Journal of Tea Science Research, 2024, Vol.14, No.4, 238-248 http://hortherbpublisher.com/index.php/jtsr 240 3 Impact of Rising Temperatures on Tea Plant Physiology 3.1 Growth patterns and yield Elevated temperatures have a significant impact on the growth patterns and yield of tea plants (Camellia sinensis). Studies have shown that moderately high temperatures can enhance certain aspects of tea plant growth, such as photosynthesis and biomass accumulation. For instance, melatonin treatment under sub-high temperature (SHT) conditions (35 °C) has been found to increase dry biomass by 40.8% and photosynthesis by 28.1% (Li et al., 2020). This suggests that while elevated temperatures can promote growth, the overall impact on yield is complex and influenced by additional factors such as the presence of growth stimulatory molecules. However, the relationship between elevated temperatures and tea yield is not straightforward. While increased CO2 levels, often associated with climate change, can enhance tea yield, this benefit can be offset by the negative effects of higher temperatures. Elevated CO2 has been shown to improve photosynthesis and respiration, leading to increased biomass production (Li et al., 2017; Ahammed et al., 2020). Yet, the same studies indicate that higher temperatures can lead to reduced water retention, chlorophyll content, and increased membrane damage, which negatively affect plant health and yield (Seth et al., 2021). Therefore, while elevated temperatures can stimulate certain growth parameters, they can also introduce stress factors that may ultimately reduce yield. 3.2 Physiological stress responses Tea plants exhibit a range of physiological stress responses when exposed to elevated temperatures. One of the primary responses is the alteration of metabolic processes. For example, high temperatures have been shown to significantly reduce the flavonoid content in tea leaves by activating specific transcription factors (CsHSFA1b and CsHSFA2) that negatively regulate flavonoid biosynthesis (Zhang et al., 2023). This reduction in flavonoids can adversely affect the quality and flavor of tea, as these compounds are crucial for the characteristic taste of tea leaves. In addition to metabolic changes, elevated temperatures also affect water usage and leaf structure. Heat stress can lead to reduced water retention and increased membrane damage, as observed in sensitive tea cultivars. This is often accompanied by a decrease in chlorophyll content, which further impairs photosynthesis and overall plant health. The expression of heat shock proteins (HSPs) and heat shock transcription factors (HSFs) plays a crucial role in mitigating these effects. For instance, the upregulation of HSPs such as CsHSP90 has been shown to enhance thermotolerance by stabilizing proteins and membranes under heat stress conditions (Seth et al., 2021). Moreover, elevated temperatures can lead to changes in leaf structure, such as altered leaf thickness and stomatal density, which impact the plant's ability to manage water loss and gas exchange. These structural changes are part of the plant's adaptive mechanisms to cope with heat stress but can also lead to reduced efficiency in photosynthesis and nutrient uptake (Li et al., 2018; Li et al., 2020). Overall, the physiological stress responses of tea plants to elevated temperatures involve a complex interplay of metabolic, structural, and molecular changes aimed at maintaining homeostasis and ensuring survival under adverse conditions. 4 Effect of Rising Temperatures on Tea Chemical Composition 4.1 Alteration in flavor compounds Rising temperatures significantly impact the chemical composition of tea, particularly affecting key flavor compounds such as catechins, amino acids, and caffeine. High temperatures have been shown to reduce the flavonoid content in tea leaves, including catechins, which are crucial for the flavor profile of tea. This reduction is mediated by the activation of specific transcription factors, such as CsHSFA1b and CsHSFA2, which negatively regulate flavonoid biosynthesis under heat stress (Zhang et al., 2023). Additionally, studies have demonstrated that catechin levels decrease significantly with increasing temperatures, leading to a decline in the characteristic bitterness and astringency of tea (Kfoury et al., 2018).
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