Allelopathic effects of tamarind husk, lemongrass and citronella residues to suppress emergence and early growth of some weeds

Abstract

The allelopathic effects were evaluated of water extracts from tamarind (Tamarindus indica L.) husk and from lemongrass (Cymbopogon citratus Stapf.) or citronella (Cymbopogon nardus Rendle.) leaves on 2 test plants: ruzi grass (Brachiaria ruziziensis Germ. & C.M. Evrard) and popping pod (Ruellia tuberosa L.). In the laboratory, the seeds were subjected to the different extracts at concentrations of 0, 1.25, 2.5, 5 and 10 % (w/v). The number of germinated seeds was counted daily for 6 days. The shoot/root length was measured 5 days after sowing (DAS). The results showed that the aqueous extract of tamarind husk did not inhibit the germination of either ruzi grass or popping pod. The lemongrass and citronella extracts had higher inhibition effects at high concentrations and clearly delayed seed germination at 10 % (w/v). With the seedling growth bioassays of the 3 extracts, only the citronella extract at the highest concentration had any effect on the shoot/root lengths of ruzi grass. Popping pod was susceptible to all 3 extracts, especially the popping pod roots treated with lemongrass and citronella extracts. The lemongrass and citronella extracts caused cell death and induced lipid peroxidation in both plants. Therefore, the primary action of these extracts on seedling growth inhibition might have been related to the loss of cell viability and triggering lipid peroxidation in affected tissues. The residue incorporation bioassay determined the effects of residue mixed in soil at concentrations of 0, 2.5, 5 and 10 % (w/w) on the growth of ruzi grass and popping pod at 30 DAS. Both plants showed an adverse effect than for the control, based on reduced leaf numbers and greater chlorosis, especially with soil incorporation of the lemongrass and citronella residues at 10 % (w/w). Mixing soil with lemongrass and citronella leaves at 2.5 % (w/w) and above resulted in reductions in the shoot/root lengths and shoot/root biomass values. Catechin, gentisic acid, syringic acids, p-coumaric acid and ferulic acid were the dominant allelochemicals found in the lemongrass and citronella residues.