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Light quality

The light outside the PAR spectrum accounts for approximately half of the incoming solar radiation including ultraviolet (UV; 10–400 nm) but mainly infrared (700–3000 nm) radiation. Additional long-wave radiation is emitted from any surrounding objects. This radiation is not directly photosynthetically active but it strongly affects the heat balance of the leaves and thereby leaf temperature. As different light sources can show large differences in the intensity and distribution of irradiance in the UV and infrared regions of the spectrum, even at the same PPFD, it is advisable to characterize not only the amount of PAR, but the full light spectrum as well. A strong modulator of plant morphology is the red (R; 660 nm) to far-red (FR; 730 nm) ratio. Sunlight has a R : FR ratio of 1.2 mol mol–1, except around sunrise and sunset, when values are lower. Plants modify this spectrum by selectively absorbing most of the PAR and UV and less of the infrared and  light transmitted or reflected by green tissue has a R : FR ratio as low as 0.2. Phytochromes, a family of photoreceptors with a common chromophore, perceive the R : FR ratio of light and thus the proximity of neighbours, leading to a multitude of photomorphogenetic responses collectively called ‘shade avoidance’. There are also other photoreceptors active in plants – cryptochrome and phototropin – that show absorption in the blue region of the spectrum (Jiao et al. 2007). These are probably more involved in the perception of light quantity than quality, but their absorption characteristics can make their action relevant in growth chambers with artificial lighting.

Further reading

Franklin KA (2008) Shade avoidance. New Phytologist 179, 930–944.

Jiao YL, Lau OS, DengXW (2007) Light-regulated transcriptional networks in higher plants. Nature Reviews. Genetics 8, 217–230

Light quality in controlled environments

Most lamps used in growth chambers have not been designed for imitating the daylight spectrum or optimizing plant growth. Some are specifically adjusted for supplemental lighting in greenhouses where the spectrum is mixed with that of daylight. The most important differences in the emission spectra of these lamps compared with daylight are that their short-wave emission is largely in the PAR region, resulting in a much higher R : FR ratio than that of daylight. Their emission may be further depleted in red or blue and they show strong emission peaks at specific wavelengths. HID lamps in particular have a very high emission of long-wave infrared as a result of their high temperatures. In growth rooms, excessive thermal radiation of HID and incandescent lamps may increase leaf and particularly shoot apex temperatures substantially above the controlled air temperature, especially with the low air turbulence present in most growth chambers. This problem can be reduced by mounting the lamps in compartments separated by an infrared-absorbing but light transparent filter. Such a filter does not, however, solve the problem completely, since re-radiation from its surface can still result in undesirably elevated leaf temperatures. Moreover, because of the small volume of the lamp compartment, it can be difficult to attain operational temperature stability. Temperature stability is an issue for fluorescent tubes, as their output strongly depends on temperature. Growth in light with a high R : FR ratio results in reduced extension growth and may alter daylength responses. Correction used to be done in the past by adding incandescent lamps; but because of their low efficiency a large wattage is required to raise the R : FR ratio to daylight values. Alternatively, end-of-day illumination with incandescent lamps has been applied. Recently, far-red emitting diodes have been used for the correction and manipulation of the R : FR ratio in growth chambers. Such LEDs have the potential advantage of a high efficiency and wavelength precision. High output white LEDs are becoming available as well. LEDs have a higher efficiency and a reduced thermal radiation per unit of PAR. Their emission spectra can be adjusted and a mixture of LEDs with different emission spectra can be used for plant growth purposes.

Further reading

Sager and McFarlane (1997) Radiation. In ‘Plant growth chamber handbook’. (Eds RW Langhans, TW Tibbitts) pp. 1–29. (Iowa State University: Ames, IA)