Session 4 Site Effects, Silviculture, and Symbiosis

Session Papers

Symbiosis

Acacia albida: Nodulation by Fast- and Slow-Growing Rhizobia

M. Gueye¹

Abstract

Up to now, Acacia albida was known to nodulate only with slow-growing Rhizobium strains which are common in most tropical soils. This paper reports for the first time the isolation of three fast-growing strains of Rhizobium isolated from A. albida nodules.

Introduction

Because of its well-known economic benefits, Acacia albida is a valuable nitrogen-fixing tree growing in the Sahelian savanna from Senegal to Sudan. Up to now, A. albida has been thought to nodulate promiscuously with slow-growing Bradyrhizobiim (like that associated with Vigna unguiculata), which are common in tropical soils. This paper reports for the first time the isolation of three fast-growing strains of Rhizobium isolated from A. albida nodules.

Materials and Methods

Isolation of Rhizobium Strains

Seeds of A. albida were sterilized with concentrated H₂SO₄ for 1 h and germinated in petri dishes containing sterile 3% water agar for 3 days. Seedlings were then transplanted into pots (one seed per pot) containing 2 kg of a soil sample (0-20 cm layer) collected from nine sites in the central region of Senegal. Before transplanting, the soil samples were sieved (1 mm mesh). The pH of the soils ranged from 5.9 to 7.4.

Forty days after germination, one well-developed, firm, pink nodule was sampled from each plant. The Rhizobium strains were isolated and grown on YEM agar according to standard procedures (Vincent 1970).

Study of Acacia Species Nodulation

Seed of four acacia species (A. albida, A. raddiana, A. Senegal, A. seyal) were germinated as described above and transplanted into Gibson's tubes (Gibson 1963). After 3 days, the seedlings were inoculated with 1 mL of a Rhizobium suspension containing 109 cells mL^-1.

After 40 days of growth, the nodulation index (NI) of the four acacia species was recorded using the method of Gueye and Bordeleau (1988). The index integrates information on the number (N), internal color (C), and size (S) of the nodules as follows:

NI = N x C x S

Nodule number is rated on a scale from 0 (no nodule) to 3 (many nodules), nodule internal color from 0 (white) to 1 (red), and nodule size from 1 (small nodules) to 2 (large nodules).

Results and Discussion

Twenty-five Rhizobium strains were isolated from A. albida nodules. They were listed in the MAO (West Africa MIRCEN) culture collection as MAO 210 through MAO 232, MAO 234, and MAO 236. All the strains were slow-growing except three (MAO 223, MAO 225, and MAO 226), which had a generation time of 3-5 h.

Table 1 shows the nodulation index of the four acacia species inoculated with the fast-growing rhizobia. All strains were infective on A. albida. Strains MAO 223 and MAO 226 had high indices; strain MAO 225 had a lower index value. On A. seyal, one small nodule was associated with strain MAO 223 and only two nodules with MAO 225. The three fast-growing strains did not nodulate A. raddiana or A. Senegal, contrary to reports by Dreyfus and Dommergues (1981) that these species do nodulate with fast-growing strains of Rhizobium.

Table 1. Nodulation index¹ of four Acacia species inoculated with three fast-growing Rhizobium strains isolated from Acacia albida, Bambey, Senegal, 1990.

Since it is known that some Bradyrhizobium strains may be fast-growing, we can assume that strains isolated from A. albida belong to this group of Bradyrhizobiwn, which would explain why they did not nodulate A. radiana and A. Senegal (NI = 0).

A. albida is reputedly a promiscuous legume tree that nodulates with a large array of typical slow-growing Bradyrhizobium strains, like those infecting V. unguiculata. Our study clearly shows that in addition to the typical slow-growing Bradyrhizobium strains, the fast-growing strains MAO 223, MAO 225, and MAO 226 can nodulate A. albida as well.

Inoculants made of fast-growing strains are easier to produce, provided their effectiveness is high. This opens up new possibilites for their utilization in management of A. albida. Investigations are under way in our laboratory to evaluate the effectiveness of our fast-growing strains.

Acknowledgment. This work was supported by the United Nations Education, Scientific, and Cultural Organization (UNESCO) agreement No. 281.172.9. We thank O. Touré for valuable technical assistance and B.L. Dreyfus and Y. Dommergues for reviewing the manuscript.

References

Dreyfus, B.L., and Dommergues, Y.R. 1981. Nodulation of Acacia species by fast- and slow-growing tropical strains of Rhizobium. Applied and Environmental Microbiology 41(1)97-99.

Gibson, A. H. 1963. Physical environment and symbiotic nitrogen fixation. Australian Journal of Biological Science 16:28-42.

Gueye, M., and Bordeleau, L.M. 1988. Nitrogen fixation in bambara groundnut, Voandzeia subterranea (L.) Thouars. Mircen Journal 4:365-375.

Vincent, J.M. 1970. A manual for practical study of root-nodule bacteria. IPB Handbook. Oxford, UK: Blackwell Scientific Publications.

Footnote__________

1 Microbial Research Center (MIRCEN)/Centre national de recherches agricoles (CNRA), Bambey, Senegal.

Gueye, M. 1992. Acacia albida: nodulation by fast- and slow-growing rhizobia. Pages 149-150 in Faidherbia albida in the West African semi-arid tropics: proceedings of a workshop, 22-26 Apr 1991, Niamey, Niger (Vandenbeldt, R.J., ed.). Patancheru, A.P. 502 324, India: International Crops Research Institute for the Semi-Arid Tropics; and Nairobi, Kenya: International Centre for Research in Agroforestry.