DISTRIBUTION AND REDISTRIBUTION OF MOLYBDENUM IN BLACK GRAM (VIGNA-MUNGO L HEPPER) IN RELATION TO MOLYBDENUM SUPPLY

The effect of seven rates of molybdenum (Mo) supply on the distribution and redistribution of Mo in Vigna mungo (black gram) cv. Regur on a Mo-deficient sandy loam was examined from flower bud appearance to pod set in one experiment and during pod filling to maturity in another. At the three lowest Mo supply rates, N deficiency symptoms typical of Mo deficiency appeared, and shoot dry matter and shoot nitrogen content were depressed. Increasing Mo supply increased Mo concentrations in all plant parts but the response varied with Mo supply and with plant part. In leaf blades and petioles, Mo concentrations increased slightly when the Mo supply increased from severely deficient to deficient levels but further increases in Mo supply markedly increased the Mo concentrations, particularly in immature and recently matured leaves. In petioles, Mo concentrations generally exceeded those in the blades which they supported at all levels of Mo supply. At Mo rates greater than that required for maximum growth, Mo concentrations in basal stem segments exceeded those in petioles. Molybdenum concentrations in nodules exceeded those in above ground plant parts except at the highest level of Mo supply where the concentrations in basal stem segments exceeded those in nodules. In Mo-adequate plants, Mo contents in the trifoliolate leaves decreased with time suggesting that Mo was readily remobilized. By contrast, in stem segments at all levels of Mo supply, and in trifoliolate leaves in Mo-deficient plants. Mo contents remained constant or increased with time suggesting that Mo was not remobilized in all plant parts or at all levels of Mo supply. Thus, the results suggest that in black gram Mo was variably mobile, being phloem immobile at low Mo supply, but phloem-mobile in all plant parts with the possible exception of stem segments at adequate Mo supply. The relevance of these results for the development of plant tests for Mo deficiency diagnosis is discussed. © 1994 Annals of Botany Company.