| Hydrilla is one of the most serious aquatic weed problems in the United States. Fluridone is the only United States Environment Protection Agency (USEPA) approved herbicide that provides systemic control of hydrilla and recently, there has been a decrease in fluridone efficacy for hydrilla control in many Florida lakes. To characterize fluridone resistance, hydrilla populations were collected from different Florida lakes with varied histories of fluridone use and grown under controlled conditions for a period of one year in the absence of fluridone. During this one year period, phenotypic measurements were performed to monitor differences in growth and reproductive physiology. In addition, shoot tissue was collected from each population at 0, 3, 6, 9, and 12 months after planting and exposed to 5, 10, 15, 20, 30 and 50 μg L-1 fluridone to assess changes in fluridone susceptibility over time. Regression analysis was performed to calculate EC50 values for phytoene and β carotene. EC50 β carotene values of 9 and 63 μg L-1 fluridone were found in the susceptible and the most resistant population, respectively. There was no significant change in fluridone resistance in hydrilla populations over time. Molecular characterization was performed by cloning the gene for phytoene desaturase (pds) from fluridone susceptible and resistant hydrilla plants. Two independent somatic mutations at the Arg304 codon of pds were observed. The codon usage for Arg304 is CGT and a single point mutation yielding either Ser304 (AGT) or His304 (CAT) was identified in different resistant hydrilla populations. Resistant populations were significantly superior to or at par with susceptible hydrilla in growth and reproductive parameters, indicating no deleterious effects of mutations. To correlate varying levels of fluridone resistance to ploidy in hydrilla, flow cytometric analysis was performed. Differential ploidy levels (diploid 2n = 2x = 16; triploid 2n = 3x = 24; and tetraploid 2n = 4x = 32), along with endoreduplication patterns were observed among different hydrilla populations, and plants within each population. Aggressive spread of fluridone resistant dioecious hydrilla in aquatic ecosystems can severely impact hydrilla management, and consequently cause substantial and long-lasting ecological and economic problems throughout the Southern USA. |
