The effects of soil water content on photosynthesis in leaves of {\sl Kmeria septentrionalis} seedlings

Net photosynthetic rate (Pn), stomatal conductance (Gs), intrinsic water use efficiency (WUE) and chlorophyll content were measured in the leaves of Kmeria septentrionalis seedlings grown in pots withvarious soil water contents during July 15-19 and October 20-25, 2001. Decrease of soil water content in summer and autumn reduced the leaf dry weight per unit area and chlorophyll content in the seedlings. In July, highestphotosynthetic rate was found at 9:00 pm in sunny day. The Pn of seedlings in treatments with soil water content at high (WH), mediam (WM), and low (WL) levels were 4.41±1.10 μmol m-2s-1, 4.28±1.23μmol m-2s-1 and 1.89±0.94μmol m-2s-1, respectively. WH, WM and WL represent the soil water contents are 60%-65%, 45%-50% and 35%-40% of the field water holding capacity, respectively. Low Pn occurred in plants grown under soil water treatments of WL. A pattern with double peaks in daily photosynthetic course was observed in leaves of the seedlings under soil with all the three treatments. Gs and intrinsic WUE daily courses also showed similar pattern. In October, daily high Pn appeared at 12:00 in plants in WH group and those in WM and WL groups were found at 9:00. A pattern with single peak in daily photosynthetic course was observed in 3 groups. Average values of Pn, Gs and intrinsic WUE were higher in autumn than those in summer in all the three groups. However, the Pn, Gs and intrinsic WUE were all low in soils with low water content, whatever in summer or autumn. The results suggest that the control of soil water content may help to promote the growth of K. septentrionalis and increase theeffectiveness of ex situ conservation.