Flumioxazin has recently (2010) been registered for aquatic use for control of hydrilla and other noxious invasive aquatic plant species. Due to the rapid degradation of flumioxazin, especially in high pH water, some hydrilla research trials have produced less than desirable results with rapid plant regrowth. Therefore, laboratory experiments were conducted to evaluate the influence of pH on flumioxazin's effect on photosynthesis. Flumioxazin applied at concentrations ≥ 200 µg ai L⁻¹ in high (9.0) pH water and ≥ 100 µg L⁻¹ in low (6.0) pH water required 68 to 123 h to reduce photosynthesis by 50% (ET₅₀). The effect of 400 µg L⁻¹ flumioxazin on photosynthesis of apical hydrilla tips was also compared at low (20 µmol m⁻² s⁻¹), medium (170 µmol m⁻² s⁻¹), and high (400 µmol m⁻² s⁻¹) light levels at pH 9.0. Low light–treated tips were still photosynthetic at approximately 73% of the nontreated control plants 168 h after treatment. Low light–treated hydrilla required an estimated 303 h to achieve a 50% reduction in photosynthesis, while high light plants only required 99 h. Chlorophyll content of hydrilla was reduced as flumioxazin concentration was increased from 100 to 1,600 µg L⁻¹. These data indicate that flumioxazin activity on hydrilla photosynthesis is influenced by herbicide concentration, water pH, and light intensity.

Nomenclature: Flumioxazin; hydrilla, Hydrilla verticillata (L.f.) Royle HYLLI.