Abstract
We used a portable spectroradiometer to measure solar photon flux densities in the spectral regions of 400-500 nm (B), 500-600 nm (G), 600-700 nm (R), and 700-800 nm (FR) throughout one year at Ayabe city, Kyoto (135°16′E, 35°17′N). The relationships between sun elevation and the respective ratios of the R, G, and B components to photosynthetic photon flux density (PPFD: 400-700 nm) were relatively constant when the sun elevation was higher than 20°. Under these conditions, the G/PPFD ratio was almost invariant at 0.35-0.36. The R/PPFD and B/PPFD ratios averaged 0.35 and 0.30, respectively, and varied within a wider range of ±0.05 that depended on the weather. The relationship between sun elevation and the R/FR ratio was relatively constant when the sun elevation was higher than 15°, under which conditions it was almost always within the range of 1.0-1.2 regardless of the weather.
