© 2000 Heron Publishing—Victoria, Canada
Effects of nitrogen source and defoliation on growth and biological dinitrogen fixation of Gliricidia sepium seedlings
Pekka Nygren (1, 2), Victor Vaillant (3), Lucienne Desfontaines (1), Pablo Cruz (1, 4) and Anne Marie Domenach (5)
1. INRA, Centre Antilles-Guyane, Unité Agropédoclimatique, Domaine Duclos, F-97170 Petit-Bourg, Guadeloupe / 2. Tropical Silviculture Unit, Department of Forest Ecology, Box 28, FIN-00014 University of Helsinki, Finland (pekka.nygren@helsinki.fi) / 3. INRA, Centre Antilles-Guyane, Unité des Recherches en Production Végétale, Domaine Duclos, F-97170 Petit-Bourg, Guadeloupe / 4. INRA, Centre Toulouse, Station d'Agronomie, B.P. 27, F-31326 Castanet Tolosan Cedex, France / 5. Laboratoire d'Ecologie Microbienne du Sol, UMR 5557, Université Lyon I, 43 Bd. du 11 novembre 1918, F-69622 Villeurbanne,
France / Received July 29, 1998
Summary
Effects of four N sources and two defoliation treatments on growth and nitrogenase activity of Gliricidia sepium (Jacq.) Walp seedlings were studied in a greenhouse. All nutrients were supplied in irrigation water to the sterile growing
medium. The N sources were: (1) 100 mg l–1 of N supplied as NO3– (high-NO3–), (2) 50 mg l–1 of N supplied as NO3– and inoculation with Rhizobium spp. medium-NO3–), (3)100 mg l–1 of N supplied as NH4NO3, and (4) inoculation with Rhizobium spp without mineral N (N2). At 35 weeks after sowing, mean total biomass was 130.5, 50.5, 22.9 and 17.4 g seedling–1 in the NH4NO3, N2, medium-NO3– and high-NO3– treatments, respectively. The root/shoot ratio was high in all of the N treatments (1.73–2.77) because the seedlings had
big taproots. The medium-NO3– treatment completely inhibited nodulation, whereas seedlings in the N2 treatment were profusely nodulated. At 32 weeks after sowing, groups of seedlings in the N2 and high-NO3– treatments were subjected to 50 or 100% defoliation. Closed-chamber acetylene reduction assays of intact root systems were
conducted to compare nitrogenase activity at 7, 14 and 28 days after defoliation (DAD). At 7 and 14 DAD, nitrogenase activity
of completely and partially defoliated seedlings was about 10 and 60%, respectively, of that of undefoliated controls. At
28 DAD, nitrogenase activity of completely defoliated seedlings was twice the predefoliation value, whereas nitrogenase activity
of partially defoliated seedlings was only 87% of the predefoliation value. Recovery of nitrogenase activity was strongly
correlated with foliage regrowth in the completely defoliated seedlings, but not in the partially defoliated seedlings. Abundant
belowground C and N reserves in the large taproot probably contributed to the rapid recovery from defoliation. Accumulation
of belowground biomass may also improve defoliation tolerance of mature trees.
Keywords:
acetylene reduction assay, amino acids, biomass allocation, nitrate, Rhizobium, symbiosis.
