Allelopathic potential of Tithonia diversifolia (Hemsl.) A.Gray and Lantana camara L. Leafy materials on growth, photosynthetic performance and yield of SB 19 and SB 97 soybean varieties

Abstract

Soybean is a legume which is widely grown in western Kenya for its edible seed but their yields are reduced by the effect of low soil fertility. The use of forest tree species and weeds as green manure has exacerbated soil fertility issues, potentially due to the allelopathic chemicals present in these organic materials. In Western Kenya, farmers are being encouraged to use shrubs as green manure such as Tithonia diversifolia and Lantana camara which have shown to improve soil fertility by 24% to 54%. Soybean varieties, SB97 and SB19 are known to be good yielders, whereas the two plants are being used as organic manure to increase nutrients in the soil for better crop growth and productivity, they are however also known to have allelopathic effects on some crops. Information available on their allelopathic potential on photosynthetic pigment, chlorophyll fluorescence and growth of soybean is not exhaustive. The main objective of this study was to investigate the allelopathic effects of fresh leaf aqueous extracts and fresh leaf material of T. diversifolia and L. camara on germination of seeds, growth, photosynthetic performance and yield of SB97 and SB19 soybean varieties. The specific objectives were to determine allelopathic effects of aqueous extract of L. camara and T. diversifolia on germination of SB97 and SB19 soybean varieties, to determine the allelopathic effects of fresh leaf segments of L. camara and T. diversifolia on growth and yield of SB97 and SB19 soybean varieties and to determine allelopathic effects of fresh leaf segments of L. camara and T. diversifolia on chlorophyll pigments and chlorophyll fluorescence of SB97 and SB19 soybean varieties. The experiments were carried out in the laboratory and greenhouse at Maseno University. In the lab, different concentrations of aqueous extracts, (25%, 50%, 75%, and 100%) and the control- tap water was used. Germination experiment was set in petri dishes and growth, yield and physiological parameters was carried out in the green house. Fresh leaf material of T. diversifolia and L. camara of 4g, 8g, 12g and 16g was added to soil in pots. The control only composed soil. This was replicated four times and laid out in Completely Randomized Design (CRD). Data on germination count were recorded daily, while growth and physiological parameters were recorded weekly. Data were subjected to analysis of variance (ANOVA) using SAS 9.1 to establish significant differences among treatments, varieties and plant materials. The treatment means were separated and compared using the Least Significance Differences (LSD) at P = 0.05. The results revealed that leaf extracts from Lantana camara and Tithonia diversifolia reduced the germination of both soybean varieties. These extracts contained bioactive compounds that inhibited germination, root and shoot growth and yield of Soybean varieties SB 19 and SB 97. Lantana camara exhibited stronger allelopathic activity than Tithonia diversifolia. On growth parameters, including shoot height, leaf number, leaf area, fresh and dry weights, and yield, the presence of allelochemicals in the biomass was suggested to have hindered growth by disrupting enzyme activities and nutrient uptake. Chlorophyll fluorescence and chlorophyll concentration analysis revealed that both leaf biomass materials impacted negatively on photosynthetic activity of the soy bean varieties. Allelochemicals in the fresh leaf segments of these plants decreased maximum photochemical efficiency and quantum efficiency, potentially reducing photosynthetic activity. Farmers are therefore advised not to incorporate Lantana camara and Tithonia diversifolia leafy biomass in farms where soy bean varieties SB 19 and SB 97 are growing.