UC Davis

The success of the Insect Sterile Technique (IST) in managing insect pests raised the hypothesis that a similar approach could be employed to control weed populations. This research delved into the potential of using irradiated sterile pollen to disrupt seed production in dioecious weeds, with a focus on Palmer amaranth (Amaranthus palmeri S. Watson). The overall objective this project is to understand the reproductive biology of dioecious weeds and to examine the possibility of using sterile pollen to disrupt seed production in dioecious weeds. In Chapter 1, we characterized phases of flower development in A. palmeri and compared organogenesis of flower development in female and male plants. Understanding the reproductive biology of this species is crucial for guiding the development of novel weed management strategies, as it enables the identification of specific vulnerabilities that can be targeted to disrupt seed production. Results showed the distinction between the two flower types became apparent at stage four by the formation of stamen primordia in staminate flowers, which developed both the female and male reproductive organs initially, as contrasted to pistillate flowers which produced carpel primordia only. Our study suggests that the evolution of A. palmeri from a cosexual ancestral to complete dioecy is still in progress. Chapter 2 examined the optimal irradiation dose to reduce seed production in A. palmeri. An irradiation dose of 300 Gy seems to be the most effective in reducing seed set in Palmer amaranth. Furthermore, the greatest reduction in seed set was achieved when irradiated pollen was introduced to the stigma through artificial pollination prior to open pollination. It appears that irradiated pollen exerts a preventive effect on naturally occurring pollen that arrives later. Furthermore, in order to increase the efficiency of SPT applications, Chapter 3 focused on investigating an ideal dry (inert) diluent and a most effective mix ratio of pollen/diluent and identifying the optimal strategy of sterile pollen applications to minimize seed production in Palmer amaranth. The findings showed that the optimal formulation was a 25%/75% mixture of irradiated pollen and talc powder by volume, successfully reducing seed set in A. palmeri while efficiently utilizing the limited resource of irradiated pollen. The most effective application strategy was initiating the application 7 days after anthesis and repeating it three times at 7-day intervals. This study also addressed the potential trade-off between inflorescence growth and fertilization rate, hypothesizing that high fertilization could divert resources away from inflorescence development to seed production. We found massive pollination of irradiated pollen or non-irradiated pollen did not have an effect on inflorescence growth, but it did on the sex ratio in the progeny population, resulting in female-biased progeny as predicted by certation theory.