Manganese (Mn) is an essential micronutrient for plant growth which is required in photosynthesis, nitrogen metabolism, chloroplast formation and synthesis of various enzymes. Its deficit is dangerous for chloroplasts as it affects the water-splitting system of photosystem II, which provides the necessary electrons for photosynthesis. However, its chemistry in acid soils is complex and in the excess amount it acts as a toxic element for plants. Its toxicity is more prevalent in acid upland soils, lowland soils containing large amounts of easily reducible Mn, acid sulfate soils and areas affected by Mn mining. Mn soil contents are also influenced by environmental conditions, thereby soluble and exchangeable Mn is found in high amounts after hot, dry weather.
Mn is taken up as divalent cation Mn2+ via an active transport system in epidermal root cells. Its uptake by roots is characterised as a biphasic process. The initial and rapid uptake phase is reversible and non-metabolic, whereas, the second phase is slow. The amount of exchangeable manganese -mainly Mn2+ form increases in the soil solution with decrease in soil pH. This form is available for plants and can be readily transported, which can finally accumulate in shoots. Whereas, at higher pH, other forms of Mn such as Mn (III) and Mn (IV) are prevalent, which are unavailable and cannot be accumulated in plants.
Effects of Mn toxicity
Mn toxicity is more prominent in shoots where it accumulates and become toxic, although at very high concentrations; root growth may also be affected. The morphological symptoms for both deficiency and toxicity of Mn are similar to a certain extent. These include dark-brown, necrotic spots on lower leaves, distortion of expanding leaves and chlorosis of young leaves. Due to excess Mn, there is reduced iron (Fe) uptake by roots and therefore, chlorosis is often confused with Fe deï¬ciency. The interactions of Mn toxicity with other ions are very important for plant growth and development. Excess Mn can interfere with the absorption, translocation and utilisation of other mineral elements such as calcium (Ca), magnesium (Mg), iron (Fe), sodium (Na) and phosphorus (P).
Mn toxicity has effect on several physiological and biochemical parameters of plants. These include destruction of auxins, increase in peroxidise activity, inhibition of photosynthesis, decreased chlorophyll content and increase in organic acid levels. Also, formation of callose is a sensitive and reliable indicator for Mn toxicity. These biochemical and physiological methods can be used to assay Mn toxicity in plants. These include measuring the indices of Mn status in plants which are provided by changes in metabolic rates of Mn-specific enzymes or processes, as they respond directly to variations in Mn supply. For e.g., peroxidase activity is measured by oxidation of pyrogallol by leaf extract in the presence of hydrogen peroxide.
These tests measure absolute enzyme activity in both healthy and deficient tissue to make a diagnosis by comparison. Also, chlorophyll ‘a’ fluorescence has been used as a measure of photosynthetic dysfunction to permit early diagnosis of Mn deficiency. Enhanced fluorescence at low Mn concentrations in leaves reflects a functional association between leaf Mn and electron transport from water to photosystem II.
Mn phytotoxicity is dependent on several factors which include soil pH, redox potential, soil moisture and microbial activity. Different plant species or even varieties within a species have different degrees of tolerance to Mn. Its concentrations in the range of 0.2 to 12 m M can cause severe growth limitations and at low concentration of 1 ppm it can be toxic to various crop plants. Two prominent mechanisms operate for Mn tolerance in plants viz., exclusion and inclusion mechanisms. In the first case there is prevention of its uptake and translocation while in the second one, the detoxification of Mn occurs in vacuole. Superior tolerance to excess Mn is due to reduced transport of Mn from roots to leaves. Plants that tolerate high levels of Mn in their tissues may oxidise it to a metabolically inactive form.
Reducing Mn toxicity
To reduce Mn toxicity, two approaches can be used. The ï¬rst is soil amelioration through application of lime or gypsum. The second is to find genotypes which are better adapted to Mn toxicity. Considerable genetic differences exist among plant species for tolerance to high concentrations of Mn which is assumed to be controlled by one or more genes. Such variation may occur via differences in root exclusion of excess Mn, complexation of Mn within roots or shoot tolerance of high Mn. Studies of tolerance mechanisms may help to select or breed plants having greater tolerance to Mn toxicity which is realised as a promising approach for improvement of Mn tolerance in crop plants.