Abstract

   Tea quality in greenhouse was certain gap with open air. Metabolites
   and foliar microorganisms were investigated under seaweed fertiliser
   (CF) and gibberellin (CH) treatments using sensory evaluation, HPLC,
   untargeted metabolomics, 16S rDNA, and Internal Transcribed Spacer. CF
   tea was mellow, less astringent, and of better quality compared to CH.
   Catechin, −(−)Epicatechin, and Epigallocatechin were notably lower in
   CF. Differentially accumulated metabolites (DAMs) were notably enriched
   in Flavonoid and Phenylpropanoid biosynthesis, both involved in
   Catechin synthesis. DAMs in these pathways appeared down-regulated in
   CF. The CF improved quality by down-regulating metabolites in
   Phenylpropanoid biosynthesis in conjunction with microbial community
   metabolism enriched in amino acid and secondary metabolite
   biosynthesis. Metabolite- microbial correlation analysis indicated that
   the highest correlation with phenylpropane pathway metabolites was in
   bacteria Variovorax and Pseudomonas, and in fungi Filobasidium. The
   study provides theoretical basis for regulating flavour quality of
   greenhouse tea.

   Keywords: Quality, Greenhouse tea, Metabolomics, Microorganisms

Graphical abstract

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Highlights

     * •
       CCF exhibited umami, mellow and fragrant and sweet flavour, while
       CH showed slightly astringent, mellow and thick
     * •
       Catechin, −(−)Epicatechin, and Epigallocatechin flavour metabolites
       were notably lower in CF
     * •
       Phenylpropanoid biosynthesis was identified as a key pathway
       driving the flavour differences
     * •
       Strong correlations were observed between signature microorganisms
       and flavour substances
     * •
       A greenhouse tea quality improvement measure was proposed

Chemical compounds studied in the article

   Catechin (PubChem CID 9064)

   (−)-Epicatechin (PubChem CID 72276)

   Epigallocatechin (PubChem CID 72277)

   Gallocatechin (PubChem CID 65084)

   (−)-Catechin gallate (PubChem CID 6419835)

   (−)-Epicatechin gallate (PubChem CID 107905)

   (−)-Epigallocatechin gallate (PubChem CID 65064)

   Gallocatechin gallate (PubChem CID 199472)

   Caffeine (PubChem CID 2519)

   L-Theanine (PubChem CID 439378).

1. Introduction

   Tea plants are normally cultivated in tropical and subtropical regions
   ([33]Wang et al., 2020) and are susceptible to low temperatures in mid-
   to high-latitudes regions and at high altitudes ([34]Tong et al.,
   2024). Winter freezes or spring frosts can cause massive deaths of tea
   plants ([35]Di et al., 2024). Some regions in northern China are still
   subject to spring cold affecting tea plants even into March ([36]Hao et
   al., 2018). In order to safely survive the winter and prevent spring
   cold, plastic greenhouse facilities are used to artificially control
   environmental factors such as temperature and humidity, promoting tea
   plants growth or breaking dormancy to allow for earlier harvest. Inside
   the greenhouse, temperatures are warmer, and the humidity is higher
   ([37]Li et al., 2016), leading to faster leaf development, with
   elongated buds and thinner leaves. However, the accumulation of key tea
   compounds, particularly quality-related metabolites, may be limited.
   After brewing, the fresh flavour remains fine, but the taste is bland,
   and the aroma is common ([38]Ma et al., 2024). Appearance of both the
   dry tea and the leaf bottom is clearly distinct from the of tea grown
   in the open air.

   In traditional tea garden management, gibberellin (GA) significantly
   promotes the spring sprouting of overwintering tea plants by activating
   the phenylpropanoid biosynthesis pathway ([39]Di et al., 2019),
   accelerating internode elongation and leaf expansion ([40]Yue et al.,
   2018). However, the increase in yield may lead to an imbalance in
   quality, prompting researchers to explore more comprehensive tea garden
   management strategies. Meanwhile, the accumulation of various seaweeds
   along China's coastline is often treated as waste ([41]Civelek Yoruklu
   et al., 2022). These seaweeds are rich in organic compounds, serving as
   a natural biological resource for organic fertilizers ([42]Gibilisco et
   al., 2022). Foliar application of seaweed extracts has been shown to
   enhance crop quality and growth ([43]Shang et al., 2024), yet their
   potential in tea cultivation remains underexplored.

   High quality tea is highly regarded for its smooth taste, low
   bitterness and astringency, and rich umami flavour. However, the bitter
   and astringent notes of tea remain unappealing to many consumers. For
   instance, flavonoid glycosides and catechin compounds are the primary
   contributors to bitterness and astringency ([44]Ye et al., 2022).
   Flavour, as a key indicator of tea quality ([45]Zhang et al., 2020), is
   largely determined by the composition and content of non-volatile
   compounds ([46]Wang et al., 2022). These compounds not only shape the
   unique flavour profile of tea but also influence its leaf morphology,
   colour, aroma, and functional properties ([47]Wang et al., 2021). In
   addition, microorganisms play a crucial role in the formation of tea
   flavour ([48]Han et al., 2024). The involvement of microbes can impart
   characteristic flavours to tea ([49]Tian et al., 2024). For example,
   the collaboration between bacteria and fungi promotes dynamic changes
   in flavonoid compounds and amino acids ([50]Zhang et al., 2024), and
   the enzymes they secrete can catalyze the metabolic transformation of
   important secondary metabolites. In different types of tea, microbial
   activities exhibit diverse characteristics. In the production of Fu
   brick tea, core functional microorganisms significantly reduce the
   content of catechins and flavonoid compounds, thereby lowering
   bitterness and astringency while enhancing mellowness ([51]Li et al.,
   2021a). In Wuyi rock tea, the bacterium Chryseobacterium influences the
   production of catechin compounds, shaping the distinctive rock flavour
   ([52]Wu et al., 2024). In Liubao tea, Sphingomonas improves flavour by
   modulating the biosynthesis of flavonoids and flavonols ([53]Liu et
   al., 2024). Furthermore, in Shui Xian and Pu’er teas, microorganisms
   regulate metabolic products to create unique flavour profiles ([54]He
   et al., 2023; [55]Yuan et al., 2024). Therefore, it is essential to
   study the impact of microorganisms on the quality of greenhouse tea.

   To mitigate quality deficiencies in greenhouse tea due to faster
   growth. In this study, sensory evaluation, HPLC, metabolites with 16S
   and Internal Transcribed Spacer (ITS) techniques were used to study the
   gap between microbiome and metabolites due to seaweed fertiliser and
   gibberellin treatments and explored the best model suitable for quality
   improvement of greenhouse tea plants. Changes, effects, and
   interactions patterns of tea leaves microbes and metabolites were
   analysed. The key metabolites and core microorganisms responsible for
   improving tea quality were identified, and their functions were fully
   explored, with the goal of providing theoretical references for