In legumes, both increases and decreases in nodule number in response to Mo deficiency have been reported, but reasons for the different responses have not been proposed. The present study examined nodule initiation and development in black gram seedlings using two levels of seed Mo to induce Mo deficiency. In the first 11 days after inoculation, low levels of Mo in seed had no effect on nodule initiation or the number of nodules. At 13 days after inoculation, low Mo in seed depressed bacteroid concentration, leghaemoglobin concentration, nodule number and nodule fresh weight. Acetylene reduction activity was delayed by 2 days in plants grown from low Mo seed. We suggest that the delay in N 2 fixation in plants grown from low Mo seed was due to slower incorporation of Mo of soil origin into nitrogenase. We further suggest that restricted supply of essential metabolites to the nodules on plants from low Mo seed resulted in the slower maturation of early initiated nodules and the repression of formation of new nodules.
Diagnosis and prognosis of molybdenum (Mo) deficiencies in black gram crops by plant analysis is difficult because Mo standards have not been set and tested in the field. Therefore, critical Mo concentrations, for the diagnosis of Mo deficiency at early flowering and for diagnosis and prognosis at pod filling in black gram, were determined in two glasshouse experiments by examining the relationship of Mo concentrations in young leaves and nodules to shoot nitrogen content or seed dry matter in plants treated with seven levels of Mo supply on a Mo-deficient sandy loam. In severely Mo-deficient plants, shoot dry matter (DM) and shoot nitrogen (N) content were depressed. Molybdenum concentrations in plant parts increased with increasing Mo supply and were closely related to shoot N content. shoot DM, and seed DM. Critical Mo concentrations for diagnosis of hi0 deficiency were obtained from the relationship between N content and &lo concentrations in leaves and nodules. and for prognosis of Mo deficiency were obtained from the relationship between seed yield and Mo concentrations in plant parts. Critical Mo concentrations were much higher in nodules than in leaves, and among young leaf blades, they increased with decreasing leaf age. For diagnosis of Mo deficiency, blades of the leaf immediately older than the youngest fully expanded leaf (YFEL+lb) and nodules are recommended plant parts. Their respective critical concentrations were 22 and 9600 ng Mo/g DM at flowering, and 22 and 3378 ng Mo/g DM at initial pod set. Molybdenum concentrations in the YFEL+lb and nodules at podding were also related to seed production at maturity. Recommended critical h10 concentrations in the YFELflb and nodules at initial pod set for the prognosis of IbIo deficiency for seed DM were 18 and 3000 ng Mo/g DM respectively.
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