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Microflora of bulk soil from coconut garden and that of rhizosphere of pepper and nutmeg possibly modulate the allelopathic interactions in coconut based cropping system

Murali Gopal1 and Alka Gupta1

1Central Plantation Crops Research Institute, Kudlu P.O., Kasaragod – 671 124, Kerala, India. www.icar.org.in Email mgcpcri@yahoo.co.in

Abstract

    The effect of leaf and root leachate of adult coconut palm was studied on the growth parameters and rhizosphere microflora of pepper and nutmeg, when grown in unsterile and sterilized bulk soil taken from a coconut garden. Application of coconut leachate at 1:10 and 1:20 concentrations significantly suppressed the growth and rhizosphere microflora of pepper, while it killed the nutmeg when grown in sterile bulk soil. In unsterile bulk soil, the leaf leachate at 1:20 concentration promoted the growth of pepper and nutmeg; whereas, the root leachate at 1:20 concentration enhanced the rhizosphere microflora population. The result suggests that rhizosphere microflora of pepper is able to overcome the inhibitory allelopathic effect of coconut leachate as evidenced by its growth in sterilized and unsterile bulk soil. The inability of nutmeg to grow in sterile bulk soil on application of leachate indicates the failure of its rhizosphere microflora to prevail over the inhibitory effect of coconut leachate, whereas growth in unsterile soil implies that bulk soil taken from the coconut garden already contains microflora that can detoxify the suppressive nature of coconut leachate and support the growth of nutmeg.

Media summary

    The effects of coconut palm leachate on the growth of pepper and nutmeg was investigated

Key words

coconut, pepper, nutmeg, microflora, allelopathy, soil

Introduction

Coconut (Cocos nucifera L.) is an important oil seed crop grown in 1.91 million hectares in India. Majority of the farmers cultivating this crop possess land holding lesser than 0.5 ha area. When cultivated as monocrop, the economic returns are not uniform and generally insufficient due to the influence of biotic/abiotic and market factors. In such conditions, a high density multiple species cropping system including spices, fruits, vegetables and cash crops like vanilla are suggested to be grown as intercrops in coconut garden to improve the economic returns. It is estimated that around 7000 kg of dry leaves, sheaths, spadices/ inflorescences and coconut husk is generated from 1 ha area having 175 coconut palms. During monsoon, tannins, which are potential allelochemicals, ooze out of such crop residues possibly creating a plant growth interference (Bidappa et al.,1996). In addition to this, it has been reported that approximately 12 g/g of phenol is released by root exudates of coconut (Bopaiah et al., 1987). Availability of these allelochemicals in the soil provides an ideal situation for occurrence of allelopathic interactions in coconut based cropping system (Jacob John & Nair, 2001). In this paper, we present the results of the allelopathic studies on effect of coconut leaf and root leachate on black pepper (Piper nigrum) and nutmeg (Myristica fragrens), two important export-oriented spice crops that are generally grown in coconut gardens by farmers. As soil is the ecological matrix in which allelopathy interactions take place (Inderjit, 2001), we conducted experiments in such a way that the role of soil microflora is also investigated through these studies.

Methods

Preparation of leachate

Leaves from young non bearing ; adult and yielding coconut palm of West Coast Tall var. were washed with deionized water and then air-dried for 48 hours at room temperature (30 oC). About 200 g of these leaves were taken (70 % green and 30 % senescent) and soaked in 1000 ml of deionized water for 72 hours. Similarly, fresh growing roots from the same palms were collected, and their leachate prepared. This gave 1:5 concentrations of leaf and root leachate.

Laboratory bioassay

Laboratory bioassay was performed in Petri dishes to compare the seedling vigour index of cowpea seeds (test crop for dicot plants) at 1:5, 1:10, 1:15 and 1:20 concentrations of coconut leaf and root leachate. Bottom plates of Petri dish were layered with Whatman No. 1 filter paper and then were sterilized in an autoclave. Ten cowpea seeds (uniform size and weight) were washed once with deionized then with sterilized water and placed on the filter paper of sterilized dishes. Ten replications per treatment were maintained. Irrigation with 5 ml of leachate/ deionised water was given twice a day. Once the seeds germinated and started growing, the lids of the dishes were removed. This was followed by irrigation at three times a day. After 10 days, the length of the individual seedlings was measured and their fresh and dry weights were recorded. The vigour index of the seedlings was calculated by multiplying germination % with seedling length (Abdul Baki & Anderson, 1973). The physical, chemical and biochemical characters of coconut leachate from adult coconut palm of WCT variety were also analysed.

Soil microcosm studies

The effect of coconut leachate on black pepper and nutmeg was conducted in soil microcosm experiment in green house from Sept. 2002 to Feb. 2004. Bulk soil was collected from coconut garden where coconut palms of WCT variety were growing. Ten kilo of sieved soil was packed in clean perforated black poly-bags of dimensions 22 x 12.5 inches. One lot of the bulk soil was sterilized by moist heat autoclaving (Wolk and Skipper, 1994) and filled in similar poly-bags after ensuring that the soil was totally free of microorganisms. Black pepper (Piper nigrum) variety Panniyur-1 cuttings of 3 months age and nutmeg (Myristica fragrens) grafts same age of Viswasree variety was then replanted into the above poly-bags. Before replanting, the plant roots were thoroughly washed with deionised water to remove the soil particles. Pepper cuttings were planted with the support of bamboo poles. Both pepper and nutmeg were allowed to establish in the bulk soils (unsterile and sterilized) taken from coconut garden.

Five treatments i.e coconut leaf and root leachate at 1:10 and 1:20 concentrations and water were used for irrigating pepper and nutmeg in both categories of soil. The experiment was set up in Completely Randomized Design. Ten replications per treatment were maintained for each plant. Application of leachate (200-250ml/plant) extracted in plain as well as sterile water was carried out approximately equal to the number of rainy days received in a year in the locality. All other times plain/sterilized water was added.

Observations

Plant growth parameters of both crops were recorded immediately after the pepper and nutmeg established properly in the bulk soil. At the end of the study, the vine/shoot and root length and the total fresh and dry weights were recorded. Soil microflora viz. bacteria, fungi, actinomycetes, free living N2-fixers, phosphate solubilizers and cellulose degraders were analyzed from each treatment as and when the plant growth observations were recorded. Standard microbial media, serial dilution and spread plate methods were used for the enumeration.

The statistical analysis was carried out by adjusting the covariate wherever initial and final observations where recorded. The others were analysed by normal CRD. In case of soil microorganisms, square root transformation was done and then covariance analysis was done.

Results

The results of laboratory bioassay showed that leaf leachate from young coconut palm increased the cowpea seedling vigour index (9.8 to 10.6) as compared to control (7.8). However, leaf and root leachate from adult coconut palm at 1:5 and 1:10 concentrations produced a significant suppressive effect on seedling vigour. Based on these results soil microcosm studies in green house were conducted with 1:10 and 1:20 concentrations, as the possibility of 1:5 concentration reaching the soil was considered to be remote. The effect of coconut leachate on growth parameters of pepper is given in Fig 1(a). In sterilized soil the coconut root leachate reduced the growth of vine, but the total fresh and dry weights remained on par in all the treatments. In unsterile soil, the 1:20 concentration of leaf leachate resulted in maximum growth of pepper vine (188cm) whereas, the root leachate at both the concentrations significantly enhanced the root length of pepper (43 and 46 cm respectively). The initial rhizosphere population of different microbial communities in pepper was significantly less in the sterile soil when compared to unsterile soil except in case of N2-fixers which was on par in all the treatments in both the categories of soil (data not shown). The final adjusted data showed similar trends (Table 1). Coconut root leachate at 1:20 concentration significantly enhanced the population of most of rhizosphere microbial communities in unsterile soil when compared to other treatments.

Fig. 1. Effect of coconut leaf and root leachate on growth parameters of (a) pepper and (b) nutmeg

Application of coconut leachate killed the nutmeg grafts after 6 to 7 months in sterilized soil; however the plants survived in control treatment (sterile water) and continued to grow at slower rate. In unsterile soil, the leaf leachate at 1: 20 concentration appeared to improve the nutmeg growth parameters. The root leachate as in the case of pepper, significantly improved the root growth of the nutmeg (Fig 1b).

The initial microbial numbers in the rhizosphere was significantly higher in unsterile soil as compared to the sterilized soil (data not shown). The adjusted and transformed data shows that the bacterial population was on par in all the treatments in both the categories of soil. In sterilized soil the control treatment (sterile water) had significantly higher counts of actinomycetes, free living N2 fixers, phosphate solubilizers and very low counts of fungi and cellulose degraders when compared to leachate treatments in same soil category. In the unsterile soil, again the coconut root leachate at 1:20 concentration significantly boosted the rhizosphere microflora of nutmeg (Table 1).

Table 1: Effect of coconut leachate on rhizosphere microflora of pepper and nutmeg (adjusted and transformed data)

Pepper

Nutmeg

Treatments

Bacteria
(nX103)

Fungi
(nX102)

Actino.
(nX102)

N2-fixer
(nX103)

Phos.sol
(nX102)

Cell. deg
(nX102)

Bacteria
(nX103)

Fungi
(nX102)

Actino.
(nX102)

N2-fixer
(nX103)

Phos.sol
(nX102)

Cell. deg
(nX102)

S-Cont

669.0

77.0

85.0

399.0

60.7

34.0

485.0

33.0

83.0

120.0

237.0

74.0

S-L 10

480.0

82.0

92.0

244.0

9.0

34.0

488.0

64.0

53.0

14.0

5.0

155.0

S-L 20

467.0

95.0

91.0

287.0

12.0

26.0

398.0

38.0

37.0

16.0

10.0

177.0

S-R 10

463.0

61.0

75.0

200.0

12.0

37.0

473.0

48.0

40.0

19.0

8.0

157.0

S-R 20

422.0

74.0

62.0

273.0

15.0

31.4

428.0

39.0

41.0

21.0

10.0

180.0

U-Cont

1010.0

62.0

54.0

248.0

40.0

59.0

335.0

139.0

236.0

308.0

246.0

280.0

U-L 10

1207.0

96.0

67.0

598.0

22.0

48.0

504.0

147.0

260.0

234.0

138.0

284.0

U-L 20

1570.0

107.0

76.0

585.0

22.0

55.0

396.0

187.0

262.0

313.0

164.0

307.0

U-R 10

1200.0

99.0

97.0

533.0

22.0

45.0

393.0

140.0

233.0

311.

122.0

245.0

U-R 20

1542.0

113.0

109.0

691.0

28.0

64.0

490.0

237.0

228.0

318.0

204.0

311.0

CD
(p=0.05)


119.0


12.00


18.0


120.0


4.0


4.0


------


15.0


7.0


11.0


14.0


27.0

The root leachate contain higher proportions of sugars and phenols when compared to leaf, while the latter are rich in potassium, calcium, copper and magnesium (Table 2).

Table 2. Some of the chemical and biochemical properties of leachate of coconut palm of WCT variety (in ppm)

Properties:

pH

N

P

K

Ca

Mg

Fe

Cu

Zn

Mn

Total
sugars

Total
phenols

Leachate

                       

Leaf

4.5-5.0

1.7

7.5

43.8

42.4

3.8

0.2

2.1

Trace

1.2

124.8

13.4

Root

5.0-5.5

1.7

11.2

27.0

17.0

3.5

0.3

0.1

0.1

0.1

413.5

56.1

The coconut palm is mostly grown in coastal, humid regions in India where the average annual rainfall ranges from 1000 to 3000 mm. Rainfall partitioning studies in coconut has shown that 85-90% of rain passes as through fall from the coconut canopy and more than 50% as stem flow that carry the leachate to the soil. The radial spread of coconut roots to a distance of 5 to 7 m aid the translocation of the root leachate to more ground area. Pepper is usually planted in basin area of coconut and is allowed to twirl around the trunk whereas nutmeg is grown in the interspaces. Our studies show that both pepper and nutmeg grow well in unsterile bulk soil taken from coconut garden when irrigated with coconut leaf and root leachate. It could be deduced that the presence of nutrients, organic acids and proteins in leachate act as substrates, which is efficiently used by the microflora in unsterile bulk soil than the rhizosphere microflora of pepper in sterile soil. Increase in numbers of N2-fixers, phosphate solubilizers and cellulose degraders probably improve the soil nutrient pool and enhance the plant growth. Though the root exudates contain very high phenol which are attributed with allelochemical property, soil microbes capable of overcoming them must be resident in the bulk soil as well as native inhabitant in pepper rhizosphere. Ages of cultivation of pepper in the coconut basin would have resulted in almost similar microbial make up as that of coconut which has adapted well to the leachate.

In nutmeg, death of plants in response to coconut and leaf leachate when grown in sterilized soil (Fig. 2), and significant suppression of rhizosphere microflora barring the bacteria suggests that, the native rhizosphere microbes of nutmeg are not up to the stress put by the leachate. It is seen that the nutmeg is able to grow in sterile soil on irrigation with sterile water, the rhizosphere microflora (N2-fixers, phosphate solubilizers, etc) also showing better activity and supporting the growth of crop. The cellulose degraders are recorded in large numbers in the leachate applied sterile soil, possibly consuming the detritus added because of the death of the grafts. In the unsterile soil we see that the growth of nutmeg is enhanced in presence of coconut leachate, implying that the bulk soil contains the microflora that are capable of utilizing the leachate, multiply and improve the growth of the plant.

Fig. 2 : Effect of water (W), leaf (L10, L20) and root leachate (R10, R20) on growth of nutmeg in sterilized (S) and unsterilized (U) soils

Allelochemicals are present in every plant; their presence per se does not necessarily demonstrate the occurrence of allelopathy in nature (Heisey, 1990).One important reason could be the role played by the soil microorganisms in detoxifying or utilizing the allelochemicals resulting in minimized plant to plant interference (Stark and Hyvarinen, 2003)

Conclusions

Microflora of bulk soil taken from coconut garden and that of rhizosphere of pepper and nutmeg play an important role in the possible allelopathic interactions in coconut based cropping system. The rhizosphere microflora of pepper is able to overcome the inhibitory allelopathic effect of coconut leachate as evidenced by its growth in sterilized and unsterile bulk soil. The inability of nutmeg to grow in sterile soil on application of leachate indicates the failure of its rhizosphere microflora to prevail over the inhibitory effect of coconut leachate, whereas growth in unsterile soil implies that bulk soil taken from coconut garden already contain microflora that can detoxify the suppressive nature of coconut leachate and support the growth of nutmeg.

References

Abdul-Baki AA, Anderson JD (1973) Vigour determination in soyabean and seed multiple criteria. Crop Science 13, 630-633.

Bidappa CC, Upadhyay AK, Hegde MR, Palaniswami C (1996) Organic matter recycling in plantation crops. Journal of Plantation Crops 24, 71-85.

Bopaiah BM, Shekhara Shetty H, Nagarajan KV (1987) Biochemical characterization of root exudates of coconut palm. Current Science 56, 832-833.

Heisey RM (1990) Evidence for allelopathy by tree-of-heaven (Ailanthus altissima). Journal of Chemical Ecology 16, 239-225.

Inderjit (2001) Soil: Environmental effects on allelochemical activity. Agronomy Journal 93,79-84.

Jacob John, Nair AM (2001) Allelopathy a cause of concern in coconut based home stead gardens in Kerala, India. Indian Coconut Journal 31, 11-12

Wolf DC, Skipper HD (1994) Soil sterilization. In ‘Methods of Soil Analysis, Part 2- Microbiological & Biochemical Analysis’ (Ed. RW Weaver, JS Angle, PS Bottomley) pp. 41-51. (Soil Science Society of America Inc.)

Stark S, Hyvarinen M (2003) Are phenolics leaching from the lichen Cladina stellaris sources of energy rather than allelopathic agents for soil microorganisms. Soil Bioiology and .Biochemistry 35, 1381-1385

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