Contribution to understanding the mechanism of titanium action in plant

The biological effects of titanium (Ti) in the form of Ti(IV)-ascorbate on oats (Avena sativa L. cv. Zlatak) were studied in hydroponic experiments on defined nutrient solutions, determining the influence (i) of the chemical form of nitrogen (N) in nutrient solution on the Ti effects on plants (nitrate and an ammonium salt (acetate) were the only N species used) and (ii) of various Ti concentrations in nutrient solution while changing the magnesium (Mg) concentration on the Ti, Mg, iron (Fe), and potassium (K) content in tops and roots, top and root dry weights, chlorophyll a and b content, top height, and root length. It was found that (i) Ti was beneficial for plants grown on the nitrate-containing nutrient solutions as compared to the ammonium-containing nutrient solutions where Ti results in inhibitory effects on plant health status (decrease of top and root dry weights, chlorophyll a and b contents, top height, and root length), therefore, it was deducted that the increase of nitrate reductase activity was mainly responsible for the Ti beneficial effect on plants and (ii) Ti increased the Fe, Mg, and Ti content in plant tissues and this effect was independent of N form in the nutrient solution, chlorophyll a and b content or plant health status. It is suggested that the biological effects of Ti (the synthesis of the Ti chelating alpha-hydroxy carboxylic acids (citric and malic), and ascorbic acid, the increase of the Fe and Mg contents in plant tissues, the increased nitrate reductase activity, the increased chlorophyll a and b biosynthesis, and the effects on the other enzymatic activities) are the defense mechanisms of the plant against Ti replacing some essential elements from their binding sites (probably mainly from the phosphate-based ones). These defense mechanisms are, in the case of the usual Ti application doses, much stronger than adequate for the elimination of the Ti toxic effects (this "paradox" effect called hormesis has already been described for lead (Pb) and its effect on the hemoglobin content in blood of animals and humans).