Molybdenum nutrition of black gram (Vigna mungo L. Hepper)

Black gram (Vigna mungo L.Hepper) is an important legume crop in India, Burma, Bangladesh, Pakistan and Thailand and of minor, bur increasing importance in Sri Lanka and Australia. Although molybdenum (Mo) deficiency in black gram has been observed in the field, plant tests for the diagnosis of Mo deficiency have not been established. Accordingly, a series of five experiments was undertaken in a glasshouse to investigate the following aspects of the Mo nutrition of black gram on an acid sandy loam soil pH 4.3 (5 m M CaCl2) : a). The critical Mo concentrations for diagnosis and prognosis of Mo deficiency, b). The distribution and redistribution of Mo within the plant in relation to Mo supply, and c). The effect of Mo deficiency on nodule development. Critical Mo concentrations for the diagnosis of Mo deficiency were much higher in the nodules than leaves, and among young leaf blades, they increased with decreasing leaf age. For diagnosis of Mo deficiency, the blades of the leaf immediately older than the youngest fully expanded leaf (YFEL+ lb) and nodules were recommended plant parts. For YFEL+ lb, the critical concentrations were 18-22 ng Mo/g dry matter at early flowering, initial pod set and pod filling. For nodules, the critical concentrations were 9600 at early flowering and 3200 ng Mo/g dry matter at early pod filling. Since the critical Mo concentrations in the nodules declined with plant age, the recommended critical values should only be used for plants at defined growth stages. Molybdenum concentrations in the YFEL+ lb and the nodules at initial pod set in one experiment were related to seed production at maturity. Recommended critical Mo concentrations in the YFEL+ 1 band nodules, at initial pod set, for the prognosis of Mo deficiency for seed dry matter were 18 and 3000 ng Mo/g dry matter, respectively. The mobility of Mo in the phloem appeared to vary with leaf age and the external Mo supply. For example, in marginally Mo-deficient plants, there was no net redistribution of Mo from the unifoliolate leaves but there was net redistribution of Mo from the youngest fully expanded leaf blades. With minus Mo supply, there was no net redistribution of Mo from the youngest fully expanded leaf blades in the severely Mo­deficient plants but in Mo-adequate plants Mo was readily lost from the youngest fully expanded leaves. In nodulated Mo-deficient plants, roots were the major source of Mo for reproductive plant parts between full nowering and late pod fill. After late pod filling, nodules became the major source of Mo for seed fill. The other plant parts which were significant sources of Mo for seed were young stems and pod walls. By contrast, in nodulated Mo-adequate plants, the major sources of Mo were young leaves and young stems; whereas nodules, roots and old leaves were less important sources. In nodulated Mo-deficient plants supplied with NH4N0:3, the young leaves and young stems were the major sources of Mo for reproductive plant parts between early pod setting and late pod filling. After late pod filling, the roots, young stems and young leaves were the major sources of Mo for seed fill. Supplying NH4N0:3 from full nowering had little effect on Mo distribution or redistribution in black gram plants grown with external Mo supply. However, in plants grown without Mo supply, supplying NH4N0:3 from full nowering accelerated the loss of Mo from mid stems and mid leaves compared to symbiotically dependent plants, and there was earlier remobilization of Mo from stems and leaves than in plants only reliant on symbiotic N2 fixation. Supplying NH4N0:3 at full flowering also induced loss of Mo from mid stem leaves of plants grown without Mo supply whereas no such loss occurred in plants reliant on symbiotic N2 fixation. Supplying NH4N0:3 to black gram plants at either sowing or at full flowering appeared to stimulate the remobiliz.aLion of Mo. The role of Mo in root nodule development is not clear since Mo deficiency has been reported to produce either an increase or a decrease in nodule number. Low levels of Mo in seed had no effect on the number of nodule initials and nodules emerged at 11 days after sowing. However, the onset of acetylene reduction activity on plants grown from low Mo seed was delayed by two days. It is hypothesized that the delay in N2 fixation was due to slower incorporation of Mo of soil origin into nitrogenase. It is further suggested that the supply of essential metabolites to the nodules on plants from low Mo seed was restricted and this resulted in the slower maturation of early initiated nodules and the repression of formation of new nodules. The major findings of the present study were: (a) the mobility of Mo in the phloem varied with leaf age and the external Mo supply, (b) the blade of the leaf immediately older than the youngest fully expanded leaf and the nodules were suitable plant parts for diagnosis and prognosis of Mo deficiency, (c) in Mo-deficient plants, the roots and the nodules were significant sources of Mo for seed fill; whereas, in Mo­adequate plants, uptake from the soil and remobilization from mid leaves were the main sources of Mo for seed fill, and (d) early nodule development was not affected by the Mo supply from the seed