Study on daily changes of photosynthesis in {\sl Cynanchum bungei}

The daily changes of photosynthesis in Cynanchum bungei Decne. was a double peaks curve. The peak values appeared at from 9:00 to 10:00 am. and 15:00 pm.There was the distinct phenomenon of noon break under the condition of strong light. Chlorophyll fluorescent parameter Fv/Fm and #PHI#PSⅡ were reduced distinctively, and this showed that in the field light stress often occurs for Cynanchum bungei Decne.'leaves. And the main reason for the light stress was strong light and high tempeture.