Molybdenum reserves of seed, and growth and N₂ fixation by Phaseolus vulgaris L.

Summary

Plants grown from seed with high (1.5–7.3 μg Mo seed^-1) and low (0.07–1.4 μg Mo seed^-1) Mo contents were grown in the presence and absence of Mo in growth media (perlite) or in a flowing-solution culture, in a controlled environment. Neither the high (1.5 μg Mo seed^-1) nor the low (0.1 μg Mo seed^-1) Mo content in seed from a small-seeded genotype (BAT 1297) was able to prevent Mo deficiency (reduced shoot, root and nodule dry weight, N₂ fixation and seed production) in growth media without an external supply of Mo, whereas both the high (7.3 μg Mo seed^-1) and the low (0.07 μg Mo seed^-1) contents in seed were able to prevent Mo deficiency in a large-seeded genotype (Canadian Wonder). Responses to Mo treatment by the Two genotypes were inconsistent between the growth media and solution culture experiments. Seed with a large Mo content (3.5 μg Mo seed^-1) from the Canadian Wonder genotype was unable to prevent Mo deficiency (reduced shoot and nodule dry weight and N₂-fixation) in a solution culture without an external source of Mo, whereas both the large (1.7 μg Mo seed^-1) and the small (0.13 μg Mo seed^-1) contents in seed prevented a deficiency in BAT 1297. Growing plants from seed with a small Mo content, without additional Mo, reduced the seed Mo content by 83–85% and seed production by up to 38% in both genotypes. Changes in seed size and increases in shoot, root and nodule dry weight occurred, but varied with the genotype and growth conditions. These effects were also observed in some cases where plants were grown with additional Mo, demonstrating that the amount of Mo in the seed sown can influence plant nutrition irrespective of the external Mo supply. Nodule dry weight, total N content of shoots and seed production were improved by using seed with a small Mo content (1.64–3.57 μg Mo seed^-1) on acid tropical soils in Northern Zambia. Plants of both the large- and small-seeded genotypes grown from seed with a small Mo content (<1.41 μg Mo seed^-1) had a smaller nodule weight, accumulated less N and produced less seed. The viability of seed with a small Mo content was lower (germination up to 50% less) than that of seed with a large Mo content.