Allelopathic potential of cassava (Manihot esculenta var. Kuning) against selected weeds in aerobic rice (Oryza sativa L.) / Siti Aisyah Mohammad Taupik

Aerobic rice production systems require less water than conventional systems.
However, rapid growth of weeds is the major constraint in aerobic rice due to the lack
of standing water that could prevent the growth of weeds. As the result, weed control in
aerobic rice is highly dependent upon synthetic herbicide. Nevertheless, excessive use
of herbicide could lead to herbicide resistance in weed, human health and environmental
concern. Hence, this study aimed to evaluate the allelopathic potential of cassava plant
on the germination and seedling growth of selected weeds in aerobic rice and to identify
allelochemical compounds which were responsible for the inhibitory effects.
Laboratory bioassay revealed the degree of phytotoxicity of different vegetative parts
of cassava aqueous extracts can be classified in the order of decreasing inhibition as
follows: leaf > stem > tuber > tuber peel. The leaf aqueous extract at 0.5% (w/v)
concentration completely inhibited the germination and seedling growth of the
Ageratum conyzoides L. (broadleaf), Eleusine indica L. (grass), and Cyperus distans L.
(sedge) without affecting aerobic rice. In greenhouse experiment, the degree of
inhibitory effect of cassava residues followed the order of decreasing inhibition: tuber
peel > tuber > leaf > stem. Although the tuber peel was the most phytotoxic part, the
leaf residue was the only vegetative part that gave no inhibitory effect on aerobic rice.
The leaf residues at 8 t ha-1 reduced the seedling emergence and shoot growth of A.
conyzoides and C. distans by 85-100% whereas stimulation was evident on E. indica.
Fractionation of the aqueous leaf extract was subsequently carried out and the degree
of phytotoxic activity of different solvents of fractions can be classified in the following
order of decreasing inhibition: ethyl acetate > methanolic > hexane > aqueous. Seven
potential allelochemicals with herbicidal activity including coniferol, transferuladehyde,
luteolin, gigantol, anemonin, β-carboline, and procyanidin A2 were
identified from the ethyl acetate and methanolic extracts of cassava leaf. These findings
suggest that the cassava leaf has potential use for weed control in aerobic rice and might
be developed as the source of natural herbicide.