Abstract
Photon flux (PF) in the spectral regions of 300-400 nm (UV), 400-500 nm (B), 500-600 nm (G), 600-700 nm (R) and 700-800 nm (Fr) were observed almost daily with a spectroradiometer (LI-1800) at noon in from Jan. 2000 to Mar. 2001 at Gifu (lat. 35°25' N and long. 136°46' E) . The spectral ratio of B/photosynthetic photon flux (PPF) and R/PPF on clear days changed reversibly during the morning and in the evening; variability was less during the daytime. Whereas G/PPF was almost constant during the whole studies period irrespective of fine or cloudy weather. The B/PPF ratio increased rapidly from 24.6% on the winter solstice to 27.1% on the vernal equinox. After reaching a peak of 27.3% on the first day of autumn, it decreased gradually from the autumnal equinox to the winter solstice. The seasonal changes in R/PPF showed opposite tendencies for B/PPF. This cycle of spectral photon flux probably assists plants in making a distinction between spring and autumn season. We found that the PPF as well as UV, B, G, R and Fr values were nealy equal in vernal equinox and autumnal equinox. The R/PPF ratio for the fine days was high and B/PPF was low in comparison with the cloudy days.
