DIURNAL CHANGES IN CHLOROPHYLL A FLUORESCENCE AND CAROTENOID COMPOSITION IN OPUNTIA-FICUS-INDICA, A CAM PLANT, AND IN 3 C3 SPECIES IN PORTUGAL DURING SUMMER

Diurnal changes in chlorophyll a fluorescence were determined in four species, differing in life form, in Portugal during the summer of 1989. These included Opuntia ficus-indica, a CAM plant, and Helianthus annuus, Ficus carica and Arbutus unedo, three C3 species. Steady state fluorescence yield, F(S), and maximum fluorescence yield, F(M)', were determined at different times of the day. Using the model of Genty et al. (1989), the photon use efficiency of photosystem II electron transport, phi(e), was calculated from (F(M)'- F(S))/F(M)'. Diurnal changes in relative rate of non-cyclic electron transport through photosystem II, J(e), were derived by multiplying phi(e) by the incident photon flux density (PFD). When J(e), determined for each species for various points in time throughout the day, was plotted against corresponding values of PFD, the light response curves obtained showed that J(e) was linearly dependent on PFD in low light and approached saturation in high light. The highest values of J(e) were observed in Helianthus annuus, followed by Opuntia ficus-indica, Ficus carica and Arbutus unedo. The proportion of the xanthophyll zeaxanthin to total carotenoids, determined around noon, was inversely related to maximum rates of J(e).