Allelopathic potential of croton bonplandianus bail

Preparation of extracts and leachates

Croton bonplandianus was selected as a donor allelopathic invasive weed and mung bean (Vigna radiata L. var. Vaibhav) was selected as test crop. The aqueous extracts of Croton bonplandianus were prepared from 100 g fresh leaves and roots after crushing in 500 ml of distilled water and filtered through Buchner funnel using Whatman No.1 filter paper. These extracts were stored as stock solutions (20%) in amber coloured bottles, which were diluted with distilled water to make desired concentrations (5% to 20%). For preparation of leachates of leaves and roots, 100g shade dried material was soaked in 500 ml of distilled water for 48 hours at 27±2 º C and the leachates were filtered through Buchner funnel using Whatman filter paper No.1. These leachates were stored as stock solutions (20%), and diluted with distilled water as mentioned above to make different concentrations (5% to 20%).

Collection of rhizosphere soil and Preparation of its leachates

About 500g of rhizosphere soil of Croton bonplandianus was collected randomly from the field to the depth of 15 — 20cm. The soil samples were cleaned, powdered and mixed to make a composite sample. 20g and 50g of this soil sample were suspended separately in 100ml distilled water. The solutions were stirred properly and kept for 48 hrs. These were filtered through Buchner funnel using Whatman filter paper No.1. The filtrates (20% and 50%) were used for seed germination bioassay along with control. Similar studies were carried out with rhizosphere soil directly and control soil (not having any vegetation).

Seed germination bioassay

The seeds of mung bean (Vigna radiata L. var. Vaibhav) were procured from College of Agriculture, Pune (M.S. India). The healthy seeds were used for bioassay studies using sterilized Petri-plates (9cm diameter) lined with special type of seed germination paper. The seeds were surface sterilized with 0.02% aqueous HgCl2 for two minutes and thoroughly washed with distilled water. The germination papers in Petri plates were moistened with 10 ml of respective concentrations of leaf and root extracts and leachates of Croton bonplandianus. The seeds placed in Petri plates moistened with distilled water were considered as control. Each Petri plate containing 10 seeds were kept in triplicate at room temperature (27+2^oC) wrapped in black paper to avoid direct sunlight. Seed germination percentage, root and shoot length, vigour index, root: shoot ratio, fresh and dry weight of seedlings were recorded on 7^th DAS.^[8]

Phytosociological studies on weeds of baramati tahsil, dist. Pune.

Results recorded in [Table 1] revealed that about 36 weed species were dominant in irrigated, semi-arid crop ecosystems and fallow lands. Amongst these about 50% of the dominant weeds were of exotic nature. These results have clearly indicated very high rate of invasion of non-native weeds in this region. Similar results were reported by some other workers.^[5], ^[6], ^[7], ^[8], ^[9], ^[10], ^[11], ^[12], ^[13], ^[14], ^[15], ^[26], ^[16], ^[17], ^[18], ^[19], ^[20], ^[21], ^[22], ^[24], ^[27], ^[28], ^[23], ^[29], ^[30] The phytosociological survey of dominant weeds had given an alarming indication about the invasion rate of non-natives in this region. If their invasion is not controlled effectively, they may substitute and reduce the diversity of natives. These invasive alien species (IAS) may be responsible to degrade the crop ecosystems in this region, causing significant yield reduction.^[17], ^[18], ^[19], ^[20], ^[21], ^[22], ^[24], ^[27], ^[28]

Croton bonplandianus was the weed with highest density (10.1 /m²), frequency (70%) and abundance(14.4) in the field, followed by Tridax procumbens and Cynodon dactyl on [Table 2].

Effects of rhizosphere soil and its leachates

The results presented in [Table 5] clearly indicated the inhibition of seed germination percentage with 20% and 50% soil leachates over control in mungbean. The seed germination bioassay conducted in rhizosphere soil had also shown considerable reduction in seed germination percentage, root and shoot length and thereby vigour index as compared to control. This has confirmed that the rhizosphere soil leachates and the soil itself had negative influence on seed germination and seedling growth. The different allelochemicals existing in the weed might be leaching into the soil from the whole plant and exuding from the roots. These may accumulate in the soil and affect seed germination and seedling growth of recipient plant species.^[9] A similar trend was reported by other workers.^[31]

Effects of leaf and root extracts

Inhibitory effects of leaf and root extracts of Croton on seed germination and seedling growth were recorded in mung bean at higher concentrations (15, 20%). The root and shoot length of seedlings was also reduced significantly along with vigour index and dry wt. of seedlings with increasing concentrations of leaf and root extracts[Table 3].

Effects of leaf and root leachates

The leachates of Croton leaves significantly affected seed germination percentage and seedling growth in mung bean. The leaf leachates have caused full suppression of vigour index at higher concentrations (15, 20%), whilst at lower concentration (5%) stimulatory effects were recorded. Similarly, root leachates also showed positive effects on seed germination and seedling growth at lower concentration treatments while at higher concentration there was inhibition[Table 4].

Similar inhibitory allelopathic effects of Parthenium leaves on germination and seedling vigour of sunflower were reported.^[24] The allelopathic impact of extracts or leachates is more harmful to radicle.^[3], ^[4]The level of phytotoxicity was directly proportional to concentration of leachates.^[6], ^[7], ^[8], ^[9], ^[10] The results of present investigation are in agreement with the above workers. The vigour index indicates the allelopathic effects on seedling establishment same was the trend in present study.

The successful invasion and dominance of Croton over native weed species might be due to different allelochemicals existing in it. Present studies may help for understanding crop weed interaction in semiarid agro ecosystem of Baramati Tahsil.