Improved Grain Yield by Phytohormones-Driven Suppression of Pod Abscission and Revitalization of Source-Sink Relationships in Soybean

In many crops, the exogenous application of phytohormones or other chemicals can improve grain yield via modulation of carbohydrate metabolism in source and sink tissues. However, the issue is yet to be investigated thoroughly in soybean. In a field experiment, we applied auxin (indole acetic acid, IAA), cytokinin (benzyl amino purine, BAP), gibberellic acid (GA), nano-TiO2 and sucrose to soybean plants from the stage of flowering to the end of pod proliferation; and measured diurnal carbohydrate metabolism at pod filling, as well as yield related components upon harvest. Auxin appeared as the most effective treatment in reducing pod abscission by 32%, and enhancing seed size and number by 5% and 50%, respectively. The auxin-treated plants also showed an increase of about 16% in biomass, 52% in carbohydrates (leaves and pods) and 32% in pod alkaline invertase activity. Principal component analysis (PCA) indicated that carbohydrate-related parameters at sunset and sunrise correlate with yield, suggesting that diurnal photo assimilate availability-in the form of sucrose-and the nocturnal re-mobilization of starch play vital roles in yield enhancement. Of the other investigated compounds, sucrose, nanoTiO(2) and GA reduced flower abscission with slight effects on yield, while cytokinin enhanced pod abscission. Altogether, auxin seems to increase the crop yield via not only the reduction of pod abscission but also the enhancement of carbohydrate metabolism in both source and sink tissues.