Response of hydrilla (hydrilla ver ticillata(L.f.) royle) to diquat and a model of uptake under nonequilibrium conditions

Response of the aquatic macrophyte hydrilla (Hydrilla verticillata (L.f.) Royle) to the herbicide diquat was measured by dissolved oxygen concentration in the surrounding water, chlorophyll a concentration in plant tissue and membrane permeability of hydrilla cells. Dissolved oxygen concentration was the most rapid and sensitive measure of response, followed by membrane permeability. Chlorophyll a was an inadequate measure of response because of high variability in measurements and slow response time. The lethal concentration of diquat in hydrilla tissue was approximately 600 μg/g dry weight. The uptake of diquat by hydrilla was modeled under nonequilibrium conditions and assuming exponential decay of diquat from the surrounding water. The modeled uptake depended on the rate of decay of diquat from the water column, the rate of uptake of diquat by hydrilla and the rate of depuration of diquat from hydrilla. The observed uptake closely matched the calibrated, predicted uptake.