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Evaluation of allelopathic plant materials for aquatic weed control

RM. Kathiresan

Department of Agronomy, Faculty of Agriculture, Annamalai University, Annamalai nagar, Tamilnadu, INDIA 608 002.

e-mail: rm.kathiresan@sify.com.

Abstract

Exploitation of allelopathic potential of plant species as a bio-control tactic for the control of aquatic weeds has been gaining significance. Many natural compounds are reported to have a high potential to form the basis of commercially successful herbicides and their half lives in the environment often are much shorter than those of synthetic compounds. Further, they may offer improved selectivity, better toxicological and environmental safety and increased efficacy. However, achieving satisfactory control of aquatic weeds with the use of allelopathic plant products involve a complicated methodology of chain of screening procedures and application strategies. Initial screening of different plant materials involves both pot culture and laboratory experiments with the target weeds as whole plants or their parts in aqueous solution, respectively. After initial screening, dose responses to the individual product selected must be developed for different growth stages of the weed, and must be confirmed through aquatic system trials that might comprise both microponds and / or pond experiments. Once a plant material shows appreciable response, especially at low doses, then it can be evaluated in further studies regarding it’s mechanism of action, effect on other aquatic weeds and non-target organisms, formulating for application on the foliage of target weeds, integration with other control measures such as bio-control using insects or pathogens, and herbicides. The procedures and methodologies to be followed in each and every step are discussed, with several allelopthic plant materials used for the control of water hyacinth.

Media summary

Methodology of selecting allelopathic plant materials for aquatic weed control and integrating such plant materials with other bio-control tools such as insects has been developed.

Key Words

Plant product, bio-assay, water hyacinth, integrated management

Introduction

Allelochemicals reported are generally secondary plant products or breakdown products from decomposing plant tissue. Though many of these allelochemicals exhibit inhibitory responses, especially in seedling germination and growth tests, they are rarely lethal at concentrations that could be applied on a field scale for terrestrial agro-ecosystems. Also, the degree of selectivity is often not adequate for widespread commercial use, although research with allelopathic mulches and cover crops have shown some promise (Mc Laren, 1986; Fuji and Hamano, 2003). While there are few promising examples of specific allelochemicals for safe, selective and eco-friendly use in terrestrial systems, the scope of research on allelopathic phenomena in aquatic systems is still attractive. This is because of the absence of a soil interface that alters the nature of allelochemicals before they are absorbed by the target species. Further, rapid uptake of allelochemicals in aquatic systems also enhances the allelopathic activity. Thus, careful and detailed screening and evaluation of allelopathic inhibition of aquatic weeds is essential. The procedure involved is discussed with a case study on water hyacinth.

Methods

Screening of 55 different plant species for their allelopathic potential on water hyacinth (Eichhornia crassipes Mart (Solms)., the worst floating aquatic weed that has invaded all continents except temperate Europe, was taken up at Department of Agronomy, Annamalai University. The screening comprised laboratory and green house studies, simulated micropond testing, and evaluation in large watersheds under natural field conditions. However, for the sake of space limitation and precision, selected studies, plant products and observations alone are discussed. For laboratory and green house studies whole plants of the weed were grown in polyethylene containers of dimension 45 x 30 x 15 cm filled with water and standardized nutrient solution up to three fourth of the container. The dried powder of all the plants (plant products) screened were added to this water supporting the weed on the basis of weight by volume @ 30 g l-1. The observations included reduction in biomass (on fresh weight basis) and number of healthy leaves plant-1 at two days intervals. For tracing the dose dependant inhibitory response of the weed to the different plant products, the leaves of water hyacinth plants (with healthy leaves submerged in water) were detached by cutting the petiole with a razor blade with care to retain the incision point below water level. The detached portions of leaf with a part of petiole intact was kept submerged in water for 90 seconds to ensure that no air was trapped internally. Then these leaves were transferred to scintillation vials with water wherein different plant products were dissolved and individually compared with an untreated control. The percent fresh weight reduction of the cut leaves was calculated using the formula

The whole plant bio-assay was taken up for three different morpho-physiological stages of water hyacinth identified earlier (Kannan and Kathiresan, 1998). The different doses used in the incised leaf bio-assay for plotting the dose response included 30, 25, 20, 15, 10, 5, 2.5, 1.0, 0.5, 0.25 and 0.1 g l-1 of plant product in water and the reduction in fresh weight of leaves were observed 24 hrs after treatment. After selecting the most allelopathic plant product, an attempt was made to integrate this plant product with the established classical insect bio-control agents Neochetina bruchi Warner and N. eichhorniae hustuche. Both the sequences of treating the water body first with plant product followed by the release of insect agents and release of insect agents followed by spraying of plant product on the weed canopy were compared in green house conditions.

Results and Discussion

In bio-assays involving whole plants, ten of the 55 different plant products including Coleus amboinicus, Parthenium hysterophorus and Leucaena leucocephala were highly allelopathic based on fresh weight reduction (>30%) of water hyacinth within 48 hr after treatment. Another 12 including Acalypha indica Linn., Trianthema portulacastum L. and Sesbania grandiflora (L.) Pers. showed moderate allelopathy with a fresh weight reduction of water hyacinth 15-30%. Twelve other plant products including Croton sparsiflorus Morong, Cleome viscosa L. and Eclipta alba L. were less allelopathic, reducing fresh weight of water hyacinth by less than 15% (Table 1). The remaining 21 plant species including Leucas aspera Spreng. Curcuma longa L. and Euphorbia hirta L. were not allelopathic to water hyacinth in these assays.

Table 1. Percentage reduction in fresh weight of E. crassipes due to various plant products. Figures in parenthesis are original values before arc-sine transformation.

Treatments
plant products @ 30 g l-1

Days After Treatment

2 DAT

4 DAT

6 DAT

8 DAT

C. amboinicus

56.57 (69.66)

90.00 (100.00)

90.00 (100.00)

90.00 (100.00)

A. indica

29.98 (24.97)

44.24 (48.68)

90.00 (100.00)

90.00 (100.00)

L. aspera

-

-

-

-

C. sparsiflorus

20.53 (12.30)

34.51 (32.11)

45.37 (50.66)

90.00 (100.00)

C. longa

-

-

-

-

T. portulacastrum

27.06 (20.70)

45.09 (50.17)

90.00 (100.00)

90.00 (100.00)

C. viscosa

21.10 (12.96)

35.77 (34.17)

45.66 (51.16)

90.00 (100.00)

L. leucocephala

34.84 (32.65)

90.00 (100.00)

90.00 (100.00)

90.00 (100.00)

S. grandiflora

27.17 (20.86)

44.51 (49.16)

90.00 (100.00)

90.00 (100.00)

P. hysterophorous

36.92 (36.10)

90.00 (100.00)

90.00 (100.00)

90.00 (100.00)

E. hirta

-

-

-

-

E. alba

21.27 (13.17)

33.93 (31.17)

44.94 (49.91)

90.00 (100.00)

Control

-

-

-

-

SED

2.40

2.34

1.25

-

CD (p=0.05)

4.80

4.69

2.51

-

The dose response through In the incised leaf bio-assay, C. amboinicus reduced fresh weight in water hyacinth by 18% within a week of exposure at doses as low as 0.1 g L-1 (Fig.1). A stable inhibitory response caused by coleus powder applied to cut leaves of water hyacinth under controlled conditions in static water at dosages ranging from 30 g l-1 down to 1.0 g l-1 reducing fresh weight from 49% to 18%, respectively, showed that coleus dry powder is inhibitory at low dosages.

Fig.1. Effect of dry powder of Coleus amboinicus on water hyacinth leaves (% reduction in fresh weight)

In integration studies, it was observed that treating the water body first with plant product followed by the release of insect agents on the weed showed an antagonistic interaction, as the insects migrated from treated, partially killed plants to healthy plants. The second sequence of releasing the insect agents first followed by spraying of the plant product on the weed foliage produced an additive or synergistic response with rapid and complete weed control within a single season (Table 2). The plant product was also shown to be safe for the insect agents without inducing migratory behavior (Table 3).

Table 2. Impact of the integrated approach with the sequence of releasing the insect agents first followed by natural product as foliar spray on percentage reduction in fresh weight of E.crassipes. Figures in parenthesis are original values before arc-sine transformation.

Treatments

20 DAIR

30 DAIR

40 DAIR

T1 - Control

0.01 (0.00)

0.01 (0.00)

0.01 (0.00)

T2 - 5% natural product spray

0.01 (0.00)

0.01 (0.00)

0.01 (0.00)

T3 - 10% natural product spray

0.01 (0.00)

0.01 (0.00)

0.01 (0.00)

T4 - 15% natural product spray

0.01 (0.00)

0.01 (0.00)

0.01 (0.00)

T5 - 20% natural product spray

0.01 (0.00)

0.01 (0.00)

0.01 (0.00)

T6 - 25% natural product spray

0.01 (0.00)

0.01 (0.00)

0.01 (0.00)

T7 - Insect alone

31.37 (27.10)

46.91 (53.33)

54.11 (65.63)

T8 - Insect + 5% natural product spray

34.79 (32.57)

47.47 (54.30)

61.11 (76.66)

T9 - Insect + 10% natural product spray

40.84 (42.77)

53.73 (65.00)

67.21 (85.00)

T10 - Insect + 15% natural product spray

42.13 (45.00)

56.37 (69.33)

90.00 (100.00)

T11 - Insect + 20% natural product spray

46.52 (52.66)

62.03 (78.00)

90.00 (100.00)

T12 - Insect + 25% natural product spray

54.74 (66.67)

90.00 (100.00)

90.00 (100.00)

SEd

2.34

2.85

1.85

CD

4.68

5.71

3.71

Table 3. Impact of the integrated approach with the sequence of releasing the insect agents first followed by natural product as foliar spray on insect migration rate (%). Figures in parenthesis are original values before arc-sine transformation.

Treatments

3 DAS

14 DAS

T1 - Control

-

-

T2 - 5 % natural product spray

-

-

T3 - 10 % natural product spray

-

-

T4 - 15 % natural product spray

-

-

T5 -20 % natural product spray

-

-

T6 -25 % natural product spray

-

-

T7 - Insect alone

26.56 (20.00)

31.09 (26.66)

T8 - Insect + 5 % natural product spray

26.56 (20.00)

35.26 (33.33)

T9 - Insect + 10 % natural product spray

26.56 (20.00)

46.90 (53.33)

T10 - Insect + 15 % natural product spray

31.09 (26.66)

75.03 (73.33)

T11 - Insect + 20 % natural product spray

31.09 (26.66)

90.00 (100.00)

T12 - Insect + 25 % natural product spray

31.09 (26.66)

90.00 (100.00)

SEd

1.85

2.91

CD

3.70

5.83

The allelopathic injury due to C. amboinicus was observed to be effected through electrolyte leakage, membrane disruption and root dysfunction (Kathiresan, 2000) in water hyacinth. The additive or synergistic response of integrating insect agents and plant product is attributed to the insects scraping the leaves of weed, removing the cuticular barrier and thereby assisting efficient and rapid uptake of the plant product sprayed over the canopy later (Kathiresan, 2004).

Conclusion

The phenomenon of allelopathy is reflected more effectively in aquatic systems. However, plant products for aquatic systems need to be screened and selected through a series of laboratory, green house, simulated micropond and field studies. Proven plant products might yield better practicable and sustainable results if integrated with other bio-control options.

Aknowledgements.

The author acknowledges the financial support offered by Indian Council of Agricultural Research for taking up integration studies and the encouragement offered by Dr. M.A.M. Ramasamy, Founder Pro-chancellor and other authorities of Annamalai University.

References

Fujii Y and Hamano M (2003). Use of allelopathic cover crops and plant extracts with allelopathic activity for weed control in sustainable agriculture. Proceedings of the 19th Asian Pacific Weed Science Society Conference, Manila Philippines, (Weed Science Society of the Philippines).

Kannan C and Kathiresan RM (1998). Biological control at different growth stages of water hyacinth. Proceedings of the 1st IOBC global working group meeting for the biological and integrated control of water hyacinth. Harare, Zimbabwe. (Eds. M.P. Hill, M.H. Julien, T.D. Center) pp. 87-90.

Kathiresan RM (2000). Allelopathic potential of native plants against water hyacinth. Crop Protection 19, 705-708.

Kathiresan RM (2004). Integration of botanical herbicide Coleus amboinicus / aromaticus with insect biological control of water hyacinth. Completion report on research project submitted to National Agricultural Technology project – Indian Council of Agricultural Research.

McLaren S (1986). Biologically active substances from higher plants: status and future potential. Pesticide Science 17, 559-578.

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