Low temperature stress at different flower developmental stages affects pollen development, pollination, and pod set in soybean

Pod set, a critical process for legume crop productivity, is sensitive to environmental stress such as low or high temperature or drought. At higher latitudes, pod set at low temperatures is one of the most important processes for efficient soybean production. The present study was carried out to determine which flower developmental stage was most sensitive to low temperature and how low temperature interrupts the pod setting process. Soybean flowers at various developmental stages were subjected to low temperatures, and the percentage elongation of pods was subsequently measured. Two low temperature-sensitive stages were found. The first was an early developmental stage approximately 12.5 days before the anthesis of individual flowers. The second stage occurred 3–4 days before anthesis. An investigation of the pollen grain number on stigma suggests that insufficient pollination causes low pod set under low temperature stress at both temperature-sensitive stages. Tetrad-shaped abnormal pollen grains were observed when flowers were subjected to low temperature at the first sensitive stage; thus, this stage might be the tetrad stage or the stage prior to tetrad formation. Furthermore, at the first sensitive stage, pollen development deficiency was one of the causes of poor pod set under low temperature conditions.