Abstract:Hydrilla verticillata is a submerged aquatic vascular plant, distributed all over China. In the fifties, the experiments of planting Hydrilla in fish-ponds for rearing fingerling of grass carp have been carried out in Guangdong province. This paper reports the experimental results that the estimation valus of produotivity of Hydrilla; the effects of water temperature, light intensity and plant density on productivity; daily fluctuationn law of dissolved O2, CO2 and pH in fish-pond of planting Hydrilla and the cut branches propagation of Hydrlla.1. Under the condition of light intensity of 8000 lx and water temperature of 28±1.5℃, after twenty times of estimation, the gross production (mgO2/hr./0.5g fresh weight) of Hydrilla varies from 0.32 to 1.47. average 0.69; net production from 0.10 to 1.33, average 0.43; respiration from 0.07 to 0.50, average 0.26, According to the percentage of weight increase or decrease, the rate of photosynthesis in an hour varies from 0.34 to 3.04%, average 1.32%; rate of respiration in an hour varies from 0.25 to 1.65%, average 0.85%.2. Under the condition of light intensity of 8000 lx, the water temperature is 30—40℃ at the time of the higher productivity. The net production is a minus quantity at 45℃ water temperature.3. Under the condition of water temperature of 35℃, the light intensity is 4—6×104 lx at the time of the higher productivity.4. Productivity reduces after the increase of the plant density, The optimum plant density should be 1500—3600 g/m2, under this density, the collective production of Hydrilla becomes higher.5. The daily flucturation of dissolved O2, CO2 and pH in the fish-pond of planting Hydrilla is very great, the daily difference in dissolved O2 is 10 mg/l.6. In May and June, the rooting and growing of cut branches is fast. when the rate of rooting reaches 80—100% and the time needs is 5—6 days. The relative growth rate of cut branches is 49—55 mg/g/day.