Molecular and Physiological Adaptations of Tea Plant in Response to Low Light and UV Stress

The sensory quality, economic value, and health functions of tea liquor depend on several metabolites in leaves of tea (Camellia sinensis L.), such as polyphenols (the most representative quality- and function-related compounds, especially catechins), caffeine, amino acids, aroma compounds, vitamins, and carbohydrates. The biosynthesis of these molecules is significantly affected by both management practices in the field and environmental factors, such as shading and ultraviolet (UV) stress. In this chapter, therefore, we review existing literature reporting the effects of shade and UV irradiances on tea plants with the aim to reappraise morphological, biochemical, physiological, and molecular responses found in this species. Cultivation of tea plants under shade can (i) induce anatomical and ultrastructural adaptations; (ii) influence leaf color, providing characteristic vivid greenness due to incremented chlorophyll content; (iii) decrease the risk of photoinhibition; and finally (iv) improve the quality of tea beverages by causing the reduction of the concentration of phenylpropanoids and caffeine (which contribute to astringency) while increasing the level of amino acids (the compounds that contribute to sweetness). However, the shade practice in tea is still subjected to controversy, and the correct managing of the shade practice is essential. Elevated UV irradiances, especially UV-B, can alter the plant morphology and affect several biochemical and physiological processes; most plant species respond to UV in a dose dependent manner. Protracted exposition to low UV-B doses (similar to UV peaks actually perceive by plants in the ambient) leads to an enhancement of phenylpropanoid metabolism and in particular stimulates the biosynthesis of UV-protecting flavonoids (mainly catechins). Conversely, UV-B doses much higher than those usually experienced in natural environment lead to the downregulation of the phenylpropanoid metabolism likely due to irreversible (to some extent) UV-triggered damages. The understandings of the reactions of tea plants to different light conditions, including situations of low light and high UV, are of pivotal importance to enhance both plant yield and tea quality by “driving” the accumulation of desired compounds in tea leaves.