natural-products-in-plant-pest-management

Natural Products from Plants 75in their study of bioactivity of plant extracts, Deena et al. (2003) demonstratedthe inhibitory effect of leaf, stem and root leachates from Andrographispaniculata on germination and seedling growth in rice. From this it becameclear that the bioactivities of plant extracts are unpredictable and may givedifferent and often contrasting results with regard to inhibition or promotionof growth and development in other plants. The reaction of crops totreatment with plant extracts may depend on the interaction between differenttypes of plant species or even on the concentration of the extracts(Channal et al., 2002a).Research into the allelopathic activity of plant extracts has resulted in theidentification of active ingredients responsible for both inhibition or stimulationof either seed germination or seedling growth. In this respect, Chunget al. (2002) reported on the inhibitory effect of ferulic, p-hydroxybenzoic,p-coumaric and m-coumaric acids isolated from three rice cultivars on thegrowth of barnyard grass. This suggested that these compounds may be, atleast, a key factor in rice allelopathy on barnyard grass, and the informationpresented may contribute to the development of natural herbicides.Similar active allelochemicals were isolated by Sasikumar et al. (2002). Intheir study on the allelopathic effects of Parthenium hysterophorus leachateson cowpea, pigeonpea, greengram, blackgram and horsegram, the authorsreported significant seed germination inhibition for all test crops. Gas chromatographicanalysis showed the presence of phenolic acids, namely caffeic,p-coumaric, ferulic, p-hydroxybenzoic and vanillic acids in the leachatesfrom different plant parts (leaf, stem, flower and root) of P. hysterophorus. Amixture of allelopathic compounds in bioassays significantly inhibited thegermination and vigour index of all test crops. However, leachates fromflowers had no inhibitory effect on the germination of blackgram andgreengram seeds.Kato and Kawabata (2002) isolated a growth-inhibiting compound fromthe acetone extract of 30-day-old lemon balm (Melissa offinalis) shoots bymeans of silica gel column chromatography. This uncharacterized compoundinhibited the growth of cress seeds at concentrations higher than 0.3 µg ml –1 .Iqba et al. (2002) showed that living buckwheat reduced weed biomass comparedto plots without buckwheat. A laboratory study revealed that root exudatesfrom buckwheat (collected from Aomori, Japan) suppressed root andshoot growth of the weeds Trifolium repens, Brassica juncea, Amaranthus palmeri,Echinochloa crus-galli and Digitaria ciliaris but also that of lettuce, andreduced weed dry weight. Fagomine, 4-piperidone and 2-piperidinemethanolwere isolated from a chloroform extract and identified as the activeingredients.Several allelochemicals have also been characterized from Helianthusannuus that inhibit seed germination and seedling growth of Amaranthusalbus, Amaranthus viridis, Agropyron repens (Elymus repens), Ambrosia artemisiifolia,Avena fatua, Celosia cristata (C. argentea var. cristata), Chenopodium album,Chloris barbara (Chloris barbata), Cynodon dactylon, Digitaria sanguinalis, Dactylocteniumaegyptium, D. ciliaris, E. crus-galli, Flaveria australasica, P. hysterophorus,Portulaca oleracea, Sida spinosa, Trianthema portulacastrum and Veronica