Introduction

Weeds have general purpose genotypes. They decrease crop yields primarily by competing for light, water and nutrients. They influence the growth of associated plants through root exudates, leachates and competition for growth resources (Sundaramoorthy and Sen, 1990). There are different control mechanisms of weeds. The chemical herbicides are effective in controlling weeds, yet risks are involved in their usage. Due to increased awareness about the risks, nowadays there is much emphasis to search for alternative methods of weed control. In this connection, allelopathy has been recognized as a natural weed control approach (Rice, 1984; Narwal, 1994; Akhtar et al., 2001; Khan et al., 2006).

Within last decade Parthenium hysterophorus Linn. has become one of the most dreaded weeds of the world (Trivedi et al., 1991). This weed of Asteraceae family is a native of West Indies and Middle and North America. This has been spreading very rapidly in our whole India. The reason of its rapid growth all around is its achenial fruit which disperses along with air very far off because of its special structure. It is now invader in crop fields also. The day is not far when it will occupy our fertile lands and will compete with our native crops. Cassia tora is a dominant weed of family Caesalpiniaceae during rainy season and loves shade. It was introduced originally from Tropical America and is a very common all over the study area (Patna and Vaishali) along roadsides and in wastelands. Croton bonplandianum belongs to family Euphorbiaceae and is found as ruderals on roadsides and canals.

In India, 4-7% of human population suffers from clinical symptoms associated with Parthenium hysterophorus (Shelke, 1984; Towers and Subba Rao, 1992). Reports are there for replacement of Parthenium through the use of other plants like Cassia species, Croton bonplandianum and others (Joshi, 1991). In this context, this study was undertaken to unravel the competive aggressiveness of Parthenium hysterophorus vis-à-vis Cassia tora and Croton bonplandianum in terms of germination and radicle length.

Materials and Methods

The materials for present investigation are Parthenium hysterophorus, Cassia tora and Croton bonplandianum and were locally collected from Patna. Leaves, stems and roots of all three species were used for judging allelopathic potential among themselves. They were chopped into small pieces and crushed in distilled water to get mother solution. From mother solution, desired aqueous concentrations 0.5%, 1%, 2%, 3% and 4% were prepared by dilution with distilled water. The pH of different concentrations of leaf extract of Parthenium hysterophorus ranged from 6 – 7.0, stem extract from 5 – 6.0 and root extract from 5 – 5.5. In Croton bonplandianum the pH of leaf and stem extract ranged from 6 – 7.0 and root extract from 5 – 5.5. The pH of different concentrations of leaf and stem extract of Cassia tora ranged from 5 – 5.5 while root extract 5.0. Now for germination and radicle growth, 25 seeds (mechanically scarified in Cassia tora and Croton bonplandianum) of species concerned were taken and surface sterilized by 0.2% HgCl₂. Seeds were finally put in petridishes on filter paper backed with cotton wool and were moistened with extracts of desired concentrations. The treatment was replicated thrice. A separate control was also maintained. The petridishes were kept in diffuse light and at temperature 26 + 3^oC. At the end of the experiment, percentage germination and radicle length were recorded and analysed statisticially.

The experiments were done in the Departmental laboratory of Botany, Patna University.

Results and Discussion

The mean weight of the fresh 100 seeds was 0.11 gm in Parthenium hysterophorus, 0.73 gm in Croton bonplandianum and 1.55 gm in Cassia tora.

The initiation period for germination was 2 days in Parthenium, 3 days in Cassia tora and 11 days in Croton bonplandianum. The percentage germination was highest in Parthenium hysterophorus (Table 1).

Table 1 : Germination percentage of freshly harvested seeds

 The rate of germination per day was 3.75% in Cassia tora, 4.95% in Croton bonplandianum and 10.55% in Parthenium hysterophorus. The root, stem and leaf extract of Parthenium had no remarkable effect on germination and radicle length of other two species (Tables 2 and 3). 

Table 2: Effect of aqueous leaf, stem and root extract of Parthenium hysterophorus on Cassia tora

Table 3 : Effect of aqueous leaf, stem and root extract of Parthenium hysterophorus on Croton bonplandianum.

 But aqueous extracts obtained from roots, stems and leaves of Cassia tora and Croton bonplandianum, significantly suppressed the germination of P. hysterophorus. Generally increase in the extract concentration increased the inhibitory potential.

Aqueous leaf, stem and root extracts of Cassia in every concentration (i.e. 0.5 – 4%) were inhibitory for seed germination of Parthenium. At 0.5%, the germination percentage was 53.33 to 57.33% but at 4%, it declined to 37-40% (Table 4). The trend of inhibition was similar.

Table 4 : Effect of aqueous leaf, stem and root extract of Cassia tora on Parthenium hysterophorus

The effect of aqueous leaf extract of Croton on Parthenium is similar to Cassia. The root extract was more potent and it declined the germination upto 29.33% at 4% (Table 5). The stem extract was intermediate in effect.

Table 5 : Effect of aqueous leaf, stem and root extract of Croton bonplandianum on Parthenium hysterophorus.

Aqueous extracts of Cassia tora and Croton bonplandianum also suppressed the radicle length of the Parthenium seedlings. Progressive inhibition was observed in the linear growth of radicle with proportionate increase in the concentration of the extract. Radicle length was maximum at earlier and then there was a continuous decrease ie. 1.73 cm and 0.53 cm (in 0.5% and 4% aqueous leaf extract of Cassia tora), 1.83 cm and 0.8 cm (in 0.5% and 4% aqueous stem extract of Cassia tora), 1.77 cm and 0.73 cm (in 0.5% and 4% aqueous root extract of Cassia tora) whereas 1.63 cm  and 0.6 cm (in 0.5% and 4% aquous leaf extract of Croton bonplandianum), 1.83 cm and 0.93 cm (in 0.5% and 4% aqueous stem extract of Croton bonplandianum) and 1.97 cm and 0.42 cm (in 0.5% and 4% aqueous root extract of Croton bonplandianum).

Correlation coefficient (r) between different concentrations and germination percentage was worked out. The leaf extract of Cassia tora on Parthenium hysterophorus shows significant negative correlation at 1% level. The leaf, stem and root extract of Croton bonplandianum on Parthenium hysterophorus shows negative and significant correlation at 1% level. Only the root extract of Parthenium hysterophorus on Croton bonplandianum was significant at 5% level (Table 6 ).

Table 6 : Correlation coefficient (r) between concentrations and germination percentage.

Significant at P=0.05

Significant at P=0.01

Ph=Parthenium hysterophorus, Ct= Cassia tora,

Cb=Croton bonplandianum 

There was negative correlation between concentrations and radicle length. It was significant in Cassia tora and Croton bonplandianum on Parthenium hysterophorus. The leaf extract of Parthenium hysterophorus on Cassia tora was negatively correlated and significant at 5% level (Table 7).

Table 7 : Correlation coefficient (r) between concentrations and radicle length.

 Significant at P=0.05

Significant at P=0.01

Ph=Parthenium hysterophorus, Ct=Cassia tora,

Cb=Croton bonplandianum

Any direct or indirect, inhibitory or stimulatory effect of one plant on another through the production of volatile chemical compounds has been termed as allelopathy. Chemicals that impose allelopathic influences are called allelochemicals. The harmful effect of higher extract concentration on growth parameters might be due to excess of allelochemicals which inhibit gibberellin and IAA – induced growth. It is hoped that exploitation of these natural mechanisms may lead to novel approaches to weed management with undoubted environmental advantages.

Croton bonplandianum with well developed root and shoot systems are able to suppress the vegetative growth of Parthenium hysterophorus. The chemical constituents of Croton bonplandianum are palmitic acid, stearic acid, oleic acid, linoleic acid, linolenic acid and β-linoleo-di-α-lanolenin while of Cassia tora are glycoside, oleic acid, emodin, stearic acid, palmitic acid, lignoceric acid and sitosterol (β-sitosterol-β-D-glucoside). Further Cassia tora and Croton bonpladianum are valued as fodder.

Acknowledgements

The authors are extremely thankful to Prof. U.K. Sinha, Head, Department of Botany, Patna University for providing laboratory facilities.

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