Deficiency 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 (89–91), differences in excess cation uptake thus affecting the pH of the rhizosphere (92,93), and changes in root density (94). The genotypic variation within species for manganese efficiency can be utilized by breeding programs to develop more efficient varieties (95,96).

Tolerance to manganese deficiency may be attributed to one or more of the following five adaptive mechanisms (96):

1. Superior internal utilization or lower functional requirement for manganese.


2. Improved internal re-distribution of manganese.


3. Faster specific rate of absorption from low manganese concentrations at the root-soil interface.


4. Superior root geometry.


5. Greater extrusion of substances from roots into the rhizosphere to mobilize insoluble manganese utilizing: (i) H⁺; (ii) reductants; (iii) manganese-binding ligands; and (iv) microbial stimulants.

The importance of, and evidence for, each mechanism has been reviewed extensively by Graham (98), and so will not be re-analyzed here. It is concluded that mechanisms 1 and 2 are not important mechanisms of efficiency generally, mechanism 3 may be important in certain situations, while breeding for mechanism 4 is not thought to bring about rapid progress in improving tolerance. Mechanism 5 is thought to have some role, though this area requires further investigation.