PHOTOSYNTHETIC RESPONSES OF TWO SUBTROPICAL WOODY PLANTS GROWING UNDER SUN AND SHADE ENVI RONMENTS

Abstract:

Photosynthesis and fluorescence parameters involving photosynthetic process in the leaves of 2-year-old subtropical tree species Castanopsis fissa and understory shrub Psychotria rubra were determined by using gas exchange and fluorescence techniques，which were transplanted in 25 plots for 6 months under flIl sunlight and shade (17％ of full sunlight)for each treatment The daily course variation of net photosynthesis rate (Pn)，stomatal conductance (g_s) ratio of intercellular to ambient CO₂ concentration (Ci/Ca)，ratio of carotenoid to chlorophyll content (Car/Chl)，and the efficiency of photochemical reaction (Fv/Fm) for both plants under shade were relatively stable and less difference between species．However，in full sunlight，the Pn，g_s and Ci/Ca for the two species generally declined with time, consistently with the decrease of the q_P and Ф_PSII．q_N and Car/Chl for Castanopsis fissa increased with time， but decreased for Psychotria rubra． The decrease of q_P and Ф_PSII in leaves of Castanopsis fissa under full sunlight around the noon time indicated that the numbers of tlle open PSII reaction center were possibly reduced，but its photochem ical efficiency still maintained at a relativdly stable level indicated by Fv/Fm．Meanwhile，the increase in Car/Chl and q_N suggested that the excessive activated energy could be safely dissipated．While for Psychotria rubra, the g_s q_N and Fv/Fm decreased and were significantly lower, but Ф_PSII and q_P maintained relatively stable and were higher as compared to Castanopsis fissa, implying that its PSII reaction center may change a little， but CO₂ exchange， photochemical efficiency, and heat dissipation capacny had been weakened to acertain degree．In general，Psychotria rubra showed more sensitivi to strong light intensity than Castanopsis fissa.