Abstract
The effects of photosynthetic photon flux density (PPFD) fluctuations in sunlight have already been investigated; however, the spectral photon flux density distribution (SPD) has hardly been considered. Here, sunlight SPD fluctuations recorded for 200 min in October in Tokyo, Japan were artificially reproduced using an LED-artificial sunlight source system. The net photosynthetic rate (Pn) of cucumber leaves under reproduced sunlight was measured and compared with the Pn estimated from a steady-state PPFD–Pn curve for the same leaves. The measured and estimated Pn agreed except when the PPFD was low, where the measured Pn was lower than the estimated Pn. The ratio of measured Pn to estimated Pn was 0.94–0.95 for PPFD ranges of 300–700 μmol m–2 s–1, while the value was 0.98–0.99 for 900–1,300 μmol m–2 s–1, and the overall ratio was 0.97. This 3% reduction in the measured Pn compared with the Pn estimated from a steady-state PPFD–Pn curve was significantly smaller than the approximately 20–30% reduction reported in previous experimental and simulation studies. This result suggests that the loss of integral net photosynthetic gain under fluctuating sunlight can vary among days with different fluctuation patterns or may be non-significant when fluctuations in both PPFD and relative SPD of sunlight are taken into consideration.
Highlights
The spectral photon-flux-density distribution (SPD) is a distribution of photon flux density (PFD) per unit wavelength within a defined wavelength range
Occasional reductions in photosynthetic photon flux density (PPFD) by clouds that covered the sun enhanced the fraction of diffuse solar radiation from the sky in global solar radiation and the diffuse radiation was rich in light with a shorter waveband compared with direct radiation (Kume et al, 2018)
The photosynthetic rate (Pn) of cucumber leaves measured under the reproduced sunlight and that estimated from the steady-state PPFD–Pn curve of the same leaves were compared
Summary
The spectral photon-flux-density distribution (SPD) is a distribution of photon flux density (PFD) per unit wavelength within a defined wavelength range. The SPD can be characterized by two aspects: the integral of spectral PFD and the relative SPD. As an index of the former factor, the photosynthetic PFD (PPFD), with an amount of PFD between 400 and 700 nm, is often used. The latter factor is the “shape” of the SPD curve and may sometimes be called light quality. Photosynthesis Under Artificially Reproduced Sunlight of the light environment, both PPFD (Boardman, 1977; Björkman, 1981) and relative SPD (McCree, 1972; Inada, 1976) significantly affect the net photosynthetic rate (Pn) of leaves
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