Assimilation, Stomatal Conductance, Specific Leaf Area and Chlorophyll Responses to Elevated CO₂ of Maranthes corymbosa, a Tropical Monsoon Rain Forest Species

Abstract

Seeds of Maranthes corymbosa Blume, a monsoon rain forest species of northern Australia, were sown under ambient or elevated CO₂ concentrations in tropical Australia. Seedlings were grown under conditions of photon flux density, temperature and atmospheric vapour pressure deficit which followed ambient variations as closely as possible. Specific leaf area, chlorophyll, stomatal density, stomatal conductance and assimilation responses to photon flux density were measured after 30 weeks growth. Gas exchange characteristics were divided into morning and afternoon data sets and analysed separately. Stomatal density decreased and leaf area:dry weight ratio decreased in response to elevated CO₂. In 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 was 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 gs between morning and afternoon for ambient grown trees, but gs declined significantly between morning and afternoon for elevated CO₂ grown trees. Instantaneous transpiration efficiency (ITE) was higher for elevated CO₂ grown trees compared with control trees. There was a significant increase in ITE between morning and afternoon data for ambient grown 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 conductance increased for plants grown under elevated CO₂. These data are discussed and compared with the responses of plants adapting to different photon flux densities.