2.1. Total Phenolics (TP) Content
The result obtained from the preliminary analysis of TP is shown in Table 1
. It is apparent that TP accumulation and partitioning in the plant was considerably affected by the differing light intensity. The different light intensities had a significant (p ≤ 0.01) effect on the TP produced (Table 1
). The results show that in both varieties of ginger, the leaves had a higher TP content (33 mg/g dry weight in Halia Bentong and 39.06 mg/g dry weigh in Halia Bara) under a light intensity of 790 μmol m−2
compared with other light intensities. The partitioning of TP at 16 weeks after planting was: leaves > rhizomes > stems. The results obtained from the preliminary analysis of TF and TP are shown in Table 1
. According to the results in this table, factors (light, varieties and plant parts) had a significant effect (p ≤ 0.01) on TP accumulation and partitioning in both varieties. On the other hand, different light intensity had a significant effect on the production, accumulation and partitioning in different part of plants and also different varieties had a different concentration of TP in different part of plants. TF and TP content in ginger grown in a glasshouse under normal light intensity (790 μmol m−2
) were reported in a previous study by present authors [12
] and the results from this light intensity (790 μmol m−2
) is shown in Table 1
as a standard condition to compare with the results of lower light intensities.
Ginger is a semi shade plant, and the results of this study showed that 790 μmol m−2
is suitable light intensity for maximum TP production. The increased TF in Halia Bentong with increasing shade could be associated with significantly higher leaf chlorophyll and carotenoid contents under lower light levels. TP levels were higher in leaves of both of varieties under a light intensity of 790 μmol m−2
. Contrary to our results, higher phenolic contents and antioxidant activity have been reported in rhizomes than in leaves of Z. officinale
]. These studies involved one or two ginger species and it is not known whether their comparisons were based on plant samples from the same or different locations. However, the results of Chan et al.
] supported our findings. High levels of TP was reported in ginger leaves when compared with rhizomes in Zingiber officinale
]. The quality and yield of ginger rhizomes increased when grown in the shade because nutrient uptake increased [28
Increases in soluble phenolics, such as the intermediates in lignin biosynthesis, can reflect the typical anatomical change induced by stressors: an increase in cell wall endurance and the creation of physical barriers preventing calls against harmful action [29
]. However, some products in plants such as anthocyanin, cumarin and lignin are biosynthesized in the course of phenolic compounds being transformed into flavonoids [30
]. TP content in both varieties when compared to other medicine plants, for example Calotropis (stems: 14.97 mg/g DW; leaves 14.94 mg/g DW), Hibiscus (stems: 7.04 mg/g DW; leaves 29.96 mg/g DW), Parthenium (stems: 4.88 mg/g DW; leaves 8.29 mg/g DW) and Kigelia (stems: 33.4 mg/g DW; leaves 15.04 mg/g DW) [31
], showed good potential in different parts of the plant.
2.2. Total Flavonoids (TF) Content
It is apparent from Table 1
that TF accumulation and partitioning was considerably affected by the different light intensities. Differing light intensity also had a significant (p ≤ 0.01) effect on the TP produced (Table 1
). The results show that leaves in both of varieties have a higher content of TF (5.95 mg/g dry weight in Halia Bentong and 8.45 mg/g dry weight in Halia Bara) under 310 μmol m−2
of light intensity. A similar trend of increasing TF content with decreasing light intensity were seen in Tanacetum parthenium
and strawberry [32
] and in some medicinal plants illustrating a considerable influence of low irradiance on enhancement of plant TF [19
]. Michel et al.
] reported TF production related to plant pigments (chlorophyll and carotenoids), and in contrast to flavonoids, the xanthophyll cycle seems to be mainly relevant to the protection of photosynthesis against sudden increases in light intensity. Chan et al.
] reported much greater concentrations of flavones and flavonols in leaves of vegetables that are exposed to shade. This finding is in agreement with Bergquist’s findings [35
], which showed that the use of shade netting is acceptable for production of baby spinach in regard to flavonoid concentration and composition.
From Table 1
, it is apparent that TF partitioning to the leaves, stems and rhizomes increased steadily and substantially with decreasing light intensity to 310 μmol m−2
. With decreasing TP in the leaves and rhizomes from 790 to 310 μmol m−2
, TF content increased significantly in the leaves and rhizome of both varieties. According to the shikmic acid pathway, phenolic compounds are produced first and after that phenolic acids (elagic acid, tannic acid and vanillin), hydroxy cinnamic acid (caffeic acid, ferulic acid, curcumin, cournarins), lignas and flavonoids produced from phenolic compounds in other cycles [36
]. Therefore, according to Table 1
data, it can be hypothesized that the biosynthesis of TF from phenolic compounds in plants grown under light intensity of 310 μmol m−2
was more than other phenolic components (phenolic acid, hydroxyl cinnamic and lignas) and low levels of TF in plants grown under 790 μmol m−2
of light intensity is related to more biosynthesis of other phenolic components compared with flavonoids. It can be seen from the data in Table 1
that the TF content—when compared with other plant for example onion leaves (2.72 mg/g DW), bird chili (1.66 mg/g DW), papaya shoots (1.26 mg/g DW), guava (1.12 mg/g DW), semambu leaves (2.04 mg/g DW) and black tea (1.49 mg/g DW) [37
]—shows high level in Halia Bara leaves and rhizomes when grown under 310 μmol m−2
of light intensity. The results of our study are consistent with other studies and suggest that high content of some phenolic components such as cinamic acid can inhibit flavonoid biosynthesis with inhibition of PAL (phenylalanine ammonia lyase) enzyme activity [38
The synthesis of isoflavones and some other flavonoids is induced when plants are infected or injured [27
], or under low light and low nutrient conditions [39