Allelopathic compounds are released directly or indirectly into the rhizosphere by leaching, residue incorporation and decomposition, volatilization, root exudates etc. Research indicates that bio chemical interaction takes place when allelochemicals produced by one plant escape into environment and influence the growth and development of another plant.
Factors influencing the allelopathic potential
- Allelopathic response depends largely on the qualitative and quantitative composition of allelochemicals in the soil and their degraded by-products by abiotic and biotic soil factors.
- Knowing sensitive stage of susceptible plants.
- Release of allelopathic compounds in bioactive concentrations from crop plant is important rather than mere presence of allelochemicals in crop.
- The time of crop sowing, weed emergence, crop harvesting and weed maturation are important considerations in using allelopathic rice cultivars.
Allelopathic effect of weeds on rice
Rice germination, growth and yield were significantly reduced by leachates from C. rotundus and this inhibition might be due to the presence of allelopathins. Among the whole plant extracts of Lantana camara, Cyperus rotundus, Ageratum conizoides, Echinochloa crusgalli, Commelina benghalensis and Xanthium strumanium, except for those of E. crusgalli and C. benghalensis, all others delayed rice seed germination. The allelopathic effect of methonol extract from goose weed (Sphenoclea zeylanica) using bio assay techniques and phytotoxic effect of the extract increased with the age of S. zeylanica. Extracts from inflorescences (seed) were the most phytotoxic, followed by extracts from leaves, stems and roots.
The extracts of Ceratophyllum sps., Hydrilla sps. increased rice seed germination.
Eleocharis acicularis suppressed the growth of Scirpus juncoides, Cyperus difformis and Monochoria vaginalis and increased rice yield.
Cyperus rotundus, C. iria and Fimbristylis miliacea does not influence rice (variety Jyoti) seed germination, but C. difformis, Panicum repens, Brachiaria ramosa, Monochoria vaginalis, Scirpus sps., and Ludwigia parviflora significantly reduced the germination.
Allelopathic effect of rice on weeds
In rice there are 60 cultivars from 15 countries which have known allelopathic activity and some of these cultivars control weeds at a rate of about 90% with in a certain radius.
Rice hull extracts contained more allelopathic toxic substances than leaf extracts and germination of Echinochloa crusgalli was inhibited as the extract concentration increased.
In rice most common allelopathic compounds (Kim Kil Ung et al. 1997) have been identified viz., p-hydroxy benzoic, vanillic, p-coumaric and ferulic acids.
Plant breeding for genetic manipulation and allelopathic potential against weeds, selecting for allelopathic crops and including them in the rotation, identifying, modifying natural plant products could give more active, selective and persistent herbicides.