Calcium modifies Cd effect on runner bean plants
Introduction
Calcium is a well-known component of signal transduction pathways. It can activate and regulate many processes in eukaryotic cells. Small intracellular changes of Ca concentration can regulate such processes as elongation and division of cells (Hepler and Wayne, 1985), the metabolism (Kauss, 1987, Bush, 1995), transport and secretion (Bush, 1995, Marschner, 1995). Although Ca can constitute more than 10% of dry weight of mature leaves, without showing distinct symptoms of toxicity or inhibition of plant growth (Marschner, 1995), only its small part defined as ‘free cytosolic calcium’ participates in regulation of metabolism, growth and development of plants. Calcium is taken up by plants passively and only by root tips (Shuman, 1994). However, in general, the mechanisms of trans-plasmamembrane metal transport in higher plants are poorly understood (Ernst et al., 1992). Moreover, transport of Ca can be competitively impeded by other elements, especially by univalent cations. Attention is also given to the fact that this element can be displaced by other ions, for example, by Cd ions, which are toxic to plants. Cadmium is taken up by plants, using the paths of Ca transport. It is therefore reasonable to suppose that the presence of Cd in the medium can change Ca level in plant cells and thereby influence the processes of plant growth and metabolism. Attention was drawn earlier by Greger and Lindberg (1987)to the fact that effects of toxic Cd action, found in sugar beet roots, resemble symptoms of Ca deficit. Although toxic effects of Cd on plants are well documented (Barceló and Poschenrieder, 1990, Krupa and Baszyński, 1995, Prasad, 1996) the mechanism of its cellular action is still unclear.
From our earlier studies it is known that the stage of plant growth differs in the sensitivity during Cd treatment and Cd exposure time (Skórzyńska-Polit and Baszyński, 1995, Skórzyńska-Polit and Baszyński, 1997). We observed disturbances mainly in growth processes of young bean plants and changes in the activity and efficiency of the photosynthetic apparatus in older plants (Skórzyńska-Polit and Baszyński, 1997). Taking into account these results and the significance of Ca to plants we considered that disturbances in calcium management influenced by Cd are possible. Searching for elucidation of the mechanism of Cd action on bean plants at their different growth stages we hypothesized that a lower or higher amount of Ca than normal, present in the Knop solution can either enhance the toxic effect of Cd on plants or diminish it.
Changes in plant metabolism resulting from disturbances in supply of mineral elements may induce generation of reactive oxygen species. Excess of free radicals in plant cells leads to oxidative stress. Its effects can be reduced through the mediation of enzymatic and nonenzymatic antioxidants (Scandalios, 1990, Foyer et al., 1994), among which is GSH or hGSH. According to this assumption, in plant cells there is a significant increase in the level of thiol compounds under stress conditions and their utilization for diminishing the negative effects of stress (Walker and McKersie, 1993). We hypothesized that different concentrations of Ca in the medium are responsible for changes of hGSH level as an antioxidant factor in plant cells and utilization of this tripeptide in the synthesis of Cd-binding hPCs.
Section snippets
Plant material
Runner bean plants (Phaseolus coccineus L., cv. Piękny Jaś) were germinated from seeds on wet filter paper in a thermostated darkened chamber (23°C, 95% relative humidity). From day 5 the seedlings were cultivated hydroponically (five plants per pot, three pots per treatment) in aerated modified Knop nutrient solution. The nutrient solution was composed of (mg l−1): Ca(NO3)2·4H2O-1000; KH2PO4-280; MgSO4·7H2O-250; KCl-120; H3BO3-0.2; MnSO4-0.5; CuSO4-0.05; ZnSO4-0.1; (NH4)6Mo7O24·4H2O-0.025.
Responses of young plants to Cd/Ca interaction
Changes of the Ca content in the nutrient solution resulted in an accumulation of this element in the various organs of bean plants (Table 1). Cadmium treatment of younger plants growing in the nutrient solution with a low and high level of Ca caused changes in Ca accumulation and its distribution among the plant organs. In plants growing at a low level of Ca its accumulation increased in roots after 12-day Cd exposure. The roots of Cd-treated plants growing in the nutrient solution with a high
Discussion
The similarity of the physical properties of the ions (a valence, ionic radius) present in the medium can cause disturbances in their uptake and distribution in plants. The possibility of Cd/Ca interaction was given attention by Greger and Lindberg (1987), suggesting that Cd presence in the medium facilitated Ca transport to shoots. Thus, it seemed to us probable that a different calcium content in the medium (essentially not influencing the growth and development of control plants) could
Acknowledgements
This paper is a part of a project supported by the State Committee for Scientific Research (KBN) grant No. 6P04C.037.09. It was also supported by The Swedish Institute and The Swedish Natural Science Research Council.
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