As for perennial plant, it is difficult to calculate the N retained in the lignified stem and root system and no account is taken of N amount for these compartments within tea plant. The N behavior is monitored for the entire above-ground leaves throughout growth period. N concentration (g kg–1 DW) was quantified during the growing seasons for the three organs. The different phases of leaves harvest were defined spring tea for the two rounds of April and May, summer tea for the two rounds of July and August, autumn tea for one turn in October, which indicates the rounds of new leaves development. Generally, one bud with two leaves are considered as the harvest part and as sinks for N accumulation, and then they grew and reached the physiological maturity, being as the leaves attached to the green-red stem at next picking harvest time, whether they serve as sources or sinks depends on N status of tea plant. The most of leaves attached to the grey stem are usually leaves developed last year. N concentration determination in a series of 20 tea plant varieties, starting spring tea season in early April and ending autumn tea season in October before pruning, were carried out.
N repartition within tea plants has not yet been well characterized during each round of new leaves development. All source and sink organs undergo N remobilization, but the efficiency with which it can be transferred to new developed leaves and the rate of N remobilization may vary according to the organs. In tea plant, N remobilization associated with vegetative growth in summer and dormancy in winter is considered as crucial for tea production potential and quality, and also related with NUE.
Dynamic changes of N concentration in different leaves during the growing seasons
N concentration is correlated to N availability within plants. The dynamic changes of N concentration for one bud with two leaves differed between development rounds during the growth period. The N concentration decreased from first picking round with average of 44.2 g kg–1 early in April, to 41.55 g kg–1 in May, to 34.50 g kg–1 in July, and a transient increase up to 39.0 g kg–1 in August, and then decreased thereafter with the lowest of 31.05 g kg–1 in October, being ‘S’-shaped trend (Figure 2a). Natural variation of N concentration among 20 elite tea plant varieties was indicated by coefficiency variation (CV), 15.16% (October) > 9.43% (May) > 8.32% (April) >8.07% (July) >7.08 (August) (Table 2), which showed that N concentration variation is relatively limited in spring season and extensive in autumn season.
The averaged N concentration for leaves attached to green-red stems decreased from April of 27.99 g kg–1 to July of 22.67 g kg–1, whereas increased up to 23.62 g kg–1 in August and then to 29.42 g kg–1 in October, being ‘U’-shaped trend (Figure 1b). Genotypic variation of N concentration among 20 elite tea plant varieties was indicated by CV, presenting relatively high of 13.17% (April) and 12.32% (May), followed by 6.08%, 6.06% and 5.65% in October, July and August respectively (Table 2), which showed different pattern for N concentration variation among the cultivars, extensive in spring season and limited in autumn season.
Upon to the N concentration in leaves attached to grey stems, from April to May, first it increased from 20.79 g kg–1 to 24.57 g kg–1 and reached a peak level, and then decreased to 21.19 g kg–1 in July, and remained relatively constant, 21.93 g kg–1 and 21.71 g kg–1 in August and October respectively, throughout the growing seasons as ‘S-like’-shape, suggested reaching a plateau for summer and autumn seasons. Genotypes differed significantly in N concentration among 20 elite tea plant varieties with CV of 16.60% (April) > 16.16% (May) > 13.60% (July) > 11.11% (October) > 8.16% (August) (Table 2), which showed an obvious descending trend from April to August, and reversely increased from August to October.
Overall comparison pattern of N concentration in different organs
Overall, N concentrations were highest in the compartment of one bud with two leaves followed by leaves attached to green-red stems, and lowest in leaves attached to grey stems during the entire growing seasons. The differences between one bud with two leaves and the leaves attached to green-red stems were particularly obvious during the period of most rapid growth, in spring (April and May) and summer (July and August). Notably, distinctive differences between the leaves attached to green-red and grey stems were observed when tea plants re-grow in early spring (April) and stop growing in autumn (October), whereas there were no obvious differences during the vigorous growing period, from May to August. As mentioned above, N concentration dynamic changes patterns during the entire growing seasons differed among the three compartments along canopy, and it showed a largest fluctuation in the bud with two leaves, followed by the leaves attached to green-red stems, and then by the leaves attached to grey stems.
Interestingly, different compartments exhibited peak N concentration in the entire growing seasons, it was peaked in May for most of genotypes in the leaves attached to the grey stems according to Figure 2c. For the other two compartments, they exhibited similar patterns over the growing seasons between April and July, from an averagely peak level to a consecutive downward trend. Over the next growing seasons from July to October, these two organs showed distinctive trends, N concentration first increased gradually and reached a peak level in August and then declined again from August to October for the bud with two leaves, whereas for the leaves attached to green-red stems, there was a weakly rising and stayed a low level for a sustained period from July to August, followed by a substantial increase from August to October (Figure 2b), which were nearly equal to those in spring. The fluctuation of N concentration in leaves of the three compartments attached to different vertical position presumably indicated the variation of N demand concomitant with round by round growth.
Dynamic changes of N remobilization efficiency in different leaves during the growing seasons
Within tea plant, N partitioning is desirable of a maximum N concentration in harvest part and a minimum N concentration in the reserves, including canopy-leaves attached to green-red and grey stems, stems and root. The N dynamic changes of N concentration allowed us to trace the amount of N requirement and to determine the N remobilization and redistribution between the leaves with varied tenderness as the two various compartments. The bud with two leaves is always the newly developed for each round growth and harvest part, thus it is taken for granted meaningless for N remobilization. Therefore, the N remobilization efficiency (NRE) was defined as NRE = (CB-CA)/CA, and CA/CB is the N concentration of the Ath/Bth round special for the leaves attached to green-red stems or grey stems. Totally four NREs were calculated between the five developed rounds for the two parts of leaves, which are subject to obtain a dynamic view of N internal remobilization during the growing seasons. NRE is assumed as one of the most responsive traits, as it integrates N concentration of two consecutive growth rounds into a single trait. The Figure 3 represented an integrated view of the NREs for the two parallels canopy leaves throughout the growing period.
The dynamic changes of NREs were determined for the leaves attached to green-red stems among the 20 tea plant cultivars during growing period (Figure 3a). During the first period between the first two rounds (from April to May), seventeen varieties presented NREs < 0, ranging between –33.86% and –1.82%. By contrast, other three varieties showed NREs > 0, being 5.06% of cv. Shuchazao, 26.39% of cv. Fudingdahao and 38.03% of cv. Mingxuan 131 respectively. During the second period between the 2nd and 3rd round (from May to July), a series of fifteen cultivars were NREs < 0 and varied between –29.11% and –3.51%, and the remained five cultivars were NREs > 0, ranging between 1.24% and 14.30%. Compared with those of the first period, no matter what NREs < 0 or NREs > 0, the interval between NREs value was markedly reduced in the second period. During the third period between the 3rd and 4th round (from July to August), the result made entirely opposite findings towards the NREs value and there were fifteen cultivars with NREs > 0 and five cultivars with NREs < 0. The range for NREs > 0 was relatively moderate, from 0.19% to 20.14%. The range for NREs < 0 was further decreased, from –10.94% to –0.63%. During the fourth period between the 4th and 5th round (from August to October), all the cultivars exhibited NREs < 0 with the obviously extensive range between –12.21% and –51.35%. Such genotype with high NREs does not necessarily reduce harvestable leaves yield even when plants are grown under low N conditions.
The dynamic changes of NREs were determined for the leaves attached to grey stems among the 20 tea plant cultivars during growing period (Figure 3b). The leaf age is almost one year and approach senescent. During the first period between the first two rounds (from April to May), only six varieties presented NREs < 0, ranging between –29.56% and –0.15%. By contrast, other fourteen varieties showed NREs > 0 and ranged between 9.61% and 102.66%. During the second period between the 2nd and 3rd round (from May to July), totally of fourteen cultivars were NREs < 0 and varied between –44.73% and –4.50%, and the remained six cultivars were NREs > 0, ranging between 0.64% and 13.16%. Compared with those of the first period, the interval between NREs value was markedly reduced for NREs > 0 and increased for NREs < 0 in the second period. During the third period between the 3rd and 4th round (from July to August), the result showed that thirteen cultivars were NREs > 0 and seven were NREs < 0. During the fourth period between the 4th and 5th round (from August to October), twelve cultivars exhibited NREs < 0 with range between –1.95% and –17.69%, and the range for NREs > 0 was approximately comparable to that of NREs < 0.
Theoretically, NRE < 0, N was exported from the specific leaves as N source organs during the period of two consecutive rounds, thereby reusing N within plant to improve the NUE consequently. Conversely, NRE > 0, N was transported inwardly and the specific leaves acted as N sink organ to accumulate N. Therefore, the extent of N remobilization from source to sink/young developed leaves varied significant among tea plant genotypes, which is indicative for N dynamic behavior within tea plant.
Dynamic changes of N utilization efficiency (NUtE) during the growing seasons
The efficiency to N assimilation and remobilization are involved in NUtE in tea plant. The average NUtE from April to July increased from 22.76 g g–1 N to 29.05 g g–1 N, and decreased at 25.77 g g–1 Nin August, and then conversely increased up to 32.99 g g–1 N in October, and the dynamic changes of NUtE followed an ‘S’-shape (Figure 4) which is contrary to that of N concentration in bud with two leaves. With respect to the roundly harvest for sprouted young bud with two leaves among the tested cultivars, the NUtE ranged between 19.51 g g–1 and 26.39 g g–1 N in April, between 22.70–27.94 g g–1 N in August, which was in a relatively low and limited range and indicated that N was in great demand in the young part for harvest. The scopes of NUtE in May and July were 20.44–30.35 g g–1 N and 24.09–34.15 g g–1 N respectively, which were significantly extended. Particularly in October, 24.53–41.25 g g–1 N, the spectrums were further enlarged, respectively.
At the onset of genotype level, the dynamic changes of NUtE were obviously observed during the growing period and significant variations were found among the tea plant cultivars (Table 1). Interestingly, the ranges of NUtE of five cultivars, cv. Echa10, cv. Huangdan, cv. Shuchazao, cv. Jinxuan and cv. Jiaming 1, were extremely limited, showing very consistent pattern with peak in July and valley in April. By contrast, the two cultivars’ NUtE showed a broad range and were nearly two-fold between the maximum in October and minimum in April, 20.54–41.25 g g–1 N for cv. Meizhan and 20.35–38.05 g g–1 N for cv. Yingshuang respectively. For the remaining tea plant varieties, NUtE went upward trend from April to July and reversely went downward between July and August, finally went upward and peaked in October.