FUNCTION IN PLANTS
Manganese is involved in many biochemical functions, primarily acting as an activator of enzymes such as dehydrogenases, transferases, hydroxylases, and decarboxylases

involved in respiration,
 

Manganese
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amino acid and lignin synthesis, and hormone concentrations , but in some cases it may be replaced by other metal ions (e.g., Mg). Manganese is involved in oxidation-

reduction (redox) reactions within the photosynthetic electron transport system in plants . Manganese is also involved in the photosynthetic evolution of O2 in

chloroplasts (Hill reaction). Owing to the key role in  this  essential  process,  inhibition  of  photosynthesis  occurs  even  at  moderate  manganese deficiency;

however, it does not affect chloroplast ultrastructure or cause chloroplast breakdown until severe deficiency is reached .

SYMPTOMS
Characteristic foliar symptoms of manganese deficiency become unmistakable only when the growth rate is restricted significantly   and include diffuse interveinal

chlorosis on young expanded leaf blades ; in contrast to the network of green veins seen with iron
deficiency . Severe necrotic spots or streaks may also form. Symptoms often occur first on the middle leaves, in contrast to the symptoms of magnesium deficiency,

which appear on older leaves. With eucalyptus (Eucalyptus spp. L. Her.), the tip margins of juvenile and adult expanding leaves become pale green. Chlorosis extends

between the lateral veins toward the midrib . With cereals, chlorosis develops first on the leaf base, while with dicotyledons the distal portions of the leaf blade

are affected first

4
TOLERANCE
Tolerance to manganese deficiency is usually conferred by an ability to extract more efficiently available manganese from soils that are considered deficient.

Mechanisms that are involved in the improved extraction of manganese from the soil include the production of root exudates, differences in excess cation uptake thus

affecting the pH of the rhizosphere, and changes in root density . The genotypic variation within species for manganese efficiency can be utilized by breeding programs

to develop more efficient varieties .
 Tolerance to manganese deficiency may be attributed to one or more of the following five adaptive mechanisms :

TOXICITY
1
PREVALENCE
Manganese toxicity is a major problem worldwide and occurs mainly in poorly drained, acid soils owing to the interactions mentioned previously. However, not all poorly

drained soils are sources of manganese toxicity as reported by Beckwith and co-workers , who noted that flooding often increased the pH, thus reducing the availability

of manganese. Tropical, subtropical, and temperate soils have all been reported to be sources of manganese at concentrations high enough to produce visible symptoms of

toxicity. In the tropics, toxicity has been reported in tropical grasses grown in the Catalina (basalt) and the Fajardo (moderately permeable) clayey soils of Puerto

Rico , and in ryegrass (Lolium spp. L.) grown on red-brown clayey loam and granite-mica schists in Uganda, Africa . Among the subtropical regions, toxicity has been

reported in subtropical United States in poorly drained soils and soils on limestone  and on ultisols. However, the impermeability of soils does not seem essential for

manganese toxicity . In southeastern Australia, manganese toxicity has been reported in fruit trees grown in neutral-pH duplex soils , in French beans (Phaseolus

vulgaris L.) grown in manganese-rich basaltic soil , and in pasture legumes . There is very little information available on manganese toxicity in temperate regions,

though one report found toxicity on soils characterized by low pH and high concentrations of readily exchangeable manganese .

2 INDICATOR PLANTS
A number of crops are considered sensitive to manganese toxicity, and these include alfalfa, cabbage, cauliflower (Brassica oleracea var. botrytis L.), clover

(Trifolium spp. L.), pineapple (Ananas comosus Merr.), potato (Solanum tuberosum L.), sugar beet, and tomato (Lycopersicon esculentum Mill.). An excess of one nutrient

can aggravate a deficiency of another, and so symptoms of manganese toxicity bear some features of deficiency of another nutrient. Additionally, toxicity of manganese

is often confused with aluminum toxicity as both often occur in acid soils. However, in some species such as wheat and rice , the tolerance to these two toxicities is

opposite .

3SYMPTOMS
The visual symptoms of manganese toxicity vary depending on the plant species and the level of tolerance to an excess of this nutrient. Localized as well as high

overall concentrations of manganese are responsible for toxicity symptoms such as leaf speckling in barley , internal bark necrosis in apple , and leaf marginal

chlorosis in mustard (Brassica spp. L.) .
 The symptoms observed include yellowing beginning at the leaf edge of older leaves, sometimes leading to an upward cupping (crinkle leaf in cotton, ), and

brown necrotic peppering on older leaves. Other symptoms include leaf puckering in soybeans and snap bean ; marginal chlorosis and necrosis of leaves in alfalfa, rape

(Brassica napus L.), kale (Brassica oleracea var. acephala DC.), and lettuce (Lactuca sativa L.) ; necrotic spots on leaves in barley, lettuce, and soybeans ; and

necrosis in apple bark (i.e., bark measles) . Symptoms in soybeans include chlorotic specks and leaf crinkling as a result of raised interveinal areas ; chlorotic leaf

tips, necrotic areas, and leaf distortion  in tobacco (Nicotiana tabacum L.).
 

4TOLERANCE
Reduction of manganese to the divalent and therefore more readily absorbed form is promoted in waterlogged soils, and tolerance to wet conditions has coincided with

tolerance to excess manganese in the soil solution. Graven et al. suggested that sensitivity to waterlogging in alfalfa may be partially due to manganese toxicity, and

alfalfa has been shown to be more sensitive to manganese toxicity than other pasture species such as birdsfoot trefoil (Lotus corniculatus L.) . In support of this

suggestion, several other pasture species have also been reported to have a relationship between waterlogging and manganese toxicity . For example, manganese-tolerant

subterranean clover (Trifolium subterraneum cv. Geraldton) was reported to be more tolerant to waterlogging than the manganese-sensitive medic (Medicago truncatula

Gaertner) . Increased tolerance to manganese toxicity by rice when compared with soybean is combined with increased oxidizing ability of its roots .