ASSIMILATION, STOMATAL CONDUCTANCE, SPECIFIC LEAF-AREA AND CHLOROPHYLL RESPONSES TO ELEVATED CO2 OF MARANTHES CORYMBOSA, A TROPICAL MONSOONRAIN-FOREST SPECIES

Seeds of Maranthes corymbosa Blume, a monsoon rain forest species of n orthern Australia, were sown under ambient or elevated CO2 concentrati ons in tropical Australia. Seedlings were grown under conditions of ph oton flux density, temperature and atmospheric vapour pressure deficit which followed ambient variations as closely as possible. Specific le af area, chlorophyll, stomatal density, stomatal conductance and assim ilation responses to photon flux density were measured after 30 weeks growth. Gas exchange characteristics were divided into morning and aft ernoon data sets and analysed separately. Stomatal density decreased a nd leaf area:dry weight ratio decreased in response to elevated CO2. I n contrast there was no effect of elevated CO2 upon chlorophyll (total or ratio of a:b). Apparent quantum yield and rates of light saturated assimilation (A(max)) increased in response to elevated CO2. There wa s a significant decline in apparent quantum yield for both treatments between morning and afternoon. Stomatal conductance (g(s)) declined in response to elevated CO2. There was no significant difference in g(s) between morning and afternoon for ambient grown trees, but g(s) decli ned significantly between morning and afternoon for elevated CO2 grown trees. Instantaneous transpiration efficiency (ITE) was higher for el evated CO2 grown trees compared with control trees. There was a signif icant increase in ITE between morning and afternoon data for ambient g rown trees; in contrast a significant decline in ITE was observed for elevated CO2 grown trees between morning anf afternoon data sets. The slope of the regression between assimilation rate and stomatal conduct ance increased for plants grown under elevated CO2. These data are dis cussed and compared with the responses of plants adapting to different photon flux densities.