Selection of Cocoa Tree ( Theobroma cacao Linn) Endophytic Bacteria Solubilizing Tri-Calcium Phosphate, Isolated from Seedlings Grown on Soils of Six Producing Regions of Côte d’Ivoire

Abstract

Phosphorus (P) is an essential macronutrient for the growth and development of cocoa tree (Theobroma cacao Linn). Most of the soils used for cocoa farming in Côte d’Ivoire are low in phosphorus. But cocoa farmers generally have a widespread reluctance to invest in chemicals fertilizers due to high costs and environmental associated risk. Phosphate Solubilizing Bacteria (PSB) are kwon to play an important role in supply of phosphorous to plants in a sustainable manner in P deficient soils. The aim of this research was to screen the endophytic bacteria of cocoa nurseries able to solubilize tri-calcium phosphate. Seedlings of two varieties of cocoa (P7 and NA32) and seedlings of an all-comer, were grown on eighteen (18) samples soils collected in six producing regions of Côte d’Ivoire. A total of 218 endophytic bacteria were isolated and tested on the Pikovskaya’s agar medium, containing Ca3(PO4)2. The colonies with clear zone around the microbial growth were suspected as phosphate solubilizing bacteria. Out of 218 bacteria, 90 (41.28%) showed a clear zone around colonies after 7 days of incubation. The Phosphate Solubilization Index (PSI) ranged from 20% to 200%. Bacterial isolated from the soils of Duékoué locality showed the highest mean index of 137.67%. Five PSB (CEBSP5, CEBSP6, CEBSP7, CEBSP8, and CEBSP9) from Duékoué soils and two PSB (CEBSP12 and CEBSP13) from Soubré soils have a PSI ranged from 150% to 200%. Further study in greenhouse and in field condition will confirm the use of these PSB as biofertilizer to increase the available P content in soils, reduce environmental pollution and promote sustainable agriculture.