Absorption and Mobility

Absorption Mechanisms

As mentioned previously, manganese is preferentially absorbed by plants as the free Mn²⁺ ion from the soil solution (41-43). It readily complexes with plant and microbial organic ligands and with synthetic chelates. However, complexes formed with synthetic chelates are generally considered to be absorbed more slowly by roots than the free cation (44,45).

Manganese absorption by roots is characterized by a biphasic uptake. The initial and rapid phase of uptake is reversible and nonmetabolic, with other Mn²⁺ and Ca²⁺ being exchanged freely (46,47). In this initial phase, manganese appears to be adsorbed by the cell wall constituents of the root-cell apoplastic space. The second phase is slower; manganese is less readily exchanged (48), and its uptake is dependent on metabolism. Manganese is absorbed into the symplast during this slower phase (47,48). However, the exact dependence of manganese absorption on metabolism is not clear (46,49,50).

Uptake of manganese does not appear to be tightly controlled, unlike the major nutrient ions. Kinetic experiments have estimated manganese absorption to be at a rate of 100 to 1000 times greater than the need of plants (51). This may be due to the high capacity of ion carriers and channels in the transportation of manganese ions through the plasma membrane at a speed of several hundred to several million ions per second per protein molecule (52,53).

Distribution and Mobility of Manganese in Plants

The plant part on which symptoms of Mn deficiency is observed generally indicates the mobility of the nutrient within the plant. Manganese has been reported to be an immobile element, which is not retranslocated (54-59), and consequently symptoms do not occur on old leaves. In addition, symptoms of manganese deficiency regularly appear on fully expanded young leaves rather than on the newest leaf. This symptom may indicate an internal requirement in these leaves beyond that of the new leaves (60), or it may simply be a matter of supply and demand in what is the fastest growing tissue.

The location of manganese in plants is a significant factor in the expression of deficiency symptoms and is affected by its mobility in the xylem and phloem. Manganese moves easily from the root to the shoot in the xylem-sap transpirational stream (61). In contrast, re-translocation within the phloem is complex, with leaf manganese being immobile, but root and stem manganese being able to be re-mobilized (62). The net effect of the variable phloem mobility gives rise to a redistribution of manganese in plant parts typical of a nutrient with low phloem mobility.

Studies into the mobility of manganese with wheat (Triticum aestivum L.) (63,64), lupins (Lupinus spp. L.) (55,65), and subterranean clover (Trifolium subterraneum L.) (56) have reported no re-mobilization from the old leaves to the younger ones. Further support for this lack of mobility was given in a study by Nable and Loneragan (57), in which plants provided with an early supply of ⁵⁴Mn failed to re-mobilize any of this radioactive element when their roots were placed in a solution with a low concentration of nonradioactive manganese. The apparent inconsistency with evidence that phloem is a major source of manganese from the roots and stems to developing seeds (59,66) can be explained by changes in carbon partitioning within the plant as Hannam and Ohki (67) reported a re-mobilization of manganese from the stem during the outset of the reproductive stages of plant development.