Toxicity 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;degC day temperature and 18 to 28;degC night temperature prevented the symptoms of manganese toxicity in both cultivars, despite the fact that manganese uptake was increased (132,133).