The Response of the C3-CAM Tree, Clusia rosea, to Light and Water Stress: II. INTERNAL CO2 CONCENTRATION AND WATER USE EFFICIENCY

Gas exchange in Clusia rosea has been measured under variousconditions of water status, light and leaf-air vapour pressuredeficit (_w, mbar bar^–1) which produce daytime (C₃), night-time(CAM) or 24 h uptake of CO₂. At high light levels, at a _w of6.6, well-watered plants utilized C₃ photosynthesis while CAMand 24 h uptake occurred under lower light levels and with lowto normal water availability and differing _w (13.5 and 3.4,respectively). CO₂ uptake was highest, stomatal conductanceto water vapour (gH₂o) lowest, and water use efficiency (WUE)highest in plants using C₃ photosynthesis. This latter factis contrary to the accepted view that CAM is most water useefficient, i.e. it optimizes CO₂ uptake with minimal water loss.It is suggested that the low CO₂ uptake in CAM photosynthesismay be related not only to the higher _w but also to the factthat Clusia species accumulate citrate which may originate fromß-carboxylation of fatty acids (i.e. an internal sourceof CO₂) and does not contribute to night-time external CO₂ assimilation.Curves of assimilation (A) versus internal partial pressureof CO₂ (A/c₁) for the three photosynthetic types, under atmosphericconditions, did not produce a single trend. The trends whichwere produced represent the supply function for the interaction,under differing modes of photosynthesis, of the two major enzymesystems involved in CAM. Key words: Clusia rosea, Crassulacean acid metabolism, C₃ photosynthesis, internal CO₂ concentration, 24 h carbon dioxide uptake, water use efficiency.