Manganese (Mn) toxicity in plants is often not a clearly identifiable disorder. Symptoms of Mn toxicity as well as the concentration of Mn that causes toxicity vary widely among plant species and varieties within species, perhaps because the phytotoxic mechanisms of Mn involve different biochemical pathways in different plant genotypes. Plant availability of Mn depends on soil adsorption and on root exudates for Mn chelation or reduction. Soils with higher Mn sorption capacity have lower potential for plant absorption of Mn. Great Mn tolerance is associated with restricted absorption, restricted translocation of excess Mn to the shoots, or great tolerance to high Mn levels within the plant tissue. The ability to escape Mn toxicity through limited absorption or translocation is low in plants grown in wet soils rich in organic matter, or grown under high temperature or high light intensity. Manganese toxicity is likely with plants that are fertilized with acid‐forming fertilizers, high rates of sup...

Manganese toxicity in plants

Plant stress is a major limitation to crop yield. The amelioration of crop environments is either impossible or costly. The breeding of crop varieties resistant to environmental stress is the most effective economical means to improve and stabilize yield under conditions of stress. Crop resistance to various environmental stresses is being improved by traditional and costly breeding methods that involve the stability of yield performance over different environments as a major criterion. Recent advances in stress physiology allow embarkation upon breeding programs that employ distinct physiological selection indices for stress tolerance. Using physiological selection indices in breeding work requires the definition of the importance and effectiveness of given physiological attributes under stress conditions, the design of a proper selection scheme within the logistic framework of the breeding program and the development of rapid and effective selection techniques. These requirements, the progress made and ...

Breeding crop varieties for stress environments

Manganese, a group VII element of the periodic table, plays an important role in biological systems and exists in a variety of oxidation states. The normal level of Mn in air surrounding major industrial sites is 0.03 μg/m3, in drinking water 0.05 mg/liter and in soil between 560 and 850 ppm. Manganese is an essential trace element for higher plant systems. It is absorbed mainly as divalent Mn2+, which competes effectively with Mg2+ and strongly depresses its rate of uptake. The accumulation of Mn particularly takes place in peripheral cells of the leaf petiole, petiolule and palisade and spongy parenchyma cells. Mn is involved in photosynthesis and activation of different enzyme systems. Mn deficiency may be expressed as inhibition of cell elongation and yield decrease. Mn toxicity is one of the important growth limiting factors in acid soils. Plant tops are affected to a greater extent than root systems. The toxicity symptoms are, in general, similar to the deficiency symptoms. Toxic effects of Mn on plant growth have been attributed to several physiological and biochemical pathways, although the detailed mechanism is still not very clear. Higher O2 uptake and loss of control in Mn activated enzyme systems have been associated with Mn toxicity. Mn interferes with the uptake, transport and use of several essential elements including Ca, Fe, Cu, Al, Si, Mg, K, P and N. Excess of Mn reduces the uptake of certain elements and increases that of others. pH plays an important role in Mn uptake. Acidic pH causes a lack of substantial amount of nitrate as an alternative electron acceptor and leads to a high amount of Mn in leaves. High microbial activity, water logging and poorly structured soils cause severe Mn toxicity even in neutral soils. The molecular mechanism of Mn-tolerance is not yet clear. The level of tolerance is different in different species and seems to be controlled by more than one gene. Further information is required on the factors affecting the distribution, accumulation and membrane permeability of the metal in different plant parts and different species. Understanding of the genetic basis of Mn-tolerance is necessary to improve adaptation of crops against acid soils, water logging and other adverse soil conditions.

Manganese in cell metabolism of higher plants

Acid soil (A1) tolerance in certain wheat (Triticum aestivum L.) genotypes has been associated with high protein levels in the grain. Superior Al tolerance in some wheat genotypes is also characterized by the ability to use NO‐ 3‐N in the presence of NH+ 4‐N and to increase the pH of the growth medium. Such evidence suggests that nitrogen metabolism is involved in differential A1 tolerance. In the absence of mineral stress, UC 44–111 wheat is significantly higher in nitrate reductase (NR) activity than Anza wheat. Our objectives were to test the hypothesis that differential NR activities in these two genotypes are associated with differential Al tolerance and to determine the specific effects of Al on their NR systems. The high NR UC 44–111 genotype was significantly more tolerant to an Al‐toxic Tatum soil at pH 4.5 and 4.9 and to Al in nutrient cultures than was the low NR Anza. The UC 44–111 also induced a higher pH in nutrient solution than did Anza when both were under Al stress. Greater sens...

Aluminum tolerances of two wheat genotypes related to nitrate reductase activities

Investigations of Al phytotoxicity, including the identification of the Al species responsible for toxicity, require a rapid assay procedure employing very low concentrations of Al and a chemically simple rooting medium Root elongation in newly germinated red clover (Trifolium pratense L cv Kenland) was inhibited by submicromolar concentrations of Al Ca2+ at concentrations of at least 02 mM was essential for optimal elongation in control seedlings Ca2+ also relieved Al toxicity with the net effect that maximum reduction of elongation by 1 μM Al was achieved at 02 mM Ca2+ Elongation in control seedlings was at least 90% of maximum from pH 45 to 57 Increases in pH relieved Al toxicity so that maximum sensitivity to 1 μM Al occurred at pH 47 As a consequence of these experiments and other considerations we chose for our basic assay a medium composed of 02 mM CaSO4 adjusted to pH 45 with H2SO4, variously supplemented with Al2(SO4)3



Day-old seedlings were incubated in this aerated medium in the dark at 23°C for one day No additions of other solutes increased the sensitivity of the assay, but amelioration of Al toxicity was effected by Mg2+, F-, phosphate and citrate Increases in ionic strength per se had comparatively little effect on the toxic effects of Al Two barley cultivars (Hordeum vulgare L cv Dayton and Kearney) and two wheat cultivars (Triticum aestivum L cv Hart and Thorne) known to differ in sensitivity to Al were reliably separated at submicromolar Al concentrations by the assay procedure, which was slightly modified Suggestions for the improvement of the assay and for applications to future research are offered

A rapid assay for aluminium phytotoxicity at submicromolar concentrations

Adaptation of Cotton Genotypes to an Acid, Manganese Toxic Soil1

Adaptation of Cotton Genotypes to an Acid, Aluminum Toxic Soil

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Adaptation of Cotton Genotypes to an Acid, Manganese Toxic Soil1

Adaptation of Cotton Genotypes to an Acid, Aluminum Toxic Soil