Allelopathy research in agroforestry systems of South India

In agroforestry, trees produce lot of litter the and the allelochemicals released from it influence the companion/understorey crops, however, its influence varies with the tree crop combinations. Pigeonpea was found least susceptible and sesame the most sensitive to the bark leachates of acacia, casuarina, dalbergia, eucalyptus and tectona. Acacia nilotica reduces shoot biomass of wheat, berseem, chickpea and cotton. Pearlmillet grown with Acacia tortilis showed only 12% germination at 5 m distance from the tree and germination was enhanced to 78 and 92% at 10 and 15 m distance. A.nilotica had a greater depression effect on pearlmillet than clusterbean. Plant and soil extracts of A.nilotica and Cassia siamea inhibited the nodulation and N fixation of clusterbean ( Cyamopsis tetragonoloba ). Contrarily dried leaves of glyricidia promoted the growth of tomato seedlings and the productivity of pigeonpea, sesame, castor and sorghum was increased under leuceana trees and that of sorghum, under eucalyptus. Leaf extracts of tectona, Albizia procera and Acacia nilotica showed stimulatory effects on germination, growth, chlorophyll, protein, carbohydrate and proline content of soybean. The trees in association with rice, fingermillet (Eleusine coracana) and Echinochloa frumantacea increased organic carbon by three times, available P and K status significantly in soils. Eucalyptus in association with legumes enhanced the organic carbon, total N, mineral N and exchangeable cation and available P in soils. The dehydrogenase activity in soil under tree species was in the order of Leuceana leucocephala < acacia < eucalyptus. The population of bacteria, fungi and azotobacter increased under Bambusa bambos, Casurina eguisetifolia, Ceiba pentandra, Eucalyptus tereticornis, Leucaena leucocephala, Tectona grandis. Therefore, to improve the productivity of agroforestry systems, it is essential to determine the allelopathic compatibility of crops with trees.