing shading, new shoot yield decreases.

summary

Generally speaking, various shading measures can reduce the light intensity and leaf temperature of the tree crown to a certain extent, increase the relative humidity of the air and soil, and thus improve the microenvironment for tea tree growth. After shading, the quality traits such as color, aroma, and taste of different tea tree varieties in different seasons have been improved to a certain extent, mainly manifested as a significant increase in chlorophyll and β – carotene content, a significant increase in volatile fatty acid derivatives and volatile phenylpropanoids/benzoate, a significant increase in amino acid content such as theanine and caffeine content, and a significant decrease in tea polyphenols and total catechins content. In terms of yield and traits, the impact of shading on tea trees shows inconsistent patterns, and it is speculated that the possible reasons are related to shading materials, shading intensity, tea tree varieties, shading seasons, and duration.