PRELIMINARY STUDY ON PROPAGULUM DISTRIBUTION CHARACTERISTIC AND GERMINATION OF VALLISNERIA SPIRALIS L. IN LIANGZI LAKE

Vallisneria spiralis L. is a common species of submersed macrophytes in Changjiang River Basin. Its stands offer habitats and substrates for fish and many invertebrates at lake and river underlayers. The vegetative propagulum of V. spiralis,which is turion, contain large amount of starch. Its leaves are also palatable to waterfowl and other herbivores. Thus, its population is always undergoing relative high pressure of grazing. Recent studies have shown that herbivory can greatly reduce production of aquatic plant systems and potential reproduction output of each plant species. This means, eventually, their seed bank dynamics and successive vegetation patterns can be considerably changed. However, studies also shown that plants can avoid herbivory in a certain degree by some escaping strategies. As plant-herbivory interactions are essential in aquatic systems, how V. spiralis maintain its dominated role in Liangzi Lake is an interesting question. Our study focused on the primary distribution aspects of propagulum of V. spiralis, involving in turion and seed; in Liangzi Lake, and potential differences in germination performance of the diversely distributed propagulum. We aimed at simulating influences of waterfowl herbivory on the propagulum distribution and the germination of V .spiralis, and exploring the domination mechanism of V.spiralis in Liangzi Lake.\;Firstly,we investigated the vertical distribution pattern of V.spiralis turions in silt of Liangzi Lake in December,2002.An approximate method was used for alternative measurements of turion burial depth.The stolon length was thought to be an appropriate index of the burial depth,since most of the stolons between turions and stalks extended straight downwards in the soft clay.46 random sampling was carried out. After the burial depth was recorded,turion fresh weight were measured immediately.A relationship was analyzed between the turion size and the burial depth.In germination experiment,we collected the V.spiralis turions and classified them according to the turion size(A:0.75g).Then they were buried for germination under a cross-treatment of two water level(～0 and ～3cm)and two sand depth(5 and 10cm).The germination experiment was carried out in 10 replicates.Two-way ANCOVA was performed to evaluate the germination and growth of the seedlings.We simulated the influences of waterfowl herbivory on the turions by comparing the germination differences between intact turions and cut turions.The germination differences at 20 days was compared by using non-parameter data analyses.Another experiment was carried to compare the germination of the new turions (which is formed in this year) to that of the old turions(which is formed not in this year).There was also an experiment about influences of substrate type on germination of the seeds of V.spiralis.;Results were mainly as following:Average turion weight of V.spiralis in Liangzi Lake increased with the increasing of depth from 2cm to 30cm in the sediment.Turion weight also correlated positively with bud number of the turion ( P < 0.05).Sediment depth, overlying water regime and turion weight has a pronounced impact on germination of buds. The grazing damage of turions was simulated by removing first bud and separating turion into pieces containing one bud. Results showed these removing and separating promoted the germination of the rest buds. Turions produced in last year and in this year had the same potential germination capacity, these may be helpful for V. spiralis to sustain its population size and dominance under the graze pressure during winter. Seed germination in sand is much higher than that in silt, but its survive rate did not differ significantly between the two substrates under undisturbed condition. Our results exhibits that there is an resource trade-off exists among reproductive strategy of V. spiralis.