Effect of fertilization on exudation, dehydrogenase activity, iron-reducing populations and Fe++ formation in the rhizosphere of rice (Oryza sativa L.) in relation to iron toxicity

To explain the mechanism of iron toxicity, greenhouse and growth chamber (14CO2 atmosphere) experiments were carried out. In pot experiments (with a typical iron-toxic soil and a fertile clay) we studied the effect of N, P, K and Ca+Mg fertilization (alone or in combination) on dehydrogenase activity, Fe++ formation, and the populations of iron-reducing bacteria in the rhizosphere of rice IR22 and IR42. Fe uptake by the plants was measured at regular intervals. Dehydrogenase activity, the number of N2-fixing iron-reducing bacteria, and the formation and uptake of Fe++ decreased with increased supply of K, Ca, and Mg. This effect was clearer with IR22 (susceptible to iron toxicity) than with IR42 (releatively tolerant). Increased exudation and Fe uptake by IR36 at low nutrient and high Fe supply were recorded in a growth chamber experiment. Nutritional conditions, exudation rate (a measure of metabolic root leakage), the iron-reducing activity of the rhizosphere, and Fe++ uptake by wetland rice appear to be clearly related. Iron toxicity is considered a physiological disorder caused by multiple nutritional soil stress rather than by a low pH and high Fe supply per se.