The experiment was conducted at a tea plantation base located in Zhongdong Village, Weijiang Township, Longsheng County, Guilin City, Guangxi, China (110°4′10″E, 26°0′2″N; altitude: 805.2m). The region has a subtropical humid monsoon climate, characterized by an average annual temperature of 18.2 °C, average relative humidity of 80%, average annual precipitation of 1,544 mm, average annual sunshine duration of 1,247 hours, and an average frost-free period of approximately 314 days. The physicochemical properties of the tea plantation soil were as follows: total nitrogen, 2.78 g/kg; total phosphorus, 1.37 g/kg; total potassium, 24.53 g/kg; alkali-hydrolyzable nitrogen, 236.3 mg/kg; available phosphorus, 288.1 mg/kg; available potassium, 119.0 mg/kg; organic matter, 57.04 g/kg; and pH, 4.36.

The tea cultivar used in this study was Fuyun No. 6, a six-year-old plant. Fuyun No. 6 is a first-generation clonal variety developed by the Tea Research Institute, Fujian Academy of Agricultural Sciences. It was bred via interspecific hybridization, using the Yunnan large-leaf variety as the male parent and Fusheng Dabai tea as the female parent. This cultivar is known for its high yield and early budding. Tea was planted in double rows on terraced fields, with a row spacing of 3m. The S. grosvenorii (luohan guo) variety was Dadi No. 2, propagated via tissue culture seedlings, while the passion fruit (P. edulis) variety used was Tainong No. 1, established from cutting seedlings.

A randomized block design was employed, comprising seven treatments, including a control treatment of monoculture tea plantation without intercropping. Three intercropping densities, low, medium, and high, were established for both S. grosvenorii and passion fruit within the tea plantation, as detailed in Table 1 . Each treatment plot measured 80 m² and was replicated three times.

The intercropping of S. grosvenorii and passion fruit within the tea plantation employed a box-frame root restriction method combined with a flat trellis system. For root restriction, devices measuring 30cm in height and 80cm in width were installed along the inner side of each tea plantation terrace row, according to the planting spacing requirements of each treatment. A cultivation substrate was prepared by mixing orchard soil collected from surrounding orchards with a nutrient substrate at a 6:4 volume ratio. This mixture was filled to two-thirds of the root restriction device volume, into which a single seedling was centrally planted. Subsequently, the device was filled with additional substrate, compacted, and thoroughly watered to ensure adequate moisture for seedling establishment. The flat trellis system consisted of 1.8m high structures covered with plastic mesh grids measuring 15cm × 15cm to support plant climbing and fruit load ( Figure 1 ).

The two climbing plants were initially grown in a greenhouse for approximately 40 days and reached a height of around 50cm. Then they were transplanted on May 21, 2024. About one month after transplantation, the plants were able to provide sufficient shade. Pest and disease management utilized biological pesticides, and all other cultural practices followed conventional standards.

Fertilization and management of tea plants were conducted following standard conventional practices.

After 3 months, on August 14, 2024, fresh tea samples consisting of one bud and two leaves were collected. Chlorophyll content in the tea leaves was measured following the NY/T 3082–2017 standard, “Determination of Chlorophyll Content in Fruits, Vegetables and Their Products - Spectrophotometric Method”. Tea polyphenol content was determined according to GB/T 8313-2018, “Determination of Tea Polyphenols and Catechins in Te”. Free amino acid content was assessed using the GB/T 8314–2013 standard, “Determination of Total Free Amino Acids in Tea”. Water extract content was measured in accordance with GB/T 8305-2013, “Determination of Water Extract in Tea”.

After 6 months, on November 21, 2024, fruits of S. grosvenorii and passion fruit were harvested. For each treatment replicate, fruits were collected separately, and 20 mature fruits were randomly selected from each plot for quality assessment.

Quality indicators for S. grosvenorii fruit included single fruit weight, transverse diameter, reducing sugar content, total sugar content, and mogroside V content. Single fruit weight was measured using a 1% precision electronic balance, and transverse diameter was measured with a digital caliper. Fruits were categorized by transverse diameter as follows: extra-large (>57mm), large (52–56 mm), medium (48–51 mm), small (44–47 mm), and below standard (<43mm). The proportion of fruits classified as medium or larger was calculated. Reducing sugar, total sugar, and mogroside V contents were determined according to GB 5009.7-2016 “National Food Safety Standard - Determination of Reducing Sugar in Food” (Method 1), SB/T 10203-1994 “General Test Methods for Fruit Juice”, and high-performance liquid chromatography, respectively.

Quality indicators for passion fruit included single fruit weight, peel weight, longitudinal diameter, transverse diameter, peel thickness, soluble solids content, soluble sugar content, and titratable acid content. Single fruit weight and peel weight were measured using a 1% precision electronic balance, while longitudinal diameter, transverse diameter, and peel thickness were measured with a digital caliper. Edible rate and fruit shape index were calculated using the following formulas:

Soluble solids, soluble sugar, and titratable acid contents were determined according to NY/T 2637-2014 “Determination of Soluble Solids Content in Fruits and Vegetables”, NY/T 2742-2015 “Determination of Soluble Sugar in Fruits and Their Products - 3,5-Dinitrosalicylic Acid Colorimetric Method”, and GB 12456-2021 “National Food Safety Standard - Determination of Total Acid in Food” (Method 1), respectively.

The solid-acid ratio and sugar-acid ratio were calculated as follows:

Statistical analyses were carried out utilizing Microsoft Excel 2017 in conjunction with SPSS version 19.0. For comparative assessments, one-way analysis of variance (ANOVA) was implemented, followed by Duncan’s multiple range test. All experiments were conducted in triplicate, and the outcomes were presented as mean values accompanied by the standard error. Statistical significance was determined at P < 0.05.

The single fruit weight ( Figure 4A ) and fruit transverse diameter ( Figure 4B ) were significantly greater in the SG-H treatment (94.37 g and 54.56 mm, respectively) compared with SG-L and SG-M. The proportion of medium and large fruits ( Figure 4C ) was also highest in SG-H (94.58), followed by SG-M (90.32%) and SG-L (76.71%) ( Supplementary Table S3 ).

In terms of fruit nutritional quality, both reducing sugar ( Figure 4D ) and total sugar content ( Figure 4E ) increased with intensity. SG-H yielded the highest values (6.69% and 12.01%, respectively), which were significantly higher than SG-L and SG-M. No significant differences in flavone glycoside V content ( Figure 4F ) were detected among the treatments ( Supplementary Table S3 ).

Overall, SG-H produced the most pronounced improvements in fruit weight, size, proportion of medium and large fruits, and sugar content, indicating a beneficial effect of higher intercropping intensity on S. grosvenorii fruit quality.

A key finding of this study is the significant enhancement in tea leaf chlorophyll content under all intercropping treatments, with the most pronounced increases observed at high planting densities of both S. grosvenorii and P. edulis. Elevated chlorophyll content is typically associated with improved photosynthetic capacity and leaf vitality, which may contribute to higher tea yield and quality (Liu et al., 2019; Ali et al., 2024). The greater increase in chlorophyll b relative to chlorophyll a, reflected by a decreased chlorophyll a/b ratio, suggests that intercropping, particularly at higher densities, may induce adaptive modifications in the photosynthetic apparatus, potentially in response to altered light environments created by the climbing canopy (Brooker et al., 2015). Such shifts in pigment composition are consistent with previously reported shade-adaptation strategies of tea plants intercropped with taller shade trees (Qiao et al., 2019).

In addition to chlorophyll content, intercropping treatments led to significant changes in key biochemical constituents of tea leaves. Polyphenol content, a major determinant of tea astringency and antioxidant activity (Zhao et al., 2022), was significantly reduced across all intercropping treatments, with the most substantial decrease observed at higher planting densities. Notably, this reduction was more pronounced in the P. edulis intercropping system. In contrast, free amino acid content, which imparts umami flavor (Tang et al., 2019) and is positively correlated with tea quality, was significantly increased under low- and medium-density intercropping, particularly in the S. grosvenorii system, but declined under high-density conditions. The water extract content, a comprehensive indicator of soluble substances contributing to tea flavor and mouthfeel (Chen et al., 2023), was also significantly elevated in all intercropping treatments.

The polyphenol-to-amino acid ratio, a widely recognized indicator of tea quality, with lower values reflecting reduced astringency and improved taste (Chen et al., 2023), was decreased in all intercropping treatments, especially at low to medium densities. These findings suggest that intercropping, particularly when planting densities are suitable, can improve the sensory attributes of tea by enhancing umami and sweetness while reducing bitterness and astringency.

The mechanisms underlying these changes are likely multifactorial. First, the climbing canopy may help establish a more favorable microclimate (e.g., lower temperature, reduced light intensity, and increased humidity) beneath the fruit trees, conditions that promote amino acid accumulation and suppress polyphenol synthesis in tea leaves (Brooker et al., 2015). Second, intercropping can improve soil fertility through increased organic matter inputs and enhanced nutrient cycling, thereby supporting the biosynthesis of nitrogenous compounds such as amino acids (Brooker et al., 2015; Huang et al., 2022). Third, competition for soil nutrients and light at higher planting densities may impose mild abiotic stress on tea plants, potentially downregulating polyphenol biosynthetic pathways. The stronger effects observed in the P. edulis system may be attributed to its denser foliage and greater shading capacity compared to S. grosvenorii.

However, the density-dependent trends observed, particularly the decline in amino acid content at excessively high intercropping densities, underscore the importance of optimizing planting arrangements to maximize benefits to tea quality.

A novel aspect of this study is the reciprocal assessment of how intercropping with tea influences the quality of the companion climbing fruit quality. For S. grosvenorii, high-density intercropping (SG-H) significantly increased single fruit weight, fruit size, marketable fruit proportion, and sugar content, without negatively affecting the content of flavone glycoside V. These enhancements in fruit quality may be attributed to the more favorable microclimate and improved soil conditions created by the tea canopy, which can mitigate heat stress, conserve soil moisture, and enhance nutrient availability (Bai et al., 2022; Duan et al., 2024). Increased sugar accumulation under shaded or moderately stressed environments has been documented in other fruit crops and is often associated with altered carbohydrate metabolism and reduced photorespiration (Guo et al., 2024a, b; Wu et al., 2022).

For P. edulis, the intercropping effects were more nuanced. While no significant differences were detected in basic physical traits such as fruit weight and diameter across treatments, the highest levels of soluble solids and sugar content, as well as the most favorable solid-acid and sugar-acid ratios (key determinants of fruit flavor), were observed in the high-density intercropping treatment (PE-H). These findings suggest that, similar to S. grosvenorii, moderate shading in tea intercropping systems may promote sugar accumulation and enhance flavor attributes. However, certain morphological traits such as edible ratio and fruit shape index were maximized at low or medium planting densities, indicating that excessively high intercropping density may adversely affect fruit morphology, likely due to intensified competition for light and soil nutrients (Brooker et al., 2015).

Interestingly, titratable acidity was highest in the medium-density treatment, suggesting that fruit acidity may respond to intercropping-induced microclimatic and physiological changes differently from sugar content. This result highlights the complexity of fruit quality responses under intercropping systems and underscores the need to balance planting density to improve both yield and quality traits in companion fruit crops.

The findings of this study offer valuable insights for the development of multifunctional and sustainable tea-based agroecosystems. First, the observed enhancements in tea quality, including elevated chlorophyll and free amino acid contents, reduced polyphenol-to-amino acid ratios, and increased water extract levels, indicate that intercropping with climbing fruit species can support the production of high-quality tea with improved sensory and health-related attributes. Second, the notable improvements in fruit quality and marketability of both S. grosvenorii and P. edulis provide economic incentives for growers by diversifying income sources and enhancing agroecosystem resilience.

Ecologically, such intercropping systems promote greater plant diversity, improve resource use efficiency, and reduce reliance on external inputs such as synthetic fertilizers and pesticides, thereby contributing to environmental sustainability. The use of climbing fruit species is particularly advantageous, as vertical stratification enables efficient spatial utilization without direct competition for ground-level resources with tea plants.

Nevertheless, the density-dependent responses observed in both tea and fruit crops underscore the importance of site-specific optimization of intercropping configurations. Excessively high planting densities may result in excessive shading and resource competition, offsetting the benefits to tea quality and certain fruit characteristics. Conversely, low densities may fail to fully capitalize on the advantages of intercropping. Future research should investigate the long-term performance of these systems, including their effects on soil health, pest and disease dynamics, and overall system productivity.