Abstract: To determine impacts of water depth on growth of Hydrilla verticillata,top branches or telomes taken from the plants were planted in plastic  pots placed at water depths of 0.5,2.0,2.5,3.0,and 4.0 meters,separately.Results show that water depth significantly affected survival rate of H.verticillata.In water deeper than 2.5 meters,significant retardation of H.verticillata in growth was found due to shortage of light.With the water depth reaching 3.0,3.5 or 4.0 meters,H.verticillata died on D55,D33 and D21,separately.Moreover,water depth also altered morphological characteristics of H.verticillata.The effect was especially significant on plant height(P<0.01)and on number of nodes,node spacing and number of branches as well(P<0.05).On D80 within the water 0.5-2.0 meters in depth,the plant height of H.verticillata and the number of nodes increased gradually with the depth,however,within the water deeper than 2.0 meters,they declined with depth.In addition,within the water 0.5-2.5 m in depth,the node spacing increased with depth on D80.At the same growth time the number of branches decreased with depth.Biomass of the plants,regardless of depth,showed a similar trend,that is,rising first and then falling.Within the water 0.5-1.5 m in depth,biomass of the plant increased with depth,and peaked at 1.5 m in depth.When the plant got as deep as 2.0 or 2.5 m,its biomass declined rapidly.Biomass of the plant varied sharply between depths (P<0.01).Therefore,under the experimental conditions as such,the light compensation depth for H.verticillata ranges from 1.5 to 2.0 meters below the water mean transparency(1.0 m).

Key words: H.verticillata, water depth gradient, light intensity, growth