Abstract:

Effects of water plus heat stress on gas exchange and carbon isotope composition of Cunninghamia lanceolata,Pinus massoniana,Pseudotsuga menziesii and Thuja plicata seedlings were measured during a three-cycle stress period in spring 2001 at the University of British Columbia. The results were shown as follows:After 36 days water stress treatments, the net photosynthetic rate of four species decreased significantly. But heat stress only affected the net photosynthetic rate of C.lanceolata and T.plicata. It was only observed that the needle conductance to water vapor of P.massoniana was significantly affected by both water and heat stress. The needle conductance to water vapor of other species had just a slight change after water and heat stress treatment. Therefore, instantaneous water use efficiency, which was expressed as the ratio of net photosynthetic rate(P_n, μmol·m^-2s^-1) to stomatal conductance to water vapor (g_wv , mmol·m^-2s^-1), had no significant difference except for P.massoniana. There were significant differences in carbon isotope composition (δ¹³C) among four species under water stress treatments. Along with prolong of water stress, the δ¹³ C value of all four species increased. This meant that the water use efficiency (WUE) would be improved under water stress. Heat stress treatment had also significant effects on δ¹³ C of C.lanceolata and T.plicata. But the differences of δ¹³ C were not observed under heat stress in P.massoniana and P.menziesii.

Key words: Coniferous seedling, Water plus heat stress, Gas exchange, Instantaneous water use efficiency, Carbon isotope composition