Molecular Plant Breeding 2025, Vol.16, No.4, 221-230 http://genbreedpublisher.com/index.php/mpb 225 5.3 Mechanistic insights: hormonal and molecular crosstalk Shading can also regulate the growth, development and quality formation of tea plants by influencing light signals and hormone metabolism. For example, shading can increase the expression of genes related to chlorophyll synthesis (such as CsPORL-2), while inhibiting inhibitory factors like CsHY5, thereby promoting chlorophyll accumulation (Chen et al., 2021). In addition, shading can also affect the synthesis of substances such as amino acids and flavonoids, regulate the balance of carbon and nitrogen metabolism, and improve the quality of tea (Zhang et al., 2020; Yang et al., 2021). When shading and irrigation frequency act together, they may affect bud differentiation, growth and stress resistance by regulating some hormone signaling pathways (such as abscisic acid and gibberellin) (Chen et al., 2023a). Proteomic and transcriptomic studies have also found that shading can activate many genes and proteins related to stress coping, cell protection, and metabolic regulation. These molecular changes provide support for the normal growth of tea plants in complex environments (Han et al., 2023; Zaman et al., 2023). 6 Impacts on Tea Leaf Biochemical Composition 6.1 Total polyphenols and catechin profiles Shading treatment usually leads to a decrease in the total polyphenols and major catechins (such as epicatechin and epigallocatechin) in tea. This change helps to reduce the bitterness of tea and makes the taste milder (Xu et al., 2020). Shading also reduces the esterification degree of catechins, especially the proportion of galacylated catechins decreases, which further improves the flavor structure of tea (Shao et al., 2022; Zhu et al., 2023). In addition, shading can affect the ratio of polyphenols to amino acids. The ratio of polyphenols to amino acids decreases. This indicator is regarded as an important factor for improving the quality of green tea (Zhang et al., 2020; Chen et al., 2022). 6.2 Amino acids and aroma precursors Shading can also significantly increase the content of free amino acids in tea, including theanine, threonine, glutamic acid, alanine, etc. These amino acids make the tea fresher and sweeter (Sano et al., 2018). However, the changes in amino acids are related to the seasons. The shading effect is more obvious in spring and sometimes decreases in summer (Zhu et al., 2023). The study also found that shading can promote the synthesis of theanine in the roots and accelerate its transport to new buds (Yang et al., 2021). Furthermore, shading also regulates the synthesis of carotenoids and aroma precursors, enhancing the accumulation of aroma substances, which lays the foundation for the aroma of high-end tea (Fu et al., 2022; Elango et al., 2023). 6.3 Balance of secondary metabolites for marketable tea quality In addition to polyphenols and amino acids, shading can also affect other secondary metabolites, such as flavonoids, caffeine and lignin (Shao et al., 2022). Shading can reduce bitter flavonoids and lignin, while increasing the levels of caffeine and pigments (chlorophyll, carotenoids), thereby making the color and taste of tea better (Teng et al., 2020; Chen et al., 2021). In addition, shading further affects the synthesis and distribution of these substances by regulating the expression of related genes and DNA methylation (Wakamatsu et al., 2024). These changes combined have made the sensory quality of tea better and enhanced its competitiveness in the market (Xu et al., 2020). 7 Agricultural and Economic Implications 7.1 Yield improvement under combined treatment Shading can improve the micro-environment around tea plants, reduce the pressure brought by high temperatures and strong light, help buds and leaves grow better, and enhance the quality of tea. Many studies have found that appropriate shading (such as using double-layer shading nets) can increase the amino acid content in tea, improve the flavor, and at the same time keep the soil moisture more stable, which is conducive to the continuous growth of buds and leaves (Ge et al., 2024; Hu et al., 2024). Shading can also improve the photosynthetic efficiency of tea plants, increase the chlorophyll content, and enhance the growth ability of the entire tea plant (Sano et al., 2018; Zhang et al., 2022). Although some studies have pointed out that shading may lead to a decrease in yield per unit area (Chen et al., 2023a), as long as the intensity of shading is reasonably adjusted and combined with an
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