The effect of near-infrared radiation on plants

Added on 26 May 2022

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The effect of near-infrared radiation on plants

Look at any textbook on botany and you will find this maxim: plants respond to optical radiation in the spectral range of 280 nm to 800 nm. Period, end of discussion. The question is, how was this spectral range (sometimes referred to as Photobiologically Active Radiation, or PBAR) determined?

This question addresses issues beyond mere academic curiosity. Recent studies, both in the laboratory and in the field, have shown that ultraviolet-C radiation - optical radiation with wavelengths shorter than 280 nm - offers significant economic benefits for horticultural applications. The effects and potential economic benefits of near-infrared radiation - optical radiation with wavelengths longer than 800 nm - have yet to be explored.

Ultraviolet Radiation

The reason for choosing 280 nm as the lower limit of PBAR is simple: plants in the wild are simply not exposed to ultraviolet-C radiation. As shown by the terrestrial solar spectrum in Figure 1, the atmospheric ozone layer effectively blocks any significant amount of ultraviolet-C radiation (100 nm to 280 nm) from reaching the Earth's surface.

The logic of excluding UV-C radiation from the definition of PBAR may be sound, but it has had unintentional consequences. We have had the ability to produce UV-C radiation for ultraviolet germicidal irradiation (UVGI) applications for over a century (e.g., Kowalski 2009), and low-pressure mercury vapor lamps generating 254 nm UV-C radiation have been used in hospitals and food processing facilities for disinfection purposes since the 1930s. However, it has only been in the past decade or so that UV-C irradiation (mostly using 254 nm UV-C lamps) has been studied and commercialized for pre- and post-harvesting applications in horticulture (e.g., Aarrout et al. 2020, Urban et al. 2016).

The use of UV-C radiation in commercial horticultural applications, including in open fields, greenhouses, and enclosed vertical farms, is proving to have important economic benefits in terms of plant health and reducing spoilage post-harvest. This therefore begs the question: by excluding near-infrared radiation (NIR) from the definition of PBAR, what (if anything) are we missing?

Far-red and the Phytochromes

To understand why 800 nm was chosen, we first need to look at the phytochromes, a class of photoreceptors that control numerous functions in higher plants, including seed germination, shade avoidance, photomorphogenesis, stem elongation, branching, circadian rhythms, root growth, and flowering times (e.g., Smith 2000 and Wang et al. 2015).