The role of cobalt (Co) as an essential nutrient in plants has not been demonstrated, despite being an essential element for animals and prokaryotic organisms, such as blue-green algae (cyanobacteria). However, Co promotes growth in some plant species by enhancing the growth of symbiotic root organisms. Therefore, Co has been considered a beneficial element for plants, especially for legumes, for almost 60 years. Co is absorbed as Co2+ ion by the roots and is transported by the transpiration flow, accumulating in the margins and tips of the leaves. It has greater mobility when it is complexed in organic substances. Over time, Cobalt-based products such as Cobalt sulfate heptahydrate CAS# 10026-24-1 began to flood the market.
Functions of cobalt
The role of Co in plants is still poorly understood. Some effects of this element that have been observed are: 1) delay in leaf senescence, 2) increase in seed resistance to drought, 3) regulation of alkaloid accumulation in medicinal plants, and 4) blocking of the ethylene synthesis. Despite these effects, the best-known function of Co is as a constituent of the cobalamin coenzyme (vitamin B12 and derivatives), on which different enzymes depend for nitrogen (N2) fixation by bacteria of the genera Rhizobium and Bradyrhizobium that live in legume root nodules.
Cobalamin helps in the synthesis of leghemoglobin, a protein that helps protect the enzymes involved in N2 fixation, which are sensitive to oxygen (O2). Co has chemical properties similar to nickel (Ni2+) and is even absorbed within cells through the same channels. Based on the dry weight of the plants, Co is found in concentrations of 0.1 to 10 mg/kg in plants, but in legumes (Co accumulator plants) they can contain over 1000 mg/kg per plant.
Effects of cobalt deficiency
Co deficiency affects the development and function of root nodules because methionine synthesis is decreased, which leads to low protein synthesis. Leguminous plants grown in soils with low levels of Co, have less activity in their nodules compared to those that were provided with this element. Low nodule activity also reduces nitrogenase activity or N content in plants. Similarly, Rhizobium infection is often lower than in plants supplied with Co and the onset of N fixation is delayed by several weeks. In legumes, Co deficiency is associated with N deficiency symptoms (yellowing and growth retardation), since these depend on N2 fixation. Under Co-deficient soils, the concentration of the element in the plants depends on the species and the place where it is grown, since the concentration in the seeds varies within the same species when grown elsewhere.
Seed treatment with Co is an effective procedure to help fix N2 and the growth of legumes grown in soils deficient in this element. The response to field fertilization with Co in legumes is rare, but it has been demonstrated in sandy soils. On the other hand, foliar applications can be effective but their combined effect with seed treatment is better. In forage plants, Co application not only increases nitrogen fixation but also improves its nutritional quality. Fine-textured (clay) soils usually have a higher Co content than coarse-textured (sandy) soils. The dose of Co necessary to improve the growth of legumes is low and ranges from 40 to 140 g/ha. However, the critical Co concentration needed in soils to meet plant needs varies between species. Co doses from either soil applications, foliar applications, or seed treatments are very low, so additional research is needed to improve understanding of Co behavior.