Gibberellic Acid (GA(3)) Applied to Flowering Heracleum sosnowskyi Decreases Seed Viability Even If Seed Development Is Not Inhibited

Sosnowsky's hogweed (Heracleum sosnowskyi Manden.), an important invasive species in Eastern Europe, is a monocarpic perennial plant that propagates exclusively by seeds. Hence, interfering with seed viability could help control its spread. In the present study, we investigated the effect of exogenous GA(3) (25, 100 and 150 mg/L) sprayed twice onto flowering H. sosnowskyi plants on the development of fruits (mericarps) and their ability to germinate under field conditions over the growing seasons of 2018 and 2019. Mericarps from plants sprayed with GA(3) failed to develop normally. The width/length ratio of mericarps decreased by 23% to 25% after 150 mg/L GA(3) application and their average weight decreased between 7% and 39% under all GA(3) treatments. X-ray radiographs revealed that the internal structure was malformed, with many of the mericarps lacking well-developed seeds. Proportionally fewer well-developed mericarps were produced by GA(3)-treated plants than water-sprayed control plants in 2018. Seed germination assessed outdoors in seeds buried in the ground was also severely reduced (from 58% to 99% after 150 mg/L GA(3) application). This indicates that exogenous GA(3) sprays result in incomplete seed development and a consequent decrease in viability and germination. As the highest GA(3) dose used resulted in significantly reduced propagation of Sosnowsky's hogweed through seeds in the field, GA(3) provides a promising approach to the control of the spread of this invasive weed species.