Root nodules of Phaseolus: efficient scavengers of molybdenum for N2-fixation
1991
Brodrick, S.J. | Giller, K.E.
Molybdenum is thought to be of intermediate mobility in the phloem and this may limit N2-fixation by restricting the supply of molybdenum to the nodules of legumes. When no molybdenum was supplied to Phaseolus vulgaris nodule Mo content increased at the expense of shoots and roots even when seed molybdenum content was large. Nodules sampled from plants receiving molybdenum in the feeding solution had a concentration of 21-78 microgram Mo g-1. In the absence of molybdenum and with deficient seed content (<0.5 microgram Mo seed-1) nodule concentrations ranged from 1.9 to 3.5 microgram Mo g-1 in a small seeded genotype and 8.7 +/- 0.48 microgram Mo g-1 in a large seeded genotype. N2-fixation in these plants was not impaired except in one instance where nodule concentration was 1.9 microgram Mo g-1. Evidence that molybdenum is effectively translocated from leaves to roots and nodules was obtained using foliar treatments. All of the 3.3 microgram Mo applied to a leaf was recovered in the plant after 10 d. Mo content of the nodules increased by 81%, whilst Mo content of shoots increased by 56%. Root Mo content was eight times greater than that in plants not receiving a foliar treatment of molybdenum. We conclude that when molybdenum was scarce in the plant it was mobile and was translocated from roots and shoots to the nodules. As a result, nodule concentrations and contents of molybdenum were frequently maintained at amounts sufficient for N2-fixation even when the plant was entirely dependent on a small seed reserve of molybdenum.
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