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.
..
stated that broad spectra light was absorbed by the canopy, but only wavelengths greater than 700 nm were being detected by the crown, resulting in an Φ < 0.2 at crown level.It was hypothesized that the early flowering response was phytochrome-mediated. To test this hypothesis, high-density strawberries were established under long days in a greenhouse (Φ = 0.62) and then transferred to a controlled environment chamber with broad spectra fluorescent lamps (Φ = 0.66) under long-day conditions (16-h light/8-h dark). A strand of low-output red LEDs (λ[SUB]max[/SUB] = 662) was used to illuminate the crown and increase the Φ to 0.75 . This treatment was applied for 28 d and then the plants were transplanted in the field under a high tunnel production system.
