Abstract
Diurnal changes in chlorophylla fluorescence were determined in four species, differing in life form, in Portugal during the summer of 1989. These includedOpuntia ficus-indica, a CAM plant, andHelianthus annuus, Ficus carica andArbutus 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,φ 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 multiplyingφ e by the incident photon flux density (PFD). WhenJ 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 thatJ e was linearly dependent on PFD in low light and approached saturation in high light. The highest values ofJ e were observed inHelianthus annuus, followed byOpuntia ficus-indica, Ficus carica andArbutus unedo. The proportion of the xanthophyll zeaxanthin to total carotenoids, determined around noon, was inversely related to maximum rates ofJ e.
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