PHOTOSYNTHETIC RESPONSES TO CO2 CONCENTRATION AND PHOTON FLUENCE RATES IN THE CAM-CYCLING PLANT DELOSPERMA TRADESCANTIOIDES (MESEMBRYANTHEMACEAE)

Responses of gas exchange and photosynthesis to changes in CO2 concent ration and PPFD were examined in well watered plants of Delosperma tra descantioides Bgr. to establish the relative importance of these envir onmental changes on the photosynthetic machinery in this CAM-cycling s pecies which grows naturally in both exposed and partly shaded environ ments. Plants were grown at two PPFDs (220 [LL] and 550 [HL] mu mol m( -2) s(-1)). HL plants had larger leaves with higher specific weight, w ater content and diurnal malic acid fluctuation. Photosynthetic PPFD r esponses were typically those of sun and shade species for HL and LL p lants, both under 21% O-2 and non-photorespiratory (2% O-2) conditions . The CO2 compensation point in the absence of non-photorespirational CO2 evolution in the light (Gamma) was c. 30 mu mol mol(-1). Irradiat ion reduced mitochondrial respiration by > 50%. Comparison of the PPFD responses of linear electron flow rates derived from gas exchange mea surements and from fluorescence analysis (J(F)) indicated effective ph otosynthetic control. J(F) was always larger than electron flow rates calculated from gas exchange, indicating that processes other than car boxylation and oxygenation were consistently important in energy consu mption under all sampled environmental conditions. Regardless of PPFD during growth, electron flow to carboxylation and J(F) were linearly c orrelated, demonstrating that the photosynthetic apparatus was well ad apted to PPFD during growth. In HL plants, non-photochemical quenching increased, and photochemical quenching and the quantum yield of linea r electron transport through PS II decreased more slowly with increasi ng PPFD than in LL plants. In plants of both treatments non-photochemi cal energy dissipation seemed to be exhausted when the proportion of p hotons not utilizable by photochemistry exceeded 0.7. Results illustra te a pronounced ability of D. tradescantioides to acclimate to a 100% change in the prevailing PPFD and lend support to the hypothesis that CAM cycling might act as a photoprotective process.