REPRODUCTIVE ECOLOGY OF VALLISNERIA NATANS

Vallisneria natans is a dominant submerged macrophyte in Lake Taihu. The biomass and quantity of propagules of V. natans were investigated in Lake Taihu in October, 2003 and in March, 2004. Seed and tuber germination, and development of seedlings produced from tubers under different conditions were also studied in the laboratory. Our objectives were to understand characteristics of the reproductive ecology in V. natans. The results showed that the biomass of asexual and sexual propagules was 10.1%±7.0% and 25.0%±13.8% of total biomass, respectively, and the biomass of sexual propagules was greater than asexual propagules. The average number of female flowers was 22.9±13.8 per individual plant, and seedset rate was 73.3%±17.9%. There were 11-33 spathes per individual male plant and 364±38 male flowers inside each spathe. Based on our in situ investigation, we estimated a density of 6.29×105-1.89×106 microspores·m -2 around V. natans populations in Lake Taihu. Seedlings produced from seeds were not able to survive in Lake Taihu, which is likely caused by factors such as wind, wave and water depth. Most populations in V. natans reproduced asexually from root tubers. We estimated a mean tuber density of 90-226 per square meter. Light, temperature, sediment, thermal fluctuation and seed age had a marked influence on germination rates. Germination rates of seeds stored for 4 months under water in the lake was 8.35%±1.89%, 56.73%±6.42% and 43.55%±4.34% at 10, 20 and 30 ℃, respectively. Seed germination rates stored for one year was 35.53%±3.53% under natural light conditions and decreased to 12.63%± 2.86% under dark conditions. Germination rates of seeds stored for one or two years was 35.53%±3.53% and 12.63± 2.86%, respectively. Germination rates of seeds stored for four years was near zero. Germination rates of seeds decreased under conditions without sediment. Tubers did not require light for germination. There was an optimal temperature of 20 ℃ for tuber germination to occur that was as high as 90.2%±12.6%. Biomass accumulation in seedlings produced from tubers under light conditions was greater than that in the dark, while leaves of seedlings grown in the dark were longer. Seedling increases in length mainly depends on the stems of the lower end of leaves. We present suggestions for restoration efforts of populations of V. natans in Lake Taihu.