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Mélica nutans (tolerant)

Table 2.1 (continued)

Enzyme

Metabolic process

Metal Enzyme activity Type of interaction

Plant species

Cu-Zn superoxide dismutase Mn-superoxide dismutase Peroxidase

Protease Catalase

Destruction of superoxide Cd ions Pb

Destruction of superoxide Cd ions

Polyphenol oxidation Cd with Hj O2

Protein hydrolysis Cd

Destruction of H2O2 Pb

Zinc deficiency

Phaseolus vulgaris Lupinus luteus Phaseolus vulgaris

Phaseolus vulgaris Letrina sp„ Oryza sativa Hordeum vulgare Lemna sp„ Oryza sativa Zea mays

Note: I Indicate decrease in activity, f indicate increase in activity

Similarly if the heavy metal ions interact with native proteins, it may denaturate and change their structures. Since heavy metals change the protein structure hence it does not function properly and may cause toxicity to that particular cell.

They also check uptake mechanism of both cations (k+, Ca2+, Mg2+, Mn2+, Zn2+, Cu2+ and Fe3+) and anions (NO-) by affecting the absorption of other ions via diverse mechanisms.Their relative inputs differ in various cases, therefore we observe variations within different plant species. The two well-known mechanisms involved in the decrease of macro- and micronutrient uptake by heavy metals are; physical and chemical mechanism depending on the size of metal ion radii such as competition between Cd2+ and Zn2+ and Cd2+ and Ca2+, and metal-induced disorder in the cell metabolism leading to the changes in the membrane enzyme activity and membrane structure. For example, Cd2+ drastically changes the lipid composition of membranes and increases the contents of palmitic as well as linoleic and linolenic acids, but all classes of lipids decrease (Ouariti et al. 1997a). The overall changes in membrane permeability and inhibition of membrane enzyme could shift the ionic balance in cytoplasm.In the same way uptake of nitrate declines, when exposed to the heavy metals, resulting in lower nitrate reductase activity and disturbed nitrogen metabolism (Burzynski and Grabowski 1984; Hernandez et al. 1996; Ouariti et al. 1997b). Notable changes in ionic balances are observed in various plant species and their tissues.

It has also been reported that under heavy metal stress conditions transpiration rate and water content in treated plants declines. This process involves various mechanisms (Fig. 2.1) such as; reduction in the area of leaves due to growth retardation, smaller guard cells, decrease in the contents of the compounds maintaining cell turgor and cell wall plasticity thus leading to growth inhibition, increase in the

Fig. 2.1 Effects of Cd and Pb on photosynthesis, respiration and water uptake. Regime 1-concem only Cd; 2-concern only Pb; (—) Decrease, Inhibition; (+) Increase, Activation

Abscisic acid (ABA) content thus inducing stomatal closure, disordered respiration and oxidative phosphorylation which cause a disarray in the plant water regime. During the effects on the ABA metabolism, Cd2+ promotes the expression of ltp gene in the epidermis encoding the proteins for nonspecific lipid transfer. The latter effect leads to the accumulation of monomers arriving at the site of cutin synthesis and increase in the cuticle thickness, thus hindering transpiration (Hollenbach et al. 1997). Moreover, the water stress induced by heavy metals promotes superproduction of proline, an osmoregulating antioxidant and stress-protecting substance (Kuznetsov and Shevyakova 1999).

At a concentration of about 1 mM, Cd2+ reduces oxygen consumption by roots and tobacco cell-suspension culture. Dithiothreitol, a SH-agent, alleviated Cd2+ exerted inhibition of mitochondrial respiration and restrained their swelling. Presumably this heavy metal inhibits the transport of electrons and protons in the mitochondria and thus disorganizes the electron transport chain and remarkably affecting ATP formation. Using the labeled glucose, Reese and Roberts (1985) have demonstrated that heavy metals do not notably affect the glycolysis and the pentose phosphate pathway but considerably inhibit succinate oxidation via the Krebs cycle.

The distorted chloroplast ultrastructure generally leads to a decline of the photosynthetic rates due to restrained synthesis of chlorophyll, plastoquinone, and carotenoids; the obstructed electron transport; an inhibition in the enzyme activities of the Calvin cycle; and CO2 deficiency due to stomatal closure (Fig. 2.1). Heavy metal ions change the lipid composition of thylakoid membranes. Lower chlorophyll content is a typical effect of Cd2+ and Pb2+; in particular, chlorophyll b is more affected than chlorophyll a, apparently due to the inhibition of chlorophyll-synthesizing enzymes and the lack of Mg and Fe. The effect of one and the same metal concentration on chlorophyll content varies with the plant species. The inhibition of chlorophyll synthesis by heavy metals is often manifested as chlorosis. Cd2+ also restricts the PSII-related electron transport, probably as a result of the structural and functional changes in thylakoid membranes, the reduced ferredoxin-NADP+ oxido-reductase activity, and arrested plastoquinone synthesis.

Heavy metals produce chromosomal aberrations as well as mitotic disarrays, such as C-mitoses, resulting in a higher metaphase percentage, just like the weak effect of colchicine. When Wierzbicka (1994) followed C-mitoses in onion roots, the maximum percentage of C-metaphases was observed between 6 and 10.5 h of exposure, in the interval corresponding to the minimum mitotic index (MI), then the percentage of C-metaphases decreased. Thus, the highest level of C-metaphases is correlated with the drop in MI. The lower numbers of prophases and telophases and higher number of metaphase can be correlated with the longer mitosis.

The inhibition of cell division by heavy metals may involve different mechanisms. It is not yet clear whether or not the latter include the direct metal-DNA interactions. Though the possibility of direct interaction between metal ion and DNA has been demonstrated experimentally (Alex and Dupois 1989), it is not clear whether such ions at low concentrations can reach the nucleus. Moreover, mitoses may be affected by interactions of metals with SH-group of proteins, disruption of cell metabolism and GA functions, etc.

Diverse mechanisms are involved in a decline in the rates of cell division and elongation in the roots affected by heavy metals. These mechanisms include direct binding to DNA, metal-induced aberrations, expansion of the mitotic cycle, inhibition of microtubule development, decrease in cell wall plasticity, and reduction of the glutathione pool (Fig. 2.2). Many substances inhibit cell division and elongation, and, in this case, the two processes do not considerably differ in their sensitivity towards the inhibitory agent. The toxic effects of Cd and Pb on cell division and elongation are typical of other metals, while the alternative stress factors produce other mitotic disorders. The specific responses to heavy metals in diverse plant tissues and species depend on the extent of disorder and the capacity to synthesize metal-binding chemicals and in this way to eliminate the absorbed heavy metals from the active metabolism.

Cell division in quiescent center

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