Evaluation of the effect of selected plant growth regulators on soybean yield parameters in South Africa

Abstract

Soybean is an important protein and oilseed crop throughout the world due to the commercial use of soybean meal, oil and sub-products. South Africa is the largest soybean producer in Africa and produces on average 1.8 t ha-1. Although soybean productivity in South Africa has increased over the past decades, yield gaps can be narrowed by adopting improved agricultural practices that increase yield and decrease yield loss due to genetic barriers and environmental stresses. Available biotechnologies, in particular plant growth regulators (PGRs), have been reported to increase the tolerance of crops to environmental stresses, increase harvestable yield and to enhance growth and yield components. The aims of this study were to evaluate the efficacy of PGRs at different application times and rates to increase soybean yield in South Africa. To determine the optimal time of application, five field trials were conducted at three localities, viz. Bethlehem and Kroonstad in the Free State province and Potchefstroom in the North-West province, South Africa. Two field trials and one pot trial were conducted to investigate application rates at Potchefstroom. The cultivar PAN 1521 R was planted in the field trials while LS 6161 R was used in the pot trial. Foliar treatments consisted of a control (untreated) and three PGRs, to which a specific identification code was assigned, viz. A2019 forchlorfenuron (CPPU), B2019 naphthalene acetic acid (NAA) and C2019 gibberellic acid + abscisic acid (GA3 + S-ABA). Plant growth regulators were applied at two different growth stages (R1 and R3). To determine the optimal application time, applications were done at R1 and R3, as well as at both R1 + R3 (double application). Three concentrations of each PGR were applied at growth stages R1 + R3 (double application) to determine the optimal rate of application. No optimum application time or rate could be determined for any of the PGRs evaluated on soybean in this study. None of the three PGRs evaluated at different application times significantly increased the yield of soybean. Application of PGRs to soybean planted in Bethlehem (cold zone) were more effective in terms of nodes plant-1, pods node-1 and pods plant-1 than PGR applications at Potchefstroom and Kroonstad. Plant growth regulator treatments increased growth parameters of soybean, however these effects varied between Potchefstroom, Bethlehem and Kroonstad and were not consistent. None of the application rates of the respective PGRs, increased any of the growth and yield components in the field trials or the pot trial at Potchefstroom. It could possibly be ascribed to the low application rates used and the warm and dry climatic conditions experienced during this study. Application of PGRs at too low concentrations will have no effect on growth and yield parameters. Although increases in growth and yield parameters were reported for other crops, similar results were not found during this study. More research on the PGR, rate and application time for soybean is therefore needed in South Africa.