Beneficial Effects of Cobalt on Plants

Senescence

Senescence in lettuce leaf in the dark is retarded by cobalt, which acts by arresting the decline of chlorophyll, protein, RNA and, to a lesser extent, DNA. The activities of RNAase and protease, and tissue permeability were decreased, while the activity of catalase increased (84). Cobalt delays ageing and is used for keeping leaves fresh in vetch (Vicia spp.) (85). It is also used in keeping fruits such as apple fresh (86).

Drought Resistance

Presowing treatment of seeds with cobalt nitrate increased drought resistance of horse chestnut (Aesculus hippocastanum L.) from the Donets Basin in southeastern Europe (87).

Alkaloid Accumulation

Alkaloid accumulation in medicinal plants such as downy thorn apple Datura innoxia Mill. (88), Atropa caucasica (89), belladonna A. belladonna L. (90), and horned poppy Glaucium flavum Crantz (91) is regulated by cobalt. It also increased rutin (11.6%) and cyanide (67%) levels in different species of buckwheat (Fagopyrum sagittatum Gilib., F. tataricum Gaertn., and F. emargitatum) (89,92).

Vase Life

Shelf and vase life of marigold (Tagetes patula L.), chrysanthemum (Chrysanthemum spp.), rose (Rosa spp.), and maidenhair fern (Adiantum spp.) is increased by cobalt. Cobalt also has a longlasting effect in preserving apple (Malus domestica Borkh.). The fruits are kept fresh by cobalt application after picking (86,93–96).

Biocidal and Antifungal Activity

Cobalt acts as a chelator of salicylidine-o-aminothiophenol (SATP) and salicylidine-o-aminopyridine (SAP) and exerts biocidal activity against the molds Aspergillus nidulans Winter and A. niger Tiegh and the yeast Candida albicans (97). Antifungal activities of CO₂⁺ with acetone salicyloyl hydrazone (ASH) and ethyl methyl ketone salicyloyl hydrazone (ESH) against A. niger and A. flavus have been established by Johari et al. (98).

Ethylene Biosynthesis

Cobalt inhibits IAA-induced ethylene production in gametophores of the ferns Pteridium aquilinum Kuhn and sporophytes of ferns Matteneuccia struthiopteris Tod. and Polystichum munitum K. Presl (99); in pollen embryo culture of horse nettle (Solanum carolinense L.) (100); in discs of apple peel (101); in winter wheat and beans (102); in kiwifruit (Actinidia chinensis Planch) (103); and in wheat seedlings under water stress (104). Cobalt also inhibits ethylene production and increases the apparent rate of synthesis of peroxides and prevents the peroxidative destruction of IAA. Other effects include counteraction of the uncoupling of oxidative phosphorylation by dinitrophenol (4).

Cobalt acts mainly through arresting the conversion of methionine to ethylene (105) and thus inhibits ethylene-induced physiological processes. It also causes prevention of cotyledonary prickling- induced inhibition of hypocotyls in beggar tick (Bidens pilosa L.) (106), promotion of hypocotyl elongation (107), opening of the hypocotyl hook (bean seedlings) either in darkness or in red light, and the petiolar hook (Dentaria diphylla Michx.) (108,109). Cobalt has also been noted to cause reduction of RNAase activity in the storage tissues of potato (110), repression of developmental distortion such as leaf malformation and accumulation of low-molecular-weight polypeptides in velvet plant (Gynura aurantiaca DC) (111), delayed gravitropic response in cocklebur (Xanthium spp.), tomato and castor bean stems (112), and prevention of 3,6-dichloro-o-anisic acid-induced chlorophyll degradation in tobacco leaves (73). Prevention of auxin-induced stomatal opening in detached leaf epidermis has been observed (85). The effects of ethylene on the kinetics of curvature and auxin redistribution in the gravistimulated roots of maize are known (113). ⁶⁰Co γ-rays and EMS influence antioxidase activity and ODAP content of grass pea (Lathyrus sativus L.) (114).

Nitrogen Fixation

Cobalt is essential for nitrogen-fixing microorganisms, including the cyanobacteria. Its importance in nitrogen fixation by symbiosis in Leguminosae (Fabaceae) has been established (115–119). For example, soybeans grown with only atmospheric nitrogen and no mineral nitrogen have rapid nitrogen fixation and growth with 1.0 or 0.1 µg Co ml^-1, but have minimal growth without cobalt additions(4).