DIAGNOSIS AND PROGNOSIS OF MOLYBDENUM DEFICIENCY IN BLACK GRAM (VIGNA-MUNGO L HEPPER) BY PLANT ANALYSIS

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 Mo deficiency were obtained from the relationship between N content and Mo 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+1b) 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+1b and nodules at podding were also related to seed production at maturity. Recommended critical Mo concentrations in the YFEL+1b and nodules at initial pod set for the prognosis of Mo deficiency for seed DM were 18 and 3000 ng Mo/g DM respectively.