Manganese is a common metal in theearth crust and its presence in soils mainly results from Mn in the parent material. It activates several enzymes including superoxide dismutase, NADPH-specific decarboxylating malate dehydrogenase and nitrate reductase. However, manganese is toxic when available in excess. Eventhough the toxic effects of manganese is not generally seen in tea plants, reduction in soil pH of mature tea fields due to continuous synthetic fertilizer application and preheating of nursery soils to kill eelworm could result in manganesetoxicity. Excess manganese supply causes formation of visible brown depositions in the cell walls of leaves, which normally consists of oxidized manganese and oxidized phenols. Oxidation is believed to be mediated by an enzyme like peroxidase. The present investigation was planned to study the effect of this micronutrient on peroxidase, catalase activity, amino acids, polyphenols, catechins, chlorophyll and carotenoids.
A potted plant experiment was carried out with seven treatments including an untreated control in the clone UPASI-9. About 4 kg of soil was filled in each pot to which manganese was added externally to contain 100, 500, 1000, 3000, 5000 and 7000 mg of Mn2+ kg-1 of soil. The experiment was conducted in triplicate. The soil used for this study was sandy loam in texture having 71% sand, 10% silt and 19% clay. One year old plants of the clone UPASI-9 were grown in these pots. Table 1 provides the biochemical parameters of leaves assessed on the 10th day after imposing treatments. The plants supplied with 7000 mg Mn2+kg-1 developed toxicitysymptoms on the 4th day after imposing treatments. The plants grown in the soil containing 5000 and 3000 mg kg- 1 showed toxicity symptoms on the 7th and the 10th day, respectively. Irregular brown spots all over the leaf surface right from the petiole to the leaf tip between the marginal veins were the visible symptoms of manganese toxicity. Extensive brown spots on top surface were also noticed. Unlike iron toxicity, manganese toxicity resulted in the death of plants on the 13th day when manganese was added at 7000 mg kg-1, on the 18th day at 5000 and on the 23rd day at 3000 mg kg-1 .
There was significant decrease in amino acid content of leaves due to external addition of manganese. Eventhough the change in chlorophyll content of tea leaves was not significant at lower doses of manganese, a drastic and highly significant reduction was noted at and above 1000 mg kg-1of soil. This is because manganese inhibited chlorophyll synthesis by blocking Fe, which would ultimately reflect on productivity. So was the case with carotenoids. A significant increase in catechin and polyphenol contents was seen due to addition of Mn up to 1000 mg kg-1, beyond which there was a sharp decline. Nitrate reductase, which assimilates nitrogen in higher plants did not respond to the external addition of manganese. The activity of catalase was significantly higher than in control due to addition of manganese @ 100, 500 and 1000 mg Mn kg-1 of soil. The activity of peroxidase became stronger due to the increase in manganese. It was three fold higher than in control when 7000 mg Mn was added kg-1 of soil. This could be due to the stimulating effect excerted by manganese on an apoplastic H2O2 producing enzyme like peroxidase.
(Table 1 and pictures are available on request drsvenkatesan@gmail.com)
Authors : S.Venkatesan, K.V.Hemalatha and S.Jayaganesh
