PHOSPHORUS DEFICIENCY ENHANCES MOLYBDENUM UPTAKE BY TOMATO PLANTS

Water culture experiments are described which provide conclusive evidence that Mo uptake by tomato plants is markedly enhanced by P deficiency. In a longterm experiment, which ran for 11 days, in marked contrast to the uptake of other nutrients, a three fold higher Mo uptake rate was observed after only four days of withdrawal of P from the nutrient medium. In contrast to the gradual increase in pH of the nutrient medium of the plants supplied with P, the pH in the medium of the -P plants fell. Throughout the growth of these plants net H+ efflux could be accounted for by excess cation over anion uptake, indicating that organic acid extrusion plays no major role in the observed fall in pH. Further evidence that Mo uptake is enhanced in P deficient tomato plants is provided in short-term nutrient solution experiments (lh and 4h) using radioactive molybdenum (Mo-99). Compared with P sufficient plants, the uptake rates of Mo-99 by P deficient plants were three to five times higher after 1h and nine to twelve times higher after 4h. Resupplying P during the uptake periods to deficient plants reduced the uptake rate of Mo-99 to values similar to those of P sufficient plants. It is concluded that the uptake of molybdate occurs via phosphate binding/transporting sites at the plasma membrane of root cells. Further support for this conclusion comes from exchange experiments with non-labelled molybdenum, which show a much larger amount of Mo-99 exchangeable from the roots of P deficient plants.