Section 5
Chapter 4,878

Carbon metabolism nitrogen assimilation and seed yield of cowpea vigna unguiculata cultivar tvu 1503 plants dependent on nitrate nitrogen or on 1 of 2 strains of rhizobium

Neves, M.C.P.; Minchin, F.R.; Summerfield, R.J.

Tropical Agriculture 58(2): 115-132


ISSN/ISBN: 0041-3216
Accession: 004877871

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The C and N nutrition of cowpea cv. TVu 1503 plants nodulated with and totally dependent on one of 2 strains of Rhizobium known to produce different host plant dry weights and seed yields were compared with non-nodulated plants supplied with abundant nitrate-N. The development of both symbiotic associations was very similar until late in the reproductive period but thereafter nodules from strain R 5028 maintained larger relative fixation activity than those from strain CB 1024. Nitrate-dependent plants assimilated inorganic N more rapidly and produced more dry matter and larger seed yields than nodule-dependent plants. Remobilization of reduced N, which started sooner in non-nodulated than in nodulated plants, contributed between 23 and 31% of total seed N. Current assimilation during late pod-fill was a more important source of N for seeds in nodule-dependent plants than those which relied on nitrate. Averaged throughout crop life, roots nodulated with strain R 5028 consumed 8.0 mg C (estimated as root CO2 production) per mg N fixed and were 13% more efficient than those inoculated with strain CB 1024. Plants dependent on nitrate were much more efficient during the vegetative period than those relying on nodules and although they became very inefficient during fruit maturation, their overall root C consumption (as respired CO2) per milligram N assimilated was only 56% of the average for nodulated plants (4.5 mg). Respiration of nodulated roots utilized 26-28% of the C assimilatted throughout growth compared with 18% when non-nodulated plants relied on nitrate. The contribution of studies on C-N relationships to yield improvement in grain legume crops is discussed.

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