The effect of molybdate and tungstate in the growth medium on abscisic acid content and the Mo-hydroxylases activities in barley (Hordeum vulgare L.)

The influence of molybdate and tungstate in the nutrient growth medium on the activities of the molybdo-enzymes, aldehyde oxidase (AO) and xanthine dehydrogenase (XDH) in barley (Hordeum vulgare L.) plants was studied. Root AO and XDH activities were enhanced with increasing concentrations of molybdate in the nutrient medium, while AO and XDH activities in leaves were not significantly affected. Enzyme activities in root extracts rose slightly as the tungstate concentration was increased to 1 μM, but further elevation of tungstate concentration resulted in a reduction of AO and XDH activities. Leaf AO and XDH activities decreased with the addition of increasing amounts of tungstate in the medium. The general pattern of shoot ABA changes in response to exogeneously added molybdate and tungstate correlated with AO and XDH activity levels in roots. Shoot ABA concentration rose with increasing molybdate concentrations. Unexpectedly, similar results were obtained for exposure to tungstate, with the highest ABA level at metal concentration of 1 μmol/L. ABA contents in both the leaves and the roots declined substantially when plants were grown in media containing >10 μmol/L tungstate. The increased shoot ABA levels in response to molybdate and tungstate and the simultaneous enhancement of root AO and XDH activities, indicate that molybdate and tungstate influence ABA biosynthesis via regulation of root Mo-hydroxylases activities.