Abstract
The investigation of allelopathic effects of different plant species is important to prevent serious crop losses that would arise. The usage of such materials as mulch, cover crops or residues will be useful to implement in integrated weed management strategies of agricultural fields. Therefore, this study was conducted to screen the allelopathic effect of selected plant species using seedlings growth of lettuce (Lactuca sativa L.) as an indicator. The sandwich method was used and 20 mg of dried plant material, resembling the leaf litter, from 27 species were placed in petri dishes. Twenty-eight treatments including the control treatment were arranged in a Completely Randomized Design (CRD). On the fifth day after the establishment of lettuce seeds, hypocotyl length (cm), radicle length (cm) and total height (cm) of seedlings were measured. The inhibitory percentage was calculated and a dendrogram with single linkage was developed. Results revealed that the lowest hypocotyl and radicle lengths with the highest inhibitory percentage were recorded for Ageratum conyzoides, Cassia occidentals and Clidemia hirta when compared to the control treatment (p<0.05). Weed species expressed varying degree of inhibitory effects on growth performances of lettuce seedlings. Further studies need to be carried out to explore the effects of allelopathy on crop plants. Key words: Inhibitory percentage, lettuce, seedling growth, hypocotyl, radicle.  
Highlights
Allelopathy is considered as an interference mechanism available to plants, which release chemicals that influence the growth and development of neighboring plants in both natural and agricultural ecosystems
The present study found that leaf litter of the Ageratum conyzoides of the family Asteraceae, C. occidentalis from Fabacea and Clidemia hirta of the family Melastomataceaeca resulted in over 80% of inhibition of the growth of lettuce seedlings and ranked first among the weed species tested in this study
Leaf litter of most weed species considered in the present study showed adverse influence on lengths of hypocotyl and radicle and inhibitory percentage of lettuce seedlings under laboratory conditions
Summary
Allelopathy is considered as an interference mechanism available to plants, which release chemicals that influence the growth and development of neighboring plants in both natural and agricultural ecosystems. This process happens through leaching, root exudation, volatilization, residue decomposition and other means by creating direct and indirect effects on the adjacent microenvironment due to (Khanh et al, 2007) chemical substances released by plants (Einhelling, 2008). Functional allelopathy is the release of toxic substances, which are resultants of transformation by microorganism (Inderjit et al 2002; Jabran and Farooq, 2013) These chemicals accrue and persevere for a substantial time in the plant, thereby causing significant interference on growth and development of neighboring plants (Einhelling, 2008) which can be either a crop or a weed.
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