Typical values of
Φ under ambient solar conditions are
Φ = 0.6 for full sun and
Φ = 0.1 for dense shade under a full canopy (
Salisbury and Ross, 1992;
Nagy and Schaefer, 2002), although these values vary according to canopy type and density (
Vandenbussche et al., 2005). The range of values from electric light sources vary from
Φ = 0.1 from a far red rich light source to
Φ = 0.89 from a source with high red spectrum (
Sager and McFarlane, 1997).
The values for estimating
Φ derived from isolated phytochrome (
Sager and McFarlane, 1997;
Sager et al., 1988) are useful guides to determining the effect of any light source on the phytochrome response. When using narrow-band LEDs, the
Φ can be approximated based on the λ[SUB]max[/SUB] of the LED.
Table 2 shows the estimate of
Φ for discrete narrow-band LEDs with λ[SUB]max[/SUB] from 300 to 800 nm.
Table 2 also includes relative quantum efficiency (RQE) for photosynthesis (
McCree, 1972) to allow the photosynthetic efficiency of a given wavelength to be evaluated as well. These well-defined parameters allow the spectra to be optimized for both photosynthesis and photomorphogenesis.