Abstract: Light is a crucial factor for growth of submerged macrophytes.Its influence on growth of Elodea nuttallii seedling was analyzed.Seedlings of Elodea nuttallii were transplanted into waters exposed to 100%,5%,1%,0.5%,0.1% and 0.05% sunlight,respectively.The sunlight intensity was controlled with a shading-net.Growth of Elodea nuttallii seedlings was investigated.Meanwhile,photosynthetic fluorescence characteristics were determined with a developed,submersible,pulse-amplitude modulated(PAM) fluorometer,or diving-PAM.Results show(1)the seedlings of Elodea nuttallii varied significantly in growth between treatments different in light density.Plant height,leaf number and fresh weight of Elodea nuttallii seedlings increased significantly with duration of exposure in Treatments 100%,5%,and 1% of sunlight.However,the growth rate of the seedlings in Treatment 100% sunlight decreased in the late phase of the experiment,and the growth rate in Treatment 1% sunlight slowed down;plant heights,leaf numbers and fresh weights in Treatments 0.5%,0.1% and 0.05% sunlight declined with duration of light exposure.At the end of the experiment,plant heights of the seedlings in Treatment 0.1% and 0.05% sunlights reduced by 20% and 60%;(2)Light intensity significantly affected chlorophyll content of seedlings.Chla and Chlb increased significantly with increasing light intensity,while Chla/Chlb ratio displayed a declining trend,which demonstrated that the seedlings attempted to catch more light energy by increasing its chlorophyll content;and(3) The photo-adaptive capacity of the seedlings varied sharply between treatments different in light intensity.It was the highest in Treatment 5% sunlight.At the end of the experiment,the 6 treatments followed the order of 5%>100%>1%>0.5%>0.1%>0.05% in terms of the maximal electron transport rate.

Key words: light, Elodea nuttallii, seedling, growth, fluorescence parameter, chlorophyll, light response curve