SYMBIOSIS BETWEEN MUNGBEAN AND RHIZOBIA: INFLUENCE
OF WATER STRESS
SANGEETAPAUL
I )j vision of Microbiology, Indian Agricultural Research Institute, New Delhi- 1 10 012 Received on 31 Dec., 1996, Revised on 27 July, 1997
SUMMARY
The present investigation was carried out to isolate and evaluate symbiotic performance of stress-acclimatised •·hizobial strains. The mungbean plants were subjected to three water regimes, excess water (0.0 MPa), normal inigation (-0.035 MPa) and drought (-0.065 MPa). Mungbean cv. PS- 16 was inoculated with five strains of rhizobia. Inoculation with strains adapted to wate•· stJ·ess condition p•·oved to be useful in imp•·oving growth, yield and symbiotic nitt·ogen fixation. Howeve1·, variation was observed in plants's response to different rhizobia! strains under different water regimes. Strains which performed well under excess moisture conditions (viz, Ew 24, N 11, N 17 and D 4) did not pe•·form well under normal irrigation (exception being Nll). Rhizobial inoculation favoured plant growth under drought condtions. The standard strain M-10 performed well- under normal conditions but failed under water stress conditions. Rest of the strains did not favour plant growth under normal irrigation.
INTRODUCTION of the standard rhizobia! strain under different water
regimes. Uneven distribution of rainfall is a common feature
during Kharif season. Mung bean, an important legume crop grown in Kharif season is thus, often subjected to water logging and/or prolonged dry spells (Pawar and Bhatia, 1980). Growth and symbiotic nitrogen fixation of mungbean plants is also affected by both excess and deficiency of water, causing reduction in yield (Venna and Subba Rao, 1975, Bagga eta!., 1984, Kavimandan and Chandel, 1988). Rhizobia! inoculation significantly increases growth and yield (Guckert eta!., 1990). Strains of rhizobia introduced as inoculants generally do not survive or proliferate and exhibit effective symbiosis under unfavourable moisture conditions. It has been observed that native rhizobia! strains are well adapted to certain stress conditions and thus may be more effective inoculatns under those conditions than the more exotic rhizobia! strains (Norris, 1965, Sen, 1966, Bhardwaj, 1975). Thus root nodule bacteria were isolated from mungbean plants grown under water stress conditions and their symbiotic performance was compared with that
MATERIALS AND METHODS
Several strains of rhizobia from nodules of mungbean cv. PS- 16 grown under excess. nom1al and water defiCJet conditions were isolated and authenticated (Vincent 1970). These isolates were thus, acclimatised to a certain constant water regime. These isolates were also evaluated for their tolerance to sodium chloride. Five of the isolates exbiting comparatively more sodium chloride tolerance 'Vere selected and their symbiotic performance vvas evaluated. These
arc-(i) D 4- isolated from nodules of mung bean subjected to drought
(ii) Ew 24- isolated from nodules of mungbean subjected to excess water.
SANGEETA PAUL
(iv) M I 0- the standard mungbean strain was obtained individual effects were studied bv tactonal anal\ s1s as from tlw culture collection of the Division of described by Sukhatme and Amble (1985).
Microbiology, Indian Agricultural Research Institute, New Delhi.
Vigna radiata cv. PS 16 seeds were obtained from the Division ofGenetics, IARI, New Delhi. Twelve kg of soil was used per pot. Soil was amended with 20 kg N, 60 kg P2 05 and 40 kg K:O per hectare. Mungbean seeds were inoculated with carrier based cultures of the rhizobia) strains using sucrose as an adhesive. Three replicates were kept for each treatment. Each pot contained three plants. Appropriate uninoculated controls were maintained.
Plants were grown under nom1al \Vater conditions for 15 days (after sowing) to obtain proper gem1ination and seedling growth. Excess water(O.O MPa), nonnal irrigation (-0.035 MPa)anddrought(-0.065 MPa)weremanintained using mercurytensiometers throughtout the growth period. Soil water tension was measured using the fonnula.
Tension (in MPa) = -(12.6 Y-H-h) x 0.1
V\here Y = the height to which mercury has risen above the zero mark
H = distance between gypsum cup buried in soil and the upper level of mercury in container.
h = distance between upper level of niearcury in container and the zero mark on the mete scale
Sampling was done 45 days after sowing (DAS). Plants were harvested 75 DAS at full maturity. Acetylene reduction assay (ARA) was done using Shimadztl gas chromatograph, model GC- 14A with FlO detector (Stewart et al., 1967). Nodule weight, plant dry weight and yield were recorded. N-content of the powdered plant and grain samples was estimated by standard Kjeldahl digestion procedure followed by colorimetric analysis using Technicon N-autoanalyser.
Statistical analysis ofthe data was done by using-the technique of 'Analysis of Variance' (Fisher, 1970), as applied to completely randomised block design. Interactions between water levels x inoculation and their
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RESULTS AND DISCUSSION
Under excess moisture conditon, increase in nodule weight and nitrogen content of the plant was observed (Table I). Excess water conditon also favoured ARA. Plant growth was found to be adversely affected by water stress. Water regimes affected plant grovvth and yield Limited water supply usually mainfests into poor root, shoot and grain development (Morton eta!., 1982, Bagga eta!., 1984). Symbiotic efficiency (ARA and N-content of the plants) was greatly influenced by the availability of water and impcndence of nodulation due to scarcity of water in the present study confinns the findings ofRupela
ef a/., ( 1987). Deficiency of water affects survival and proliferation of rhizobia (Bushby, I 982), thereby affecting nodulation. In the present investigation decrease in nodule mass was observed due to water stress. Lie (I 971) has stressed the sensitivity of legume-Rhizobium symbiosis to environmental conditions and that these nmction optimally within a narrow range of environmental conditions. Adverse effect of water stress on nitrogen fixation was ascribed to limited supply ofphotosynthates to root nodules (Sprent 1976, Simonet a!, 1992). It was also inferred from the data that there were variations in all the parameters studied due to seed inoculation with rhizobia. However, at this stage of plant growth, these differences were statistically non significant.
Table I. Effect of moisture stress on Mung-Rhizobium symbiosis.
Treatments Nodule Dry N in dry
weight matter matter
(mg) (g) (mg)
Stress Excess water
(0.00 bar) 160.3 8.6 228.0
Normal
(-0.035 bar) 36.6 5.1 132.0
Drought
(-0.065 bar) 30.0 3.7 130.0
SEm± 4.0 0.2 5.1
CD0.05 11.8 N.S. 15.2
Inoculation
Uninoculated 9.21 6.6 182.0
D4 67.0 6.4 176.0
Ew24 98.1 6.9 185.0
Nil 60.2 6.7 185.0
N 17 87.8 5.5 149.0
MlO 48.5 4.6 122.0
SEm± 8.0 0.4 10.2
CD0.05 N.S. N.S. N.S.
CV% 63.4 38.1 37.5
Table II. Effect of moisture levels and Rhizobia! inoculation on mung
Treatments Yield (g per pot)
Grain Shoot Root Biological Grain
Stress Excess Water
(OOOOMPa) 6.40 14.10 1.80 22.30 234.0
Nonnal
(-0.035 MPa) 3.10 7.70 1.00 1180 116.0
Drought
(-0.065 MPa) 2.50 6.50 0.90 9.90 89.0
SEm± 10.07 0.09 0.02 0.06 2.7
CD 0.05 0.20 0.30 0.05 0.20 7.5
Inoculations
Uninoculated 3.20 8.90 I 00 13.10 118.0
D4 4.70 9.30 ] .30 15.30 177.0
E\V 24 3.80 IO.OO 1.10 14.90 139 0
Nll 4.10 11.40 1:50 17.00 143 0
N 17 4.10 9.20 1.30 14.60 148.0
M10 4.10 7.90 1.00 1300 155.0
SEm± 10.10 0.20 0.04 0.13 5.4
CD 0.05 N.S. 0.50 0.10 0.35 15.0
CV% 33.20 18.70 2720 7.80 I 0 5
ARA runol
C2H4IJ-I
268.0
195.0
62.2 17.8 N.S.
113.5 235.3 115.3 115.3 345.2 151.9 35.6 N.S. 120.7
N (mg per pot)
Shoot Root
264.0 30.00 154.0 17.00 112.0 17.00
1.8 0.33
5.1 0.90
186.0 18 00
1 W.O 24.00 194 () 19.00 205.0 29.00 167 () 20.00 140.0 17.00
3.7 0.65
102 1.80
18.7 27.60
Total
528.0 287.0 218.0 1.7 4.7
SANGEETA PAUL
ofthe plants whereas, the other rhizobia! strains produced nitrogen rich mungbean plants, thus indicating better symbiotic association. Water regimes were found to influence the symbiotic behaviour of these five strains of mung bean rhizobia (Table III-IV). Under excess moisture, significant increase in the shoot weight (Ew 24 and N 11 ), root mass (N 11 and N 17), and total N-content (D 4, Ew 24, N 11 and N 17) respectively was observed (Table III). Strain M 10 showed an adverse effect on both biomass and total N-content of the plants.
Mungbean plants grown with sufficient moisture, however, responded weakly to rhizobia! inoculation (Table V) and decrease in shoot weight was observed with strains D 4, E w 24 and N 17 undernormal water conditions. These strains, however, developed better symbiotic associations with mungbean under excess water and drought conditions as described earlier. The most significant observation recorded was, better symbiotic performance of strain M 1 0 which increased nitrogen
Table III. Effect of excess water (0.0 MPa) on Mung-Rhizobium symbiosis
Treatments Yield (g per pot) N (mg per pot)
Grain Shoot Root Biological Grain Shoot Root Total
Uninoculated 5.6 13.3 1.7 20.6
D4 8.7 14.6 1.8 25.1
Ew24 6.3 15.4 1.4 23.1
Nil 5.1 18.1 2.5 25.7
Nl7 7.3 13.1 2.2· 22.6
MlO 5.5 100 1.3 16.8
SEm± 10.6 0.5 0.1 0.4
CD0.05 N.S. 1.7 0.4 1.2
CV% 27.2 11.6 19.8 5.4
Rhizobia! inoculation favoured better plant growth then uninoculated plants under drought conditions (Table IV). There was an increase in root and shoot biomass and grain yield. The increase in plant biomass was usually accompanied by an increase in nitrogen content of vegetative parts and grain in mungbean. Most of the rhizobia! strains used for seed bacterisation enhanced various plant growth parameters under all the water regimes. There was increase in plant biomass and grain yield due to rhizobia! inoculation. However, rhizobia! inoculation proved to be more effective in ehhancing yield and N-content of plants under stress conditions as compared to normal water supply. These findings indicate that rhizobia! inculations can play a significant role in sustaining legume yield, particularly under unfavourable moisture conditions.
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201.0 271.0 28.0 500.0
334.0 279.0 29.0 642.0
233.0 291.0 25.0 549.0
176.0 316.0 47.0 539.0
256.0 254.0 29.0 539.0
204.0 173.0 22.0 399.0
21.6 10.1 2.1 14.0
N.S. 31.1 6.4 43.1
27.6 11.4 20.7 7.9
content and grain yield ofmungbean. All the other strains except N 11 failed to influence plant growth.
There was variation in the effect of inoculation with the same rhizobia! under the three water regimes. This variation could be explained from the fact that rhizobia! strains giving satisfactory perofm1ance under one set of enviromental conditions may not necessarily perfom1 well under another set of environmental conditions (Lie, 1971). As observed from the results, rhizobia! inocualtions sometimes instead of improving crop yield and plant growth may in fact have adverse effect on the plant. In the present investigation. M 1 0, the standard inoculant strain proved useful only under nom1al water conditions. The strain had adverse effect on the plant growth and yield under water stress conditions. Thus, rhizobia! strains to be used as inoculants under water stress should be selected
Table IV. Effect of drought conditions (- 0.065 MPa:) on Mung-Rhizobium symbiosis
Treatment:-; Yield (g per pot) N (mg per pot)
Grain Shoot Root Biological Grain Shoot Root Total
U ninoculated 1.0 4.6 0.6 6.2 37.0 93 0 14.0 144.0
D4 3.5 7.7 0.9 12.1 131.0 129 () 15.0 275 ()
Ew 24 1.9 8.0 1.2 11.! 74.0 Wl.O 180 252 0
Nil 3.7 6.3 1.0 110 121.0 93 0 22.0 2"lG 0
Nl7 2.5 7.1 0.9 10.5 92.5 104.0 20.0 2 1G ()
M10 2.2 5.2 0.7 8.1 810 %.0 130 190.0
SEm± 104 0.4 0.06 0.2 12.7 ()7 1.2 79
CD0.05 N.S. 1.2 0.2 04 N.S. 20.5 N.S. 24.2
CV'Yo 43.5 18.6 19.6 23.6 42.5 17.8 21.2 10.8
Table V. Effect of sufficient water ( -0.035 MPa) on Mung- Rhizobium symbiosis
Treatments Yield (g per pot) N ( mg per pot)
Grain Shoot Root Biological Grain Shoot Root Total
Uninoculated 3.0 8.8 0.9 12.7
D4 1.7 5.5 1.3 8.5
Ew24 3.3 G.5 08 106
Nil 3.5 97 l.l !4.3
Nl7 2.6 74 0.7 !0.7
M10 4G 8.3 10 13.9
SEm ± 10.3 08 0 1 0.5
CD0.05 N.S. N.S. N.S. N.S.
CV'Yo 33.5 29 (, 41.3 12.1
on the basis of their symbiotic perfonnance under similar \Vater conditions.
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