Abstract: By the method of shading, this paper studied the influence of weak light (5%, 1%, 0.5%, and 0.1% of full sunlight) on the growth and development of Vallisneria natans seedlings, with the biological variables and leaf chlorophyll fluorescence parameters measured. Under 0.1% of full sunlight, no new plants germinated; under 5%, 1%, and 0.5% of full sunlight, new plants germination was gradually inhibited with experimental time. With the increasing experimental time and the decreasing light intensity, the formation of new leaves of old plants was increasingly restrained. Meanwhile, the formation of new leaves of new plants was not restrained before the 20th day of the experiment but restrained significantly in the following days. The leaf width of both new and old plants under weak light was inhibited significantly. The leaf elongation of old plants under weak light was inhibited significantly, and the inhibition rate increased with decreasing light intensity. The leaf elongation of new plants was promoted before the 20th day of the experiment, but inhibited significantly with increasing experimental time. The leaf maximum quantum yield (F_v/F_m) of old plants decreased significantly with experimental time, and the relative electron transport rate (rETR) and nonphotochemical quenching (NPQ) decreased markedly on the 80th day of the experiment. The fresh weights of the roots, stems, and leaves of new plants and old plants under weak light were much lower than those under full light, and, with decreasing light intensity, the fresh weight ratio of stems to plants and that of roots to plants had an increasing trend, while the fresh weight ratio of leaves to plants decreased significantly. On the 80th day of the experiment, the plants still had photosynthetic capability, and the biomass of rhizome was higher than those of leaves and roots. This study showed that Vallisneria natans had stronger tolerance against ≤1% of full light.

Key words: bryophyte, diversity, TWINSPAN, canonical correspondence analysis (CCA), urban area.