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HOST SPECIFICITY OF THE ANNUAL MEDIC - RHIZOBIUM SYMBIOSIS

M.D. Denton 1, C.R. Hill 1, W.D. Bellotti 1, R.A. Ballard 2 and A.H. Gibson 3

1 The University of Adelaide, Roseworthy SA 5371
2
South Australian Research and Development Institute, PO Box 397, Adelaide, SA 5001
3
CSIRO, Division of Plant Industry, Canberra, ACT 2601

A high degree of Rhizobium strain specificity exists within the genus Medicago. Strain selection is therefore crucial for optimal symbiotic performance and medic pasture growth. For this reason controlled glasshouse experiments were conducted to test the effectiveness of 7 potentially useful strains of Rhizobium with 6 agronomically important annual medics.

MATERIALS AND METHODS

Six medic cultivars were grown in sterile, foil capped, vermiculite filled test tubes and watered with Jensen’s solution (N-free). After 3 days, tubes were inoculated with 1 of 7 bacterial strains (10 reps) and a uninoculated control and a nitrogen control were used. At 42 days shoot dry weights were determined. Data were analysed using ANOVA and Tukey HSD tests.

RESULTS AND DISCUSSION

M. littoralis SA8965, a line performing very well in upper Eyre Peninsula trials, expressed a highly strain-specific Rhizobium requirement and formed an ineffective symbiosis with the current commercial strain, WSM826 (Table 1). The good field performance of SA8965 can be attributed to its ability to form an effective symbiosis with the indigenous rhizobia in most South Australian soils (R. Ballard, unpub. data). If SA8965 is to be used commercially on soils devoid of indigenous rhizobia a compatible inoculant strain should be used. Interestingly, the two other M. littoralis cultivars formed successful symbioses with the commercial strain. Some species showed high specificity to the selected strains (M. polymorpha) whereas others (M. truncatula) expressed broad compatibility (Table 1).

Table 1. Shoot dry weights (mg DM / plant) of 6 Medicago lines (1. M. littoralis SA8965, 2. M. littoralis Harbinger AR, 3. M. littoralis Herald, 4. M. tornata Rivoli, 5. M. polymorpha Santiago, 6. M. truncatula Caliph) with 7 Rhizobium strains. Data within the same column not containing the same letter are statistically different at P = 0.05.

 

Medicago Line

Rhizobium Strain

1

2

3

4

5

6

CC2103

20.2 cd

34.7 bc

39.9 bc

42.8 d

66.3 ab

84.0 a

MSUR52a

66.1 b

63.4 ab

41.5 abc

68.2 c

12.1 c

60.8 bc

NA39

69.7 ab

57.4 ab

60.8 ab

77.7 bc

14.6 c

74.7 ab

SU47

22.4 c

16.4 d

33.1 bc

25.0 ef

51.2 b

44.9 c

U45

16.7 cde

38.9 bc

38.8 bc

66.5 c

15.0 c

68.5 b

WSM540 (1994 AM strain)

15.2 cde

21.4 cd

27.7 c

33.4 de

60.4 ab

75.3 ab

WSM826 (AL strain)

13.9 de

78.3 a

66.8 a

107.7 b

14.6 c

75.9 ab

N Control (0.027 g N/tube)

101.7 a

87.0 a

60.6 ab

166.0 a

105.9 a

103.5 a

Nil Control (no N)

12.5 e

16.3 d

10.3 d

16.1 f

12.0 c

19.4 d

ACKNOWLEDGMENTS

This research was supported by a University Research Scheme and GRDC.

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