Photosynthetic responses to CO₂ concentration and photon fluence rates in the CAM-cycling plant Delosperma tradescantioides (Mesembryanthemaceae)

Abstract

Responses of gas exchange and photosynthesis to changes in CO₂ concentration and PPFD were examined in well watered plants of Delosperma tradescantioides Bgr. to establish the relative importance of these environmental changes on the photosynthetic machinery in this CAM-cycling species which grows naturally in both exposed and partly shaded environments. Plants were grown at two PPFDs (220 [LL] and 550 [HL] μmol m⁻² s⁻¹). HL plants had larger leaves with higher specific weight, water content and diurnal malic acid fluctuation. Photosynthetic PPFD responses were typically those of sun and shade species for HL and LL plants, both under 21% O₂ and non-photorespiratory (2% O₂) conditions. The CO₂ compensation point in the absence of non-photorespirational CO₂ evolution in the light (Γ∗) was c. 30 μmol mol⁻¹. Irradiation reduced mitochondrial respiration by >50%. Comparison of the PPFD responses of linear electron flow rates derived from gas exchange measurements and from fluorescence analysis ([Jscr ]_F) indicated effective photosynthetic control. [Jscr ]_F was always larger than electron flow rates calculated from gas exchange, indicating that processes other than carboxylation and oxygenation were consistently important in energy consumption under all sampled environmental conditions. Regardless of PPFD during growth, electron flow to carboxylation and [Jscr ]_F were linearly correlated, demonstrating that the photosynthetic apparatus was well adapted to PPFD during growth. In HL plants, non-photochemical quenching increased, and photochemical quenching and the quantum yield of linear electron transport through PS II decreased more slowly with increasing PPFD than in LL plants. In plants of both treatments non-photochemical energy dissipation seemed to be exhausted when the proportion of photons not utilizable by photochemistry exceeded 0·7. Results illustrate 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.