Uptake and Transport

Absorption as Related to Properties of Plants

The molecular basis of metal transport through membranes has been studied by several workers. Korshunova et al. (31) reported that IRT 1, an Arabidopsis thaliana Heynh (mouse-ear cress) metalion transporter, could facilitate manganese absorption by a yeast mutant Saccharomyces cerevisiae Meyen ex E.C. Hansen strain defective in manganese uptake (smfl delta). The IRT 1 protein has been identified as a transporter for iron and manganese and is inhibited by cadmium and zinc. The IRT 1 cDNA also complements a Zn-uptake-deficient yeast mutant. It is therefore suggested that IRT 1 protein is a broad-range metal-ion transporter in plants (31).

Macfie and Welbourn (10) reviewed the function of cell wall as a barrier to the uptake of several metal ions in unicellular green algae. The cell walls of plants, including those of algae, have the capacity to bind metal ions in negatively charged sites. As mentioned above, the wild-type (walled) strain of the unicellular green alga Chlamydomonas reinhardtii Dangeard was more tolerant to cobalt than a wall-less mutant of the same species. In a study to determine if tolerance to metals was associated with an increased absorption, absorbed metal was defined as that fraction that could be removed with a solution of Na-EDTA and CaCl₂. The fraction that remained after the EDTA–CaCl₂ wash was considered strongly bound in the cell. When exposed to metals, singly, in solution for 24 h, cells of both strains accumulated the metals. Significantly higher concentrations of cobalt were in the loosely bound fraction of the walled strain than in the wall-less strain.

Passive diffusion and active transport are involved in the passage of CO₂⁺ through cortical cells. A comparison of concentration of CO₂⁺ in the cytoplasm and vacuoles indicates that active transport occurs outward from the cytoplasm at the plasmalemma and also into the vacuoles at the tonoplast. Light–dark cycles play an important role in transport through the cortical cells of wheat (Triticum aestivum L.) (32). A small amount of absorption at a linear rate takes place in the waterfree space, Donnan-free space, and cytoplasm in continuous light, whereas a complete inhibition of absorption occurs during the dark periods (32). In ryegrass (Lolium perenne L.), 15% of the CO₂⁺ absorbed was transported to the shoot after 72 h. Absorption and transport of CO₂⁺ markedly increased with increasing pH of the solution, but were not affected by water flux through the plants. With 0.1 µM CO₂⁺ treatment, concentration of cobalt in the cytoplasm was regulated by an efflux pump at the plasmalemma and by an influx pump at the tonoplast. Stored cobalt in the vacuole was not available for transport (33).

Cobalt tends to accumulate in roots, but free CO₂⁺ inhibited hydrolysis of Mg-ATP and protein transport in corn-root tonoplast vesicles (34). ATP complexes of CO₂⁺ inhibited proton pumping, and the effect was modulated by free CO₂⁺. Free cations affected the structure of the lipid phase in the tonoplast membrane, possibly by interaction with a protogenic domain of the membrane through an indirect link mechanism (34).

Upward transport of cobalt is principally by the transpirational flow in the xylem (35). Usually, the shoot receives about 10% of the cobalt absorbed by the roots, most of which is stored in the cortical cell vacuoles and removed from the transport pathway (32). Distribution along the axis of the shoot decreases acropetally (36). Cobalt is bound to an organic compound of negative overall charge and molecular weight in the range of 1000 to 5000 and is transported through the sieve tubes of castor bean (Ricinus communis L.) (37). Excess cobalt leads to thick callose deposits on sieve plates of the phloem in white bean (Phaseolus vulgaris L.) seedlings, possibly reducing the transport of ¹⁴C assimilates significantly (38).

The distribution of cobalt in specific organs indicates a decreasing concentration gradient from the root to the stem and from the leaf to the fruit. This gradient decreases from the root to the stem and leaves in bush beans (Phaseolus vulgaris L.) and Chrysanthemum (39,40). No strong gradient occurs from the stem to the leaves because of the low mobility of cobalt in plants, leading to its transport to leaves in only small amounts (41,42). In seeds of lupin (Lupinus angustifolius L.), concentrations of cobalt are higher in cotyledons and embryo than in seed coats (43). The distribution depends on the phase of development of the plant. At the early phase of growth of potatoes (Solanum tuberosum L.) on lixiviated (washed) black earth, large quantities of cobalt are accumulated in the leaves and stalks (44), whereas before flowering and during the ripening of beans (Phaseolus vulgaris L.), the largest amount is in the nodules. Plant organs contain cobalt in the following increasing order: root, leaves, seed, and stems (44). During flowering, a large amount shifts to the tuber of potato and, in the case of beans, to flowers, followed by nodules, roots, leaves, and stems. Movement is more rapid in a descending direction than in an ascending one (36). The cobalt content was observed to be higher in pickled cucumber (Cucumis sativus L.) than in young fresh fruit (45). In grains of lupins (Lupinus spp. L.) and wheat, the concentration varied with the amount of rainfall and soil types (46).

Absorption as Related to Properties of Soil

Soil pH has a major effect on the uptake of cobalt, manganese, and nickel, which become more available to plants as the pH decreases. Increase in soil pH reduces the cobalt content of ryegrass (Lolium spp.) (47). Reducing conditions in poorly drained soils enhance the rate of weathering of ferromagnesian minerals, releasing cobalt, nickel, and vanadium (48). Liming decreased cobalt mobility in soil (49). The presence of humus facilitates cobalt accumulation in soil, but lowers its absorption by plants. Five percent humus has been shown to decrease cobalt content by one-half or two-thirds in cultures (50).

High manganese levels in soil inhibit accumulation of cobalt by plants (51). Manganese dioxides in soil have a high sorption capacity and accumulate a large amount of cobalt from the soil solution. Much of the cobalt in the soil is fixed in this way and is thus not available to plants (52). Water logging of the soil increases cobalt uptake in French bean (Phaseolus vulgaris L.) and maize (Zea mays L.) (53).

Accumulation as Related to the Rhizosphere

Cobalt may be absorbed through the leaf in coniferous forests, but the majority is through the soil, especially in wetlands. The physicochemical status of transition metals such as cobalt in the rhizosphere is entirely different from that in the bulk soil. A microenvironment is created around the root system (e.g., wheat and maize), characterized by an accumulation of root-derived organic material with a gradual shift from ionic metal to higher-molecular weight forms such as cobalt, manganese, and zinc. These three metals are increasingly complexed throughout the growth period. Fallow soil has been shown to complex lower amounts (6.4%) of tracers (⁵⁷Co) than cropped soil, 61% for maize and 31% for wheat (54). Cobalt has a stimulatory effect on the microflora of tobacco (Nicotiana tabacum L.) rhizosphere, shown by an intensification of the immobilization of nitrogen and mineralization of phosphorus (55). Cobalt status in moist soil from the root zone of field-grown barley shows seasonal variation, being low in late winter and higher in spring and early summer. Discrete maxima are achieved frequently between May and early July, depending on the extent of the development of the growing crop and on seasonal influences. Increased concentration may result from the mobilization of the micronutrient from insoluble forms by biologically produced chelating ligands.