Genotypic Difference in Molybdenum Accumulation Affects N2-fixation in Tropical Phaseolus vulgar is L.

Differences in uptake, seed storage and distribution of molybdenumwithin plants of two genotypes of Phaseolus vulgaris L. wereobserved with and without external sources of molybdenum ina glasshouse experiment. Differences in the amount of totalplant Mo translocated into seed appeared to be under genotypiccontrol, with a consistently larger proportion of total plantmolybdenum being accumulated in seed by Kabanima compared toBaseka. The uptake efficiency (total amounts accumulated withtime) did not differ between the two genotypes, but the allocationof Mo to plant parts did. Kabanima had larger nodule dry weight(30 to 40%), and nodule molybdenum content up to three timesgreater than Baseka with concurrent higher rates of N_2- fixation,particularly when plants were grown in the absence of an externalsource of Mo. Kabanima was also more efficient in translocatingMo in roots, nodules and pod walls to the seeds, particularlywhen Mo was deficient (43% of total plant Mo allocated to seedsin Kabanima compared to 31% in Baseka). Large amounts of molybdenumwere accumulated in the pod walls of Baseka plants (11 µgplant^-1) but this was not translocated to the seed.     

Root Nodules of Phaseolus: Efficient Scavengers of Molybdenum for N2-fixation

Molybdenum is thought to be of intermediate mobility in thephloem and this may limit N₂-fixation by restricting the supplyof molybdenum to the nodules of legumes. When no molybdenumwas supplied to Phaseolus vulgaris nodule Mo content increasedat the expense of shoots and roots even when seed molybdenumcontent was large. Nodules sampled from plants receiving molybdenumin the feeding solution had a concentration of 21–78 µgMo g^-1. In the absence of molybdenum and with deficient seedcontent (<0.5 µg Mo seed^-1) nodule concentrations rangedfrom 1.9 to 3.5 fig Mo g^-1 in a small seeded genotype and 8.7±0.48µg Mo g^-1 in a large seeded genotype. N₂-fixation in theseplants was not impaired except in one instance where noduleconcentration was 1.9 µg Mo g^-1. Evidence that molybdenumis effectively translocated from leaves to roots and noduleswas obtained using foliar treatments. All of the 3.3 µgMo applied to a leaf was recovered in the plant after 10 d.Mo content of the nodules increased by 81%, whilst Mo contentof shoots increased by 56%. Root Mo content was eight timesgreater than that in plants not receiving a foliar treatmentof molybdenum. We conclude that when molybdenum was scarce inthe plant it was mobile and was translocated from roots andshoots to the nodules. As a result, nodule concentrations andcontents of molybdenum were frequently maintained at amountssufficient for N₂-fixation even when the plant was entirelydependent on a small seed reserve of molybdenum.