Investigations regarding the nature of the interaction between iron and molybdenum or vanadium in nutrient solutions with and without a growing plant

Iron offset the toxicity of molybdenum or vanadium in nutrient solutions more effectively when it was supplied at the same time as the molybdenum or vanadium than when it was given separately in alternate 3-day periods. Allowing nutrient solutions of pH 4.6 containing high concentrations of iron, with or without vanadium, to stand for 4 days before use did not delay the restoration of colour to chlorotic plants, but even 2 days' standing reduced the iron content of their roots and the vanadium content of both shoot and root. The presence of vanadium had little effect on iron uptake. In parallel experiments with molybdenum, standing the solutions for 7-9 days before use delayed colour recovery, but shorter periods had no effect. Standing for 2 days or longer greatly reduced the iron content of the root, butthemolybdenum content was unaffected or increased. High molybdenum greatly increased the iron in the root, but had little effect on that in the shoot. Precipitation of iron in the nutrient solution was delayed by high concentrations of either ammonium or sodium molybdate if the initial pH was 4.6, but not if it was 6.6. Vanadium had no influence on the precipitation of iron at pH 4.6. At least part of the compensating action of iron on molybdenum or vanadium toxicity would appear to take place outside the plant.