12.6.1 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 (99), 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 (100), and in ryegrass (Lolium spp. L.) grown on red-brown clayey loam and granite-mica schists in Uganda, Africa (101). Among the subtropical regions, toxicity has been reported in subtropical United States in poorly drained soils and soils on limestone (102) and on ultisols. However, the impermeability of soils does not seem essential for manganese toxicity (103). In southeastern Australia, manganese toxicity has been reported in fruit trees grown in neutral-pH duplex soils (104), in French beans (Phaseolus vulgaris L.) grown in manganese-rich basaltic soil (105), and in pasture legumes (106). 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 (107).

12.6.2 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 como-sus Merr.), potato (Solanum tuberosum L.), sugar beet, and tomato (Lycopersicon esculentum Mill.) (74,108). 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 (109) and rice (110), the tolerance to these two toxicities is opposite (111).

12.6.3 Symptoms

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 (112), internal bark necrosis in apple (113), and leaf marginal chlorosis in mustard (Brassica spp. L.) (114).

The symptoms observed include yellowing beginning at the leaf edge of older leaves, sometimes leading to an upward cupping (crinkle leaf in cotton, (115)), and brown necrotic peppering on older leaves. Other symptoms include leaf puckering in soybeans and snap bean (116); marginal chlorosis and necrosis of leaves in alfalfa, rape (Brassica napus L.), kale (Brassica oleracea var. acephala DC.), and lettuce (Lactuca sativa L.) (116); necrotic spots on leaves in barley, lettuce, and soybeans (116); and necrosis in apple bark (i.e., bark measles) (60). Symptoms in soybeans include chlorotic specks and leaf crinkling as a result of raised interveinal areas (117,118); chlorotic leaf tips, necrotic areas, and leaf distortion (102) in tobacco (Nicotiana tabacum L.).

12.6.4 Tolerance

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. (119) 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.) (120). In support of this suggestion, several other pasture species have also been reported to have a relationship between waterlogging and manganese toxicity (121,122). 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) (123). Increased tolerance to manganese toxicity by rice when compared with soybean is combined with increased oxidizing ability of its roots (124,125).

Tolerance to manganese toxicity has also been related to a reduction in the transport of manganese from the root to the shoot as shown by comparison between corn (tolerant) and peanut (Arachis hypogaea L.) (susceptible) (126,127). Furthermore, tolerance to manganese toxicity was observed in subterranean clover (compared with Medicago truncatula) and was associated with a lower rate of manganese absorption and greater retention in the roots (123). In an extensive study comparing eight tropical and four temperate pasture legume species, it was concluded that tolerance to manganese toxicity was partially attributable to the retention of excess manganese in the root system (128). This conclusion was also reached in comparing alfalfa clones that differed in manganese tolerance (129).

In rice, tolerance to high concentrations of manganese is a combination of the ability to withstand high internal concentrations of manganese with the ability to oxidize manganese, thus reducing uptake. This is in comparison with other grasses that are unable to survive the high concentrations found in rice leaves (130).

Tolerance is also affected by climatic conditions such as temperature and light intensity (131). For example, when comparing two soybean cultivars, Bragg (sensitive) and Lee (tolerant), an increase from 21 to 33°C day temperature and 18 to 28°C night temperature prevented the symptoms of manganese toxicity in both cultivars, despite the fact that manganese uptake was increased (132,133).

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