2.1 Plant material
From June 20th to 30th, 2013, the turions were respectively collected from four distant populations ofP. crispus in Gansu Province, which is located in the warm temperate zone of Northwestern China (Table 1). For each population, over 30 turions were carefully removed from the decaying shoots and placed into labeled plastic bags separately. After the collection, the turions were immediately delivered to the National Field Station of Liangzi Lake Ecosystem in the sub-tropical region of Central South China (30°05’–30°18’ N, 114°21’–114°39’ E) and separately stored in four 1000 ml beakers with 800 ml deionized water in the refrigerator at 0 ℃. The deionized water of each beaker was replaced every three days. In December 15th, 2013, the turions from four distant populations were taken out of the refrigerator and respectively placed into 300 ml beakers with 250 ml of Liangzi Lake water. The beakers were placed on a table in the laboratory. The lake water was from an electric pump system linked to the Liangzi Lake. Each turion was placed into one beaker and the beakers were labeled to discriminate the geographic sources. The lake water of each beaker was replaced every three days. On January 21st, 2014, eight germinated turions with similar morphology and size (5-6 scale leaves, approximately 4 cm long for metamorphosed stem length and 3 cm wide for the distance between two largest opposite scale leaves, see the turion structure in Fig. 1) for each geographic source were selected for the mesocosm experiment.