1 Introduction

Tea (Camellia sinensis) is one of the most globally valued crops, second only to water as the most consumed beverage worldwide. The quality and flavor of tea are paramount to industry stakeholders, including farmers, manufacturers, and consumers. The unique taste of tea is derived from a complex mix of phytochemicals, including catechins, theanine, and caffeine, which contribute to its sensory attributes and health benefits (Kfoury et al., 2018; Lin, 2023). High-quality tea is characterized by its distinctive flavor profiles, which are influenced by the concentration of these compounds (Li et al., 2018; Li et al., 2020; Lin et al., 2024). The economic importance of tea is significant, particularly in major tea-producing regions such as China and India, where it supports the livelihoods of millions of people.

Climate change poses a significant threat to global agriculture, with rising temperatures being one of the most critical factors affecting crop production and quality (Ahmed et al., 2019; Yan et al., 2021). The impact of climate change on tea production is multifaceted, influencing both yield and quality. Extreme weather events, such as heatwaves and droughts, have been shown to reduce tea yields and alter the concentration of key secondary metabolites that determine tea quality (Buckley et al., 2014). For instance, higher temperatures can lead to a decrease in theanine content, which is essential for the umami flavor of green tea. Additionally, changes in precipitation patterns and increased frequency of extreme climate events can further exacerbate the variability in tea quality (Duncan et al., 2016).

This study aims to explore the effects of rising temperatures on the quality and flavor of tea, analyzing how temperature variations impact the biosynthesis of key flavor compounds in tea, such as catechins, theanine, and caffeine. By understanding these effects, the study seeks to offer potential mitigation strategies to maintain tea quality in the context of climate change, including examining the role of agricultural management practices and developing heat-resistant tea cultivars capable of withstanding higher temperatures.

2 Overview of Tea Plant Biology and Chemistry

2.1 Key compounds affecting tea flavor

Tea flavor is primarily influenced by several key compounds, including catechins, theaflavins, amino acids, and essential oils. Catechins, a type of flavonoid, are significant contributors to the sensory qualities and health benefits of tea. They are responsible for the astringency and bitterness in tea, with epigallocatechin gallate (EGCG) being one of the most abundant and impactful catechins (Zhang et al., 2023). Theaflavins, formed during the oxidation of catechins, contribute to the color and briskness of black tea, enhancing its overall flavor profile (Hua et al., 2021). Amino acids, particularly theanine, impart a sweet and umami taste, balancing the bitterness of catechins and contributing to the overall flavor complexity (Zhang et al., 2017). Essential oils, although present in smaller quantities, provide the aromatic qualities that are crucial for the sensory experience of tea (Qu et al., 2020).

The biosynthesis of these compounds is tightly regulated by environmental factors and genetic expression. For instance, the expression of catechin biosynthesis-related genes is influenced by temperature changes, which in turn affects the catechin content in tea leaves (Xiang et al., 2020). Similarly, theanine biosynthesis is modulated by factors such as temperature and the presence of growth stimulatory molecules like melatonin, which can enhance the expression of theanine biosynthesis genes under moderate high temperatures. Understanding the interplay between these compounds and their biosynthesis pathways is essential for optimizing tea quality and flavor.

2.2 Environmental factors influencing tea quality

The quality and flavor of tea are significantly influenced by environmental factors such as temperature, humidity, and altitude. Optimal temperature is crucial for the growth and development of tea plants, as well as for the biosynthesis of key flavor compounds. High temperatures can negatively impact the flavonoid content, including catechins, by activating specific transcription factors that repress the jasmonate pathway, which is involved in catechin accumulation. Conversely, moderate high temperatures, when combined with exogenous melatonin, can enhance the biosynthesis of catechins and theanine, thereby improving tea quality (Li et al., 2020).

Humidity and altitude also play vital roles in determining tea quality. High humidity levels can promote the growth of tea plants and the accumulation of flavor compounds, while low humidity can lead to stress conditions that may adversely affect tea quality. Altitude influences the microclimate, including temperature and sunlight exposure, which in turn affects the photosynthesis and metabolic processes in tea plants. Higher altitudes are generally associated with cooler temperatures and increased sunlight, which can enhance the synthesis of catechins and other polyphenols, leading to a richer flavor profile (Gong et al., 2020). In Wufeng Tea Garden such as those in Cangnan County, Zhejiang Province, the region's elevation is mostly above 600 meters, featuring favorable terrain, fertile soil, abundant rainfall, and ample sunlight. These factors create a microclimate highly conducive to the growth of tea plants (Figure 1). This unique ecological environment not only supports healthy tea plant development but also enhances the accumulation of flavor compounds, endowing the tea with distinctive quality characteristics.

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).

Amino acids, particularly theanine, are also affected by elevated temperatures. Theanine, which contributes to the umami flavor of tea, decreases under moderately high temperatures due to the suppression of theanine biosynthetic genes (Li et al., 2018). This reduction in theanine content results in a less distinctive umami taste in tea harvested during warmer months. Furthermore, caffeine levels are influenced by temperature variations, with higher temperatures generally leading to increased caffeine concentrations. This is attributed to the upregulation of genes involved in caffeine biosynthesis, mediated by hormone signal transduction factors (Lin, 2023).

4.2 Impact on aroma profiles

The aroma profile of tea is significantly influenced by temperature, as it affects the production of essential oils and volatile compounds. High temperatures can lead to the degradation of certain volatiles that contribute to the pleasant aroma of tea, while increasing the concentration of others that may impart undesirable odors. For instance, heating green tea liquor results in the epimerization of catechins and a decrease in volatiles such as linalool oxide and β-ionone, which are associated with floral and fruity notes. Conversely, compounds like indole and α-terpineol, which have less pleasant odors, increase in concentration (Kim et al., 2007).

Moreover, the production of specific aroma compounds is closely linked to the biosynthesis of secondary metabolites, which are regulated by temperature-sensitive pathways. For example, the roasting of green tea at different temperatures can optimize the formation of desirable odorants while mitigating the degradation of catechins. Roasting at 160 °C for 30 minutes has been shown to produce a favorable aroma profile while minimizing the loss of catechins. Additionally, the fermentation temperature of black tea significantly affects its aroma, with lower fermentation temperatures (around 28 °C) resulting in higher sensory quality scores for aroma and taste (Qu et al., 2020).

4.3 Influence on nutritional value

The nutritional value of tea, particularly its antioxidant properties, is also affected by rising temperatures. Catechins, which are potent antioxidants, decrease in concentration with increasing temperatures, leading to a potential reduction in the health benefits associated with tea consumption. Studies have shown that higher temperatures result in the degradation of catechins, thereby diminishing the antioxidant capacity of tea (Mizukami et al., 2008). This reduction in catechin levels can negatively impact the tea's ability to scavenge free radicals and protect against oxidative stress.

In addition to catechins, other bioactive compounds such as polyphenols and amino acids are influenced by temperature variations. For instance, moderate high temperatures can increase the total polyphenol concentration in tea, which may enhance its antioxidant properties. However, this is often accompanied by a decrease in free amino acids, altering the overall nutritional balance of the tea (Li et al., 2020). Furthermore, the fermentation temperature of black tea affects its bioactivity, with lower temperatures (around 28 °C) enhancing its antioxidant activities and inhibitory effects on enzymes related to glucose metabolism (Qu et al., 2020).

5 Impact on Sensory Attributes

5.1 Changes in taste profiles

Rising temperatures significantly influence the taste profiles of tea, particularly affecting bitterness, astringency, and sweetness. Bitterness and astringency are primarily attributed to phenolic compounds, caffeine, and tannins, which can be modified by temperature and brewing time. For instance, a study on coffee leaves tea demonstrated that higher brewing temperatures (91 °C-99 °C) and longer brewing times (3-5 minutes) enhanced the bitterness and astringency, which were well accepted by consumers when brewed at 95 °C for 5 minutes (Fibrianto et al., 2021). Similarly, in green tea, catechins such as (-)-epigallocatechin gallate and (-)-epicatechin gallate are major contributors to bitterness, while flavonol glycosides contribute to astringency. These compounds' concentrations and their sensory impacts are highly temperature-dependent (Xu et al., 2018).

Moreover, the sweetness of tea can also be influenced by temperature. The addition of aroma compounds like geraniol and β-ionone has been shown to enhance the sweetness of black tea infusions, indicating an odor-taste interaction that can be temperature-sensitive (Yu et al., 2021). The chemical composition of tea, including the balance of bitter, astringent, and sweet compounds, is thus intricately linked to the brewing temperature, which can either enhance or diminish these sensory attributes.

5.2 Alterations in aroma

The aroma of tea is another critical sensory attribute that is profoundly affected by temperature. Different steeping temperatures can lead to significant variations in the aroma profile of tea. For example, black tea steeped at 95 °C was found to have a more pleasant aroma with mild green, roast, and fruity notes, while steeping at 80 °C resulted in a sweeter fragrance with floral characteristics. Lower temperatures (60 °C and 70 °C) reinforced woody and fatty aromas (Wang et al., 2019). This variation is due to the differential extraction of volatile compounds at various temperatures, which include alcohols, aldehydes, hydrocarbons, and other aroma-contributing substances.

Roasting processes also play a crucial role in shaping the aroma profile of tea. For instance, heavy roasting of green tea significantly decreases catechins and flavonol glycosides while increasing aldehydes, ketones, furans, and pyrroles/pyrazines, leading to distinct nutty, roasty, and burnt odors (Zhu et al., 2021). Similarly, the roasting degree of large-leaf yellow tea (LYT) affects its flavor quality, with old fire roasting producing strong roasted, nutty, and woody odors, and retaining high levels of volatiles and heterocyclic compounds. These findings highlight the complex interplay between temperature, chemical composition, and aroma in tea.

5.3 Overall sensory experience

The overall sensory experience of tea, encompassing both taste and aroma, is intricately linked to the temperature at which the tea is brewed or processed. Higher temperatures tend to enhance the extraction of both non-volatile and volatile compounds, leading to a more intense and complex sensory profile. For instance, the sensory profiles of black tea infusions are significantly influenced by brewing conditions, with higher temperatures leading to greater variations in non-volatile compounds and a more pronounced impact on volatile compounds (Yu et al., 2021). This results in a richer and more diverse sensory experience, combining enhanced bitterness, astringency, and sweetness with a complex aroma profile.

Furthermore, the interaction between taste and aroma compounds can significantly alter the overall sensory perception of tea. The presence of specific aroma compounds, such as geraniol and β-ionone, can enhance the sweetness of tea, demonstrating the importance of odor-taste interactions. Additionally, the roasting process, which involves high temperatures, can create unique flavor profiles by altering the chemical composition of the tea, leading to distinct sensory attributes such as nutty, roasty, and floral notes (Guo et al., 2021). These findings underscore the critical role of temperature in shaping the comprehensive sensory experience of tea, making it a key factor in tea quality and consumer preference.

6 Regional Case Study: Effect of Rising Temperatures on “Anji Baicha” Quality in Zhejiang Province

6.1 Background on “Anji Baicha”

“Anji Baicha”, a renowned variety from Zhejiang Province, is celebrated for its unique flavor profile and delicate aroma. This tea is derived from the Camellia sinensis plant, specifically from a rare cultivar known as Baiye No. 1. The tea leaves are characterized by their pale, almost white appearance, which is a result of the specific growing conditions and processing methods used. “Anji Baicha” is typically harvested in early spring, when the leaves are tender and rich in amino acids, contributing to its sweet and umami flavor. The tea is highly prized not only for its taste but also for its health benefits, which include high levels of antioxidants and other beneficial compounds (Lou et al., 2020).

The unique climatic conditions of Zhejiang Province, with its cool, misty environment, have historically been ideal for cultivating “Anji Baicha”. The region's microclimate, characterized by moderate temperatures and ample rainfall, has allowed the tea to develop its distinctive qualities. However, recent changes in climate patterns pose a significant threat to the quality and consistency of “Anji Baicha”, necessitating a closer examination of these impacts (Lou et al., 2020; Yan et al., 2021).

6.2 Temperature trends in Zhejiang

Zhejiang Province has experienced notable shifts in temperature patterns over recent decades. Data indicates a trend of rising temperatures, with both average and extreme temperatures increasing. This trend is consistent with broader patterns observed across China, where climate change has led to significant alterations in weather conditions. Specifically, Zhejiang has seen an increase in the frequency and intensity of heatwaves, as well as changes in seasonal temperature distributions (Lou et al., 2020; Yan et al., 2021).

The impact of these temperature changes on tea production is profound. Higher temperatures, particularly during the critical growing and harvesting periods, can stress tea plants, affecting their growth and the quality of the leaves produced. Studies have shown that elevated temperatures can lead to earlier bud break and leaf plucking periods, which can disrupt traditional harvesting schedules and reduce the quality of the tea leaves. Additionally, increased temperatures can exacerbate drought conditions, further stressing the plants and impacting yield and quality (Lou et al., 2020; Yan et al., 2021).

6.3 Observed changes in “Anji Baicha” quality

The rising temperatures have had observable effects on the quality of “Anji Baicha”. One of the most significant impacts is on the chemical composition of the tea leaves. Higher temperatures have been associated with changes in the concentration of key metabolites that contribute to the tea’s flavor and aroma (Guo et al., 2024). For instance, elevated temperatures can reduce the levels of amino acids and increase the concentration of catechins, leading to a more astringent taste and a loss of the characteristic sweetness and umami flavor of “Anji Baicha” (Kfoury et al., 2019; Lou et al., 2020).

Li et al. (2015) applied gas chromatography-mass spectrometry combined with multivariate analysis to analyze the metabolite profiles in the different color stages during the development of “Anji Baicha” leaves. The metabolite profiles of three albescent stages, including the yellow-green stage, the early albescent stage, and the late albescent stage, as well as the re-greening stage were distinguished using principal component analysis, revealing that the distinct developmental stages were likely responsible for the observed metabolic differences (Figure 2).

Moreover, the market value of “Anji Baicha” has been affected by these quality changes. As the tea's flavor profile shifts, consumer preferences may also change, potentially leading to decreased demand and lower prices. The economic implications for tea growers in Zhejiang are significant, as they rely heavily on the premium prices that high-quality “Anji Baicha” commands. The documented changes in tea quality underscore the need for adaptive strategies to mitigate the impacts of rising temperatures (Kfoury et al., 2019; Lou et al., 2020).

6.4 Adaptation strategies

In response to the challenges posed by rising temperatures, “Anji Baicha” growers in Zhejiang have begun to implement various adaptation strategies. One common approach is the use of shading techniques to protect tea plants from excessive heat and sunlight. By providing shade, growers can help maintain cooler microclimates around the tea plants, reducing heat stress and preserving the quality of the leaves. Additionally, altering harvest times to earlier in the morning or late in the evening can help avoid the hottest parts of the day, thereby minimizing heat exposure during critical periods (Lou et al., 2020; Yan et al., 2021).

Another strategy involves the selection and breeding of more heat-tolerant tea cultivars. Research and development efforts are focused on identifying and cultivating tea varieties that can better withstand higher temperatures without compromising quality (Driedonks et al., 2016; Seth et al., 2021; Huang and Chen, 2024). Furthermore, improved irrigation practices and soil management techniques are being employed to enhance the resilience of tea plants to drought conditions. These adaptive measures are crucial for sustaining the production and quality of “Anji Baicha” in the face of ongoing climate change (Lou et al., 2020; Yan et al., 2021).

7 Adaptation Strategies and Management Practices

7.1 Agronomic practices

Agronomic practices such as altered planting times, shade-growing, and irrigation management are vital in mitigating the impact of rising temperatures on tea quality. Adjusting planting times to avoid peak heat periods can help reduce the exposure of tea plants to extreme temperatures, thereby minimizing heat stress (Khan et al, 2019). Shade-growing is another effective technique that can protect tea plants from direct sunlight and reduce leaf temperature, which in turn helps maintain higher chlorophyll content and better overall plant health.

Irrigation management is also crucial in combating the effects of high temperatures. Ensuring adequate water supply can help maintain leaf water content and reduce the negative impact of heat stress on tea plants (Minoli et al., 2019). Additionally, the use of growth regulators and fertilizers can induce acclimation and enhance the thermotolerance of tea plants. For example, brassinosteroids have been shown to attenuate the decline in tea quality caused by moderately high temperatures by enhancing theanine biosynthesis, which is essential for the unique umami flavor of green tea (Li et al., 2018). These agronomic practices, when combined, can significantly improve the resilience of tea plants to rising temperatures.

7.2 Technological innovations

Technological innovations in climate-smart agriculture play a crucial role in mitigating the impact of rising temperatures on tea quality and flavor. The use of advanced genomic, proteomic, and metabolomic approaches has provided valuable insights into the mechanisms of heat tolerance in tea plants. For instance, transcriptomic and metabolomic profiling of tea plants exposed to temperature stresses has revealed modifications in protein synthesis, photosynthetic pathways, and anthocyanin biosynthesis, which are essential for plant adaptation to heat stress (Shen et al., 2019).

Furthermore, the development of climate-resilient tea cultivars through genetic engineering and molecular breeding holds great potential. By identifying and manipulating key genes involved in heat tolerance, scientists can create tea varieties that are better equipped to withstand high temperatures (Bhardwaj et al., 2021). Additionally, the use of climate-smart technologies such as precision irrigation systems and temperature monitoring tools can help optimize water use and protect tea plants from heat stress. These technological innovations, combined with traditional agronomic practices, can significantly enhance the resilience of tea plants to rising temperatures and ensure the production of high-quality tea.

8 Future Outlook and Predictions

Climate change is expected to significantly impact tea cultivation due to projected temperature increases and associated climatic shifts. Studies indicate that rising temperatures will alter the biochemical composition of tea leaves, affecting both yield and quality. For instance, elevated temperatures can reduce the concentration of theanine, a key amino acid responsible for the umami flavor of tea, as shown by the suppression of theanine biosynthetic genes under sub-high temperature conditions (Li et al., 2018). Additionally, extreme temperature events, both hot and cold, have been linked to significant reductions in tea yields, with heat extremes posing a particularly severe threat in regions like southern China (Yan et al., 2021). The variability in temperature and precipitation patterns is also expected to affect the distribution of secondary metabolites, which are crucial for tea quality. 

As tea quality deteriorates, consumer preferences may shift, potentially leading to changes in pricing and trade dynamics. For example, a decrease in the concentration of key secondary metabolites such as catechins and methylxanthines during periods of high precipitation has been observed, which negatively impacts the sensory characteristics and market value of tea (Buckley et al., 2014). Furthermore, the variability in tea quality due to climatic factors could lead to inconsistent product offerings, affecting consumer trust and market stability. The economic impact is also significant, as reduced tea quality can lead to lower household incomes for tea farmers, particularly in regions heavily dependent on tea cultivation. 

To address the challenges posed by climate change on tea cultivation and quality, several research gaps need to be filled. There is a critical need for more comprehensive studies on the direct effects of carbon dioxide on tea quality, as current evidence is limited. Additionally, research should focus on the interaction between multiple environmental factors and their combined impact on tea plants, which would provide a more holistic understanding of real-world conditions (Jayasinghe and Kumar, 2021). The development of ensemble modeling approaches to predict climate suitability for tea cultivation is also essential for future planning and adaptation. Moreover, there is a need for evidence-based management strategies and crop breeding programs aimed at developing resilient tea cultivars that can withstand climatic stresses (Ahmed et al., 2019). Meanwhile, long-term, multi-year studies are necessary to capture the year-to-year variation in tea chemistry and better predict future trends (Kfoury et al., 2019). Addressing these research needs will be crucial for ensuring the sustainability and resilience of tea production in the face of climate change.

Acknowledgments

The author expresses deep gratitude to Professor R. Cai from the Zhejiang Agronomist College for his thorough review of the manuscript and constructive suggestions. The author also extends thanks to the two anonymous peer reviewers for their valuable revision recommendations.

Conflict of Interest Disclosure

The author affirms that this research was conducted without any commercial or financial relationships that could be construed as a potential conflict of interest.

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