The Response of the C3-CAM Tree,Clusia rosea, to Light and Water Stress

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Abstract
Gas exchange in Clusia rosea has been measured under various conditions of water status, light and leaf-air vapour pressure deficit (Δw, mbar bar−1) which produce daytime (C3), night-time (CAM) or 24 h uptake of CO2. At high light levels, at a Δw of 6.6, well-watered plants utilized C3 photosynthesis while CAM and 24 h uptake occurred under lower light levels and with low to normal water availability and differing Δw (13.5 and 3.4, respectively). CO2 uptake was highest, stomatal conductance to water vapour (gH2o) lowest, and water use efficiency (WUE) highest in plants using C3 photosynthesis. This latter fact is contrary to the accepted view that CAM is most water use efficient, i.e. it optimizes CO2 uptake with minimal water loss. It is suggested that the low CO2 uptake in CAM photosynthesis may be related not only to the higher Δw but also to the fact that Clusia species accumulate citrate which may originate from β-carboxylation of fatty acids (i.e. an internal source of CO2) and does not contribute to night-time external CO2 assimilation. Curves of assimilation (A) versus internal partial pressure of CO2 (A/c1) for the three photosynthetic types, under atmospheric conditions, did not produce a single trend. The trends which were produced represent the supply function for the interaction, under differing modes of photosynthesis, of the two major enzyme systems involved in CAM.

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