EFFECT OF SILICON SUPPLY ON GROWTH AND MALATE- METABOLIZING ENZYMES IN Mn-TREATED PEA PLANTS

The effects of exogenously applied silicon (Si) on plant growth and the isoform patterns of NAD-dependent malate dehydrogenase (NAD-MDH, EC 1.1.1.37) and NADPdependent malic enzyme (NADP-ME, EC 1.1.1.40) were studied in two pea genotypes (cultivar Auralia and mutant line 1/220) subjected to differential Mn treatments (50, 200 and 500 μM Mn) under control conditions. The excess Mn inhibited plant growth more strongly in the mutant line than in the cultivar Auralia referred to as Mn sensitive and Mn tolerant, respectively. The difference in Mn sensitivity of pea plants was related to the higher activity of the isoforms of NADP-ME and NAD-MDH in the Mn-tolerant cultivar Auralia. Mn treatment increased the activity of two isoforms of NADP-ME and one isoform of NAD-MDH in the leaves of both pea genotypes. The addition of Si improved plant growth in Mn-treated plants and its protective influence was more effective in the Mn-sensitive mutant line than in the Mn-tolerant cultivar Auralia. The alleviation of Mn toxicity by Si was related to decreased activity of the isoforms of NAD-MDH and NADPME, thus pointing to normalization of the redox status in Mn-exposed pea plants. This response was stronger in the tolerant cultivar. Noticeably, when applied to control plants Si had no effect on all parameters measured; it exerted its protective effect only in Mn-treated plants. Our data suggest a role of malate-metabolizing enzymes in Si-induced defense responses to excess Mn and in the difference in sensitivity of the two pea genotypes to Mn.