RESPONSES OF PHOTOSYNTHESIS ON WATER STRESS FOR SHRUBS FROM TROPICAL RAIN FOREST AND SUBTROPICAL MONSOON FOREST

Piper hispidum and Psychotria rubra were grown under greenhouse. Photosynthetic rates of leaves for P. hispidum and P. rubra which were subjected watering were 6.3 and 9.8μmol·m^-2.s^-1 at environmental CO₂ concentration, saturated light density and leaf temperature of 26℃, respectively. Photosynthetic rate of leaves for P. hispidum decreased by 0.38μmol·m^-2·s^-1 during leaf water potential(ψ)declined by 1 bar at lower leaf/air water vapor gradient (△w ). Photosynthetic rate (PN) decreased as △w increased and the relationship was PN=7.02-0.06△w, r²=0.7. The slope of this relationship between PN and △w, decreased during lowing ψ.For P. rubra, the similar response of PN on decreasing ψ was found, but the response of PN on changing △w was more sensitive than that of P. hispidum. The relationship between PN and △w for P.rubra was PN=11.16-0.1△w, r²=0.65.Leaf conductance (g) for these shruhs were much alike at higher ψ, such as -8 bar or -8.75 bar for each, but the change of g in leaves of P.rubra was greater than that of P. hispidum during increasing △w, Lowing ψ could increase water use efficiency(WUE)for both shrubs. And WUE decreased during increasing △w. The results showed that the response of P. rubra which habited in subtropical monsoonal forest was more sensitive than that of P. hispidum which habited in tropical rain forest.