Root signal controls pod growth in drought-stressed soybean during the critical, abortion-sensitive phase of pod development

Abstract

Abstract Root-originated chemical signals have been shown to regulate the response of vegetative shoot to drought in soybeans ( Glycine max L. Merr.). However, their roles in the growth of soybean reproductive structures under drought stress have not yet been investigated. To explore this, a glasshouse experiment was conducted in which potted soybeans were either well-watered (WW) or subjected to six levels of drought stress. Irrigation was withheld in pots at six different dates before anthesis to induce drought of different severity (D1–D6) at sampling, viz. 4 days after anthesis (DAA). Root water potential, leaf water potential, pod water potential, xylem sap [ABA], pod fresh weight (FW), and pod set percentage were determined. Soil water status in the pot was expressed as the fraction of transpirable soil water (FTSW). Pod FW started to decrease at FTSW=0.43±0.02, when pod water potential was similar to that in the WW plants, while root water potential had decreased to −0.15 MPa and xylem sap [ABA] had increased 9-fold as compared with the WW plants. Pod set started to decrease at FTSW=0.30±0.01, and coincided with the decrease in pod water potential. Pod set started to decrease only when pod FW had decreased ca. 30%. Based on the results, a potential role of drought-induced increase in xylem sap ABA in affecting pod growth was suggested. We proposed that a low pod water potential, which might have led to disruptions in metabolic activities in the pods, is important in determining pod abortion.