Tea plant shows a preferential uptake of N in the form of ammonium (NH₄ ⁺-N). Using the scanning ion-selective electrode technique, Ruan et al. (2016) found that the NH₄ ⁺ influx rate in the roots of tea plant was higher than that of NO₃ ⁻, and the presence of NH₄ ⁺ would promote NO₃ ⁻ influx rate. The yield of young shoots, total root length, N uptake rate, and the contents of caffeine, theanine (Thea), glutamine (Gln), glutamate (Glu) and aspartate (Asp) in tea leaves were significantly higher when NH₄ ⁺ was the main N source, compared with NO₃ ⁻ (Ruan et al., 2007; Ruan et al., 2010; Ruan et al., 2019; Wang et al., 2022a). NH₄ ⁺-N can also promote phosphorus (P) uptake (Chen et al., 2019) and increase iron (Fe) and chloride (Cl) contents in mature leaves and sulfur (S) content in the roots (Tang et al., 2019). As a signaling molecule, NH₄ ⁺ could induce Thea and catechin biosynthesis in a short period (Liu et al., 2017b; Huang et al., 2018). By sensing NH₄ ⁺ levels, lysine-acetylated and crotonylated proteins profoundly influenced some primary metabolic processes involved in amino acid metabolism, photosynthesis, glycolysis, and carbon fixation (Jiang et al., 2018; Sun et al., 2019).

To obtain an appropriate amount of harvestable product, i.e., young buds and leaves, multiple tender shoots are picked from the plants every year. Adequate N nutrition is necessary to increase the formation of young shoots, enhance the growth vigor and maintain the C/N balance (Ruan et al., 2010). A balanced C/N ratio is also important to promote chlorophyll biosynthesis (Yang, 2011) and to ensure adequate availability of free AAs (Liu et al., 2020; Wang et al., 2022a), thus providing N reserve for reproductive growth (Fan et al., 2019).

N metabolism of tea plant is dynamically regulated by environmental factors. Likewise, the growth of lateral roots was regulated by N levels: their length and numbers decreased with increasing N concentrations (Chen et al., 2023; Hu et al., 2023). Under N deficiency, the content of N, L-Thea, and chlorophyll decreased significantly. The activity of many antioxidant enzymes and leaf CO₂ assimilation capacity also diminished (Lin et al., 2016; Lin et al., 2019). However, low N levels positively regulated the expression of phosphate transporter genes and promoted flavonoids and polyphenols synthesis in tea leaves (Lin et al., 2023b). Appropriate N supply contributes to the aroma and flavor quality of tea infusion. The activity of the rate-limiting enzyme for N assimilation, GS, increased with N application level, and the content of total AAs, alcohols, and ketone compounds conferring aroma also increased, thus promoting tea products’ integrated quality (Ruan et al., 2010; Deng et al., 2012; Liu et al., 2021b).The accumulation of caffeine, a component of the bitter taste and a central nervous system stimulant in tea, can be increased with the increasing N supply (Ruan et al., 2010). Sufficient N also promotes flavonol glycoside biosynthesis through the expression of relevant genes and the accumulation of the corresponding substrate carbohydrates (Dong et al., 2019). Lipidomic studies revealed that the content of precursors for the formation of aroma-related substances such as monogalactosyl diaclyglycerol (36:6 MGDG) and digalactosyl diacylglycerol (36:6 DGDG) increased when the N fertilizer was applied at adequate amounts, while an excessive N application led to overaccumulation of hexenol and hexenal, compounds which cause an unpleasant grassy smell in tea (Liu et al., 2017a). With the increase in N supply, more C was allocated to N-containing compounds in mature tea leaves and roots, leading to a decrease in flavonoid concentration in the young shoots (Liu et al., 2021a). Long-term N overfertilization reduced significantly benzyl alcohol and 2-phenylethanol contents in tea leaves, as well as those of (E)-nerolidol and indoles in withering leaves, becoming not conducive to the generation of floral and fruity fragrances (Chen et al., 2021).

The membrane-localized ammonium transporter/methylammonium permease (AMT/MEP) facilitates the import and export of NH₄ ⁺ (Howitt and Udvardi, 2000). In higher plants, AMT proteins can be divided into two types: AMT1 and AMT2. Most AMT1 proteins belong to the HATS group and are synergically involved in NH₄ ⁺ transport through the apoplastic and the symplastic routes (Yuan et al., 2007). AMT2 plays a role in the translocation of NH₄ ⁺ from roots to shoots (Giehl et al., 2017). The transcription of the gene encoding this protein is tightly controlled through multiple factors, including external N level, circadian rhythm, hormone contents, and mycorrhizal symbiosis (Couturier et al., 2007; Kobae et al., 2010; Li et al., 2012; Li et al., 2016).

To cope with elevated NH₄ ⁺ concentrations, the AMT activity may be post-translationally modified via the reversible phosphorylation of the cytosolic C-terminal region, thus allowing rapid adaptation to variable environmental conditions (Yuan et al., 2013; Wu et al., 2019). In tea plants, CsAMTs expression seems to be tissue-specific: CsAMT1.2 reached the highest transcript abundance in roots, while CsAMT1.4 was mainly expressed in flower buds. However, CsAMT1.1 and CsAMT3.1 were highly expressed in all tissues, suggesting that these genes might have diverse functions in NH₄ ⁺ transport (Zhang et al., 2018; Wang et al., 2022c; Zhang et al., 2022a). Likewise, AMTs expression levels are responsive to changes in NH₄ ⁺ availability. In roots, CsAMT1.1 expression peaked at 12 h after the exogenous NH₄ ⁺ resupply, while CsAMT3.1 showed an upward trend after 24 h and CsAMT1.2 expression level increased at 10 h, with a 2.5-fold change compared to 0 h, and then decreased again by 24 h. In NH₄ ⁺-treated leaves, CsAMT1.1 expression was up-regulated only after 4 h, exhibiting a 4.75-fold increase, whereas CsAMT1.2 and CsAMT3.1 expression levels did not change until 24 h later. These data indicate that NH₄ ⁺ transport in tea roots is mainly regulated by CsAMT1.2, while in leaves, the NH₄ ⁺ induction is mainly controlled by CsAMT1.1 in the short term (Tang et al., 2020). Across different experimental NH₄ ⁺ concentrations, most CsAMTs were expressed at higher levels in leaves than roots, except for CsAMT1.2, CsAMT1.4, and CsAMT2.1a. Remarkably, CsAMT1.2 expression was significantly higher in roots than leaves under NH₄ ⁺ deficiency (0 mM NH₄ ⁺) or at 4 mM NH₄ ⁺, demonstrating the major role of this transporter in NH₄ ⁺ uptake. Other genes involved in NH₄ ⁺ transport, such as CsAMT2.1b, CsAMT3.3, CsAMT4.1a, CsAMT4.1b, CsAMT4.1c, and CsAMT4.1d, exhibited similar expression profiles, with a decreasing trend under low N supply and a notorious induction under high N supply (Wang et al., 2022c). Furthermore, this report indicates that CsAMTs expression in tea leaves is differentially regulated over time by abiotic stresses, including drought and salinity, as well as after methyl jasmonate treatments. Under these treatments, specific CsAMTs genes were up-regulated or down-regulated in different ways, suggesting different functions to cope with various stresses (Wang et al., 2022c).

Transcriptome data revealed that CsAMT1.2 expression could be highly induced by NH₄ ⁺-resupply; weighted gene co-expression network analyses and the functional validation in an NH₄ ⁺-uptake defective yeast line further corroborated that the high-affinity transporter CsAMT1.2 was a “hub gene” in the N metabolic network of tea plants, controlling NH₄ ⁺ uptake from the soil to the roots (Zhang et al., 2020). Also, Wang et al. (Wang et al., 2022c) found that 11 yeast transformant lines grew well on 0.3 mM NH₄ ⁺ as the sole N source, indicating their high affinity for NH₄ ⁺ permeation. The transcriptional regulation of CsAMTs differed even at the cultivar level (Li et al., 2017). CsAMT1.1 and CsAMT1.5 expression levels were significantly higher in the roots of the FuDingDaBaiCha cultivar than Longjin43 cultivar (Zhang et al., 2022a). After NH₄ ⁺ resupply, CsAMT1.2, CsAMT2.2, and CsAMT2.3 genes were differentially induced in tea cultivars with different NH₄ ⁺-uptake efficiency, indicating the uneven NH₄ ⁺ transport capacity among cultivars (Zhang et al., 2018; Zhang et al., 2022b).

Membrane-bound nitrate transporters (NRTs) are required for NO₃ ⁻ uptake in plants. The members of the large NRT family can be divided into four subfamilies: nitrate transporter 1/peptide transporter (NRT1/PTR), collectively known as NPF, nitrate transporter 2 (NRT2), chloride channel (CLC), and slow anion channel associated/homologue (SLAC/ SLAH) (Krapp et al., 2014).

The NRT1 subfamily harbors many members, acting in NO₃ ⁻ transport from roots to shoots (Krapp et al., 2014). NRT transport activity is also regulated through phosphorylation. AtNRT1.1 is a dual-affinity protein: phosphorylation of the Thr101 residue by the CBL-interacting protein kinase 23 changes its substrate affinity (Sun et al., 2014). NRT2 are HATS proteins and belong to the nitrate/nitrite porter (NNP) family, mainly expressed in roots. These proteins have a role in both NO₃ ⁻ accumulation and NO₃ ⁻ transport (Chopin et al., 2007; Li et al., 2007; Kiba et al., 2012). To date, four CsNRT1 and four CsNRT2/3 genes have been isolated from tea plants. These genes show tissue-specific expression patterns and are differentially induced by exogenous NO₃ ⁻. It was reported that CsNRT1.1 and CsNRT1.2 were mainly expressed in leaves. CsNRT1.7, CsNRT2.5, and CsNRT3.2 had higher expression levels in mature leaves than other tissues, while CsNRT1.5, CsNRT2.4, and CsNRT3.1 transcripts mainly accumulated in tea roots (Feng et al., 2014; Wang, 2014; Wang et al., 2014; Yang et al., 2016; Zhang et al., 2021). Further research showed that CsNRT2.4 expression was root-specific and strongly induced by N resupply. Arabidopsis seedlings overexpressing CsNRT2.4 had a significantly higher fresh weight and lateral roots length than wild-type seedlings, especially under low N availability (0.1 mM NO₃ ⁻), pointing out CsNRT2.4 as a high-affinity nitrate transporter that might improve NO₃ ⁻ uptake rate (Zhang et al., 2021). Additionally, Wang et al. (2022b) identified a total of 109 CsNPF members by analyzing the tea genome; these proteins could be divided into 8 groups according to their phylogenetic relationships, and the transcription of most of these genes responded to NO₃ ⁻ supply. Similarly, CsNRTs expression profiles varied in tea cultivars with different NUE (Wang et al., 2014). The expression of CsNRT2.4 and CsNRT3.2 in the cultivar LongJin43 was higher than that observed in ZhongCha108, indicating higher responsiveness to external NO₃ ⁻ supply in the former (Su et al., 2020).

Initially, CLC proteins were thought to be specifically involved in chloride (Cl^-) transport as channels or 2 Cl⁻/1 H⁺ antiporters (Jentsch, 2008). Further research showed that AtCLCa is a tonoplast-located 2 NO₃ ⁻/1 H⁺ antiporter that drives NO₃ ⁻ accumulation in the vacuoles (Jentsch, 2008; Monachello et al., 2009). As anion channels, SLAC/SLAH proteins showed a strong preference for NO₃ ⁻ and have been associated with CO₂ and abscisic acid-dependent stomatal closure (Negi et al., 2008; Vahisalu et al., 2008). In tea plants, Xing et al. (2020) identified eight CLC genes across the wide genome of this species and named them CsCLC1-8. Phylogenetic studies demonstrated that the proteins encoded by these genes belonged to two subclasses; further studies showed that CsCLC transporters might participate in the uptake and long-distance transport of Cl⁻ and F⁻, as their expression levels varied in response to the addition of these two ions at different concentrations. However, the role of CsCLCs in NO₃ ⁻ transport has not been elucidated. Similarly, there are no published reports related to SLAC/SLAH proteins in tea plants.

To summarize the precedent information, Table 1 lists genes involved in NH₄ ⁺ and NO₃ ⁻ sensing, uptake, and transport in tea plants reported to date. Further information about the subcellular localization, sequence data, and functional corroboration experiments is also provided.

Tea plants can directly absorb organic N and transport it to actively growing parts. The amino acid theanine (Thea) is synthesized and stored in root cells and then transported from the root to the flush shoot in spring. These movements, which include xylem loading/unloading, xylem-to-phloem transfer, and post-vascular movements into the sink cells, are driven by plasmatic membrane-localized amino acid transporters (AATs) (Fischer et al., 1995; Dong et al., 2020; Lin et al., 2023a). Studies on tea plants AAT proteins have mostly focused on amino acid permeases (AAP), lysine and histidine transporters (LHT), and cationic amino acid transporters (CAT), which play important roles in AAs acquisition and long-distance transport from source to sink (Guo et al., 2019; Guo et al., 2020; Li et al., 2020; Liu, 2020).

Six CsAAPs members, CsAAP1, CsAAP2, CsAAP4, CsAAP5, CsAAP6, and CsAAP8, were identified in tea plants through the screening of a Saccharomyces cerevisiae mutant library. The expression of genes encoding these transporters was tissue-specific and regulated by the season and N levels. Thus, CsAAP1 expression in roots increased in March and decreased by mid-April and was highly correlated with root-to-bud Thea transport in seven tea cultivars (Dong et al., 2020; Li et al., 2020). Besides, shading promoted CsAAP2, CsAAP4, CsAAP5, and CsAAP8 expression in young stems and suppressed CsAAP1, CsAAP2, CsAAP4, CsAAP5, and CsAAP6 expression in the leaves, in accordance with Thea levels in these tissues. These findings indicate that CsAAP2, CsAAP4, CsAAP5, and CsAAP8 functions may be related to Thea movements in the xylem, leading to high Thea accumulation in the stem. The up-regulated genes might induce Thea transport into the companion cells in the sieve elements for phloem loading and Thea delivery to the terminal leaves (Yang et al., 2021).

LHT proteins were investigated more deeply. The CsLHTs family comprises multiple members, among which CsLHT1 and CsLHT6, highly expressed in roots, were identified as H⁺-dependent high- and low-affinity amino acid transporters in yeast heterologous systems. The overexpression of CsLHT1 and CsLHT6 in Arabidopsis lines significantly increased the root ability to uptake exogenous nitrogen supplied as ¹⁵N-Gln and ¹⁵N-Glu, suggesting that these transporters may contribute to the use of organic N from the soil (Guo et al., 2019; Li et al., 2021). Likewise, the heterologous expression of CsLHT4, CsLHT7, and CsLHT11 in Arabidopsis was associated with a decline in aerial parts biomass compared with WT plants, but CsLHT11 overexpressing plants had increased biomass in the rosette leaves, regardless the N levels. Therefore, this protein might have a regulatory function relevant to the development of harvestable, young shoots in tea plants (Huang et al., 2023b).

Regarding the cationic acid transporters, it was reported that the CsCAT gene family includes six members, mainly expressed in roots and stems. It was also found that some CsCATs modify their expression levels in response to abiotic stress and the exogenous application of Thea, Gln, and ethylamine hydrochloride, a precursor of Thea biosynthesis (Feng et al., 2018). CsCAT2 from tea plant was homologous to glutamine permease 1 (GNP1) from yeast, and it was found to be localized in the tonoplast as an H⁺-dependent amino acid transporter. CsCAT2 was highly expressed in the roots in winter, and this was negatively correlated with Thea root-to-shoot translocation, providing evidence that this transporter may meditate Thea storage in tea cell vacuoles (Feng et al., 2021). These findings enrich our understanding of N homeostasis in the form of AAs. Table 2 lists the genes involved in AAs transport in tea plants. When available, data on subcellular localization, sequencing, tea cultivars analyzed, specific substrates, and functional corroboration experiments are supplied.

NO₃ ⁻ absorbed by plants is a nitrogen form in a highly oxidized state, which must be reduced to NH₄ ⁺ through metabolic reduction to be further utilized. In this process, nitrate reductase (NR) is the rate-limiting enzyme (Jackson et al., 2008). Both NR and NiR are substrate-inducible enzymes; their function is to transfer electrons for NO₃ ⁻ reduction. NO₃ ⁻ taken up by roots was reduced into ammonium in mesophyll cells of shoots, and the metabolic reduction can also be catalyzed in roots (Miller and Cramer, 2005). In rice, the alleles of OsNR2 present differences between the two most common subspecies, indica and japonica. Thus, OsNR2 in indica rice promotes NO₃ ⁻ uptake through OsNRT1.1B, conferring to this subspecies increased yield and greater NUE compared with japonica rice (Gao et al., 2019).

In tea plants, studies have mainly focused on the activity and expression of CsNR and CsNiR. The activity of NR was related to NO₃ ⁻ content. Besides, this activity was lower in the less vigorously growing organs, such as the larger roots, older leaves and stems. In the new shoots, the in vitro NR activity decreased with the degree of leaf development, being highest in the first leaf and lowest in the fifth one (Wang and Su, 1990; Wu and Wu, 1993). NR activity was also responsive to trace elements including copper (Cu) and zinc (Zn). Foliar spraying of Cu and Zn increased the content of N-containing compounds and the activity of NR (Han and Wu, 1992). Under the same conditions, CsNR and CsNiR expression levels in tea roots were more strongly influenced by NH₄ ⁺ than NO₃ ⁻ (Tang et al., 2020). CsNR expression in tea roots was higher than in other tissues and was up-regulated by environmental stresses (Zhou, 2014). However, the expression level of this gene significantly differed across various cultivars (Zhou et al., 2013). On the other hand, the expression of CsNiR was higher in mature leaves than in new shoots and roots, and in roots, this gene expression was up-regulated after a short period of N resupply. The change in gene expression was slower in leaves, and this responsiveness also varied in different cultivars representing diverse genotypes (Zhang et al., 2016). Most of these findings correspond to earlier experiments; the experimental evidence for CsNR and CsNiR functions in tea plants is still scarce.

Both NH₄ ⁺ absorbed directly by plant roots and NH₄ ⁺/NH₃ formed through NO₃ ^- reduction can be derived to AAs synthesis using various keto acids generated through respiration; this process is known as ammonia assimilation. In higher plants, more than 95% of the NH₄ ⁺/NH₃ pool is assimilated via the GS-GOGAT cycle. Glutamine synthetase (GS) is the key enzyme in this pathway, playing a major role in fixing NH₄ ⁺ to the δ-carboxyl group of Glu to form Gln (Thomsen et al., 2014). Tea plants have a particular ammonia-assimilation route; their roots can biosynthesize a unique amino acid, Theanine (Thea), a homolog of Gln (Lin et al., 2023a). Glutamate synthase (GOGAT) catalyzes the conversion of Gln and 2-oxoglutarate to Glu, thus providing Glu for ammonia assimilation (Bernard and Habash, 2009; Valderrama-Martín et al., 2022). When plants germinate, senesce, and begin to form seeds, glutamate dehydrogenase (GDH) can catalyze the reversible amination/deamination so that the GS-GOGAT cycle allows NH₃ reuse, necessary for ammonia detoxification (Fontaine et al., 2012; Zhou et al., 2015). Through these pathways, N absorbed by roots is incorporated into proteins, nucleic acids, and other substances needed for plant growth.

By the end of the 20^th century, it was reported that the rate of increase in tea ground stem diameter and height and dry matter production in different cultivars varied under sufficient N supply compared to no N application (Ruan et al., 1993). Under low N supply, tea plants’ height, root and shoot dry weight, and leaf SPAD values were significantly decreased (Wang et al., 2015). Wang et al. (2004) measured the added-N content in the biomass and the growth of new shoots in six tea cultivars under four N levels (based on ¹⁵N isotope labeling techniques), and redefined five interdependent traits—nitrogen use efficiency (NE), nitrogen uptake efficiency (NUE), nitrogen physiological utilization efficiency (NPE), nitrogen economic efficiency (NEE) and N responsiveness—according to growth characters and harvesting organs. They found that the biomass increase was significantly correlated with NEE, the weight of the new shoots was significantly (positively) correlated with NE, NUE, and NEE, while NUE was the main determinant of NE. These authors indicated that by comparing the NE values of different cultivars, it is possible to detect which cultivar can achieve the highest NUE for a given level of N supply.

The root is the main organ for nutrient uptake and plays a direct role in N acquisition (Lynch, 2007; Zhu et al., 2011). Root development and activity are responsive to soil N levels (Ju et al., 2015). Studies on plant response to N concentration gradients using different tea cultivars suggested that N concentration has a significant effect on root/shoot ratio, and this ratio could be used as a screening index to detect low-N-tolerant cultivars (Wang et al., 2015). On the other hand, the differences among cultivars in root-related parameters such as root dry weight, root volume, or root active uptake area were greater than those of root activity. Likewise, root volume and active uptake area varied significantly across N levels. Further correlation studies provided evidence that these parameters may be considered as promising indices for selecting and breeding tea cultivars with high NUE (Wang et al., 2005).

In the early 1950s, Epstein and Hagen (1952) applied the Michaelis-Menten equation for the first time to describe the absorption process of ionic nutrients by plants. In this equation, V_max represents the maximum uptake rate; this value is directly proportional to the uptake rate for ions, and K_m is inversely proportional to the affinity of the cell membrane for nutrient ions (Zhang et al., 2018). Because tea roots show a preference for NH₄ ⁺ uptake as the nitrogen source, the kinetic parameters of this cation are usually used to define tea adaptability to N availability. According to current studies on NH₄ ⁺ dynamics, tea cultivars may be classified into three categories: (1) cultivars with high K_m and high V_max can produce high yields in soils with elevated N contents; TeiGuanYin, HuangDan, and Yubukita cultivars belong to this type; (2) cultivars with low K_m and low V_max may display a good performance in soils with low N concentrations; YingShuang and MaoXie belong to this type; (3) cultivars with high K_m and low V_max are the most flexible concerning N levels, being appropriate for both high and low N conditions; ZhongCha#302 and FuDingDaBaiCha belong to this type (Wang et al., 2005; Liu, 2016; Zhang et al., 2018; Zhang et al., 2022b). Notably, N flux was calculated as the N content in the roots based on ¹⁵N labeling in most studies, and there are still many cultivars falling into different groups in different studies due to different number of tested cultivars and methodological approaches. Though NH₄ ⁺ influx kinetics allowed a better understanding of N use by tea plants, more precise methods, such as non-invasive procedures based on micro-test technology, will be useful for future experiments (Ruan et al., 2016; Su et al., 2020).

The leaves are the main assimilation organs of inorganic N. The accumulation of N-assimilates and the enzymes and genes regulating these metabolic processes could indicate NUE-related sub-traits (Sun et al., 2019). Some studies showed that GS activity varied among cultivars and N levels and was positively correlated with N assimilation rate and NUpE (Wang et al., 2005; Du et al., 2015). Lin et al. (2017; 2018) examined the activity of some antioxidant enzymes and found increased activities in the low N-tolerant cultivar HuangDan in a nitrogen-deficient environment. This was linked to the maintenance of high photosynthetic rates and to the adequate output of N-assimilates in the leaves. Still, by combining genes, enzymes, and assimilates and exploring their affiliation links, it was possible to evaluate NUE traits comprehensively. Zhou (2012) measured soluble sugars, soluble proteins, total N content, N-related enzymatic activities, and the expression of AAs biosynthetic genes. Their results suggested that the differences in these indicators varied in the five cultivars tested as the N concentration increased and membership function could be used to evaluated the NUE of each cultivar synthetically. Also, CsAMTs expression profiles in response to NH₄ ⁺ differed among cultivars (Zhang et al., 2018; Zhang et al., 2022b). Still, it was possible to detect that CsNRT2 participated in NO₃ ⁻ transport under low N conditions (Hu et al., 2023; Lin et al., 2023b). The AuTophaGy-related genes CsATG8e and CsATG3a were linked to an improved plant ability for N recycling and tolerance to low N levels (Huang et al., 2020; Huang et al., 2023a). These genes emerge as promising indicators and may contribute to identifying higher NUE among various germplasm resources.

Although multiple investigations have addressed NUE of tea plants, most NUE-related traits were identified based on individual morphology, physiological processes, relevant biochemical components, or gene expression patterns. Nevertheless, there are no universal standards for grading NUE in tea plants, and some cultivars have shown heterogeneous results. The measurement of biomass is time-consuming and susceptible to environmental changes. And only the processes of N uptake, transport or utilization not the comprehensive NUE have been measured in tea plants. The practicability of method also depends on the number of tested cultivars. Most importantly, NUE estimates are complicated and current evaluation methods are not comprehensive enough to cover and explain the meaning of NUE. The methodological limitations still resist our understanding of N metabolic mechanisms. Therefore, analyses combining omics data and molecular and genetic approaches will be useful to elucidate further heritability and inheritance in this species with a point of great value to improve NUE by tea plants.