H. verticillata and V. natans collected from Xihu lake in Huizhou, Guangdong province in South China were preincubated at our laboratory at Jinan University for about 5 months. For H. verticillata, apical shoots 20 cm length were separated from the mother plant, weighed to determine initial fresh weight, and washed with distilled water before use. Whole V. natans plants of approximately the same size were selected, weighed, and washed before use.