Abstract
An experiment was conducted to investigate and distinguish the relationships in the production of total phenolics, total flavonoids, soluble sugars, H2O2, O2−, phenylalanine ammonia lyase (PAL) activity, leaf gas exchange, antioxidant activity, antioxidant enzyme activity [ascorbate peroxidase (APX), catalase (CAT), superoxide dismutase (SOD) and Lipoxygenase inhibitory activity (LOX)] under four levels of foliar abscisic acid (ABA) application (0, 2, 4, 6 µM) for 15 weeks in Orthosiphon stamineus Benth. It was found that the production of plant secondary metabolites, soluble sugars, antioxidant activity, PAL activity and LOX inhibitory activity was influenced by foliar application of ABA. As the concentration of ABA was increased from 0 to 6 µM the production of total phenolics, flavonoids, sucrose, H2O2, O2−, PAL activity and LOX inhibitory activity was enhanced. It was also observed that the antioxidant capabilities (DPPH and ORAC) were increased. This was followed by increases in production of antioxidant enzymes APX, CAT and SOD. Under high application rates of ABA the net photosynthesis and stomatal conductance was found to be reduced. The production of primary and secondary metabolites displayed a significant positive relationship with H2O2 (total phenolics, r2 = 0.877; total flavonoids, r2 = 0.812; p ≤ 0.05) and O2− (total phenolics, r2 = 0.778; total flavonoids, r2 = 0.912; p ≤ 0.05). This indicated that increased oxidative stress at high application rates of ABA, improved the production of phytochemicals.
Highlights
Orthosiphon stamineus Benth from the Famiy Lamiaceae is locally known as Misai Kucing in Malaysia and Kumis Kuching in Indonesia
Total flavonoids content followed the same trend as total phenolics where the highest total flavonoids was observed in the leaf receiving 6 μM abscisic acid (ABA) that registered 2.12 mg rutin g−1 dry weight, and the lowest was in the stems with the control treatment (0 μM) that contained only 0.52 mg rutin g−1 dry weight
Showed that an increase in secondary metabolites was related to the balance between carbohydrate source and sink; the greater the source-sink ratio, the greater the production of secondary metabolites that might occur. This was reported by Guo et al [27] that found an increase in sucrose content corresponding to the enhanced production of ascorbic acid, glucosinolates, sulforaphane, anthocyanins, total phenolics and increased antioxidative activities in broccoli sprouts. This was supported by source-sink hypotheses [28] and growth-differentiation balance hypothesis [29] that assume that increase in carbon accessibility that is accumulated in total non-structurable carbohydrate (TNC) would increased the production of carbon based secondary metabolites when the provided carbon amounts exceed growth requirements [30]
Summary
ABA increased the generation of H2O2 and O2− [14,15] The free radicals such as ROS, including hydroxyl radicals, superoxide anions and hydrogen peroxide, play an important role in promoting tissue damage in living organisms. They may lead to cell damage through membrane lipid peroxidation and DNA mutations and as a consequence of that diseases such as cancer may develop [16]. Water stress-induced ABA accumulation has been shown to trigger increased generation of H2O2, which in turn led to the up-regulation of secondary metabolites and antioxidant enzyme activities in maize leaves [22,23]. ABA might enhance the production of specific secondary metabolites and increase antioxidant activity in Orthosiphon stamineus.
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