We have known that light is responsible for driving plant growth via photosynthesis for many years; however, the influence of light on plant development has only become well understood in the last century. The color of light is not only an important variable for photosynthesis, but also acts as a packet of information to signal light mediated developmental responses in plants, such as: seed germination, stem elongation, and flowering. The term used to describe these responses in plants is photomorphogenesis. Plant morphology is extremely important in controlled environment agriculture where vertical or horizontal growing space may be limited. Depending on the plant architecture you desire, there may be other aspects of horticulture lighting systems to consider beyond providing a source of photosynthetic light.

Photosynthetically active radiation (400 – 700 nm) is mainly used for photosynthesis, however, plants can sense wavelengths ranging all the way from UV-C (260 nm) to far-red (730 nm) using separate photoreceptors that are not utilized for photosynthesis. These photoreceptors direct an adaptive response in plants under changing environmental conditions to regulate key stages of plant development which depend strongly on the spectrum of light, and in some cases timing, periodicity, and the overall exposure. The latter is usually called fluence, and is measured in micromoles of photons per square meter of surface. There are very low, low, and high fluence responses, with the corresponding sufficient light levels ranging from those of star light (for very low light levels) to direct sunlight (for high light levels). The purpose of this article is to describe photomorphogenic responses in plants.

VYPRx PLUS with PhysioSpec Greenhouse compared to a typical high pressure sodium (HPS) grow light.

There are long-day plants (which require short nights to flower), short-day plants (requiring long nights), and day-neutral plants which have no specific requirement for the photoperiod. This dependence on the photoperiod is referred to as photoperiodism. However, it is the length of the dark period (scotoperiod) that regulates flowering of photoperiodic plant species. In the absence of light, P_fr slowly converts to P_r, and as the scotoperiod increases, so does the relative amount of P_r. Long day plants (which have a short scotoperiod) will not flower if P_fr converts to P_r during the scotoperiod, while short day plants (which have a long scotoperiod) will only flower if P_fr converts to P_r during the scotoperiod. Photoperiodic phytochrome responses occur in the low fluence range (as low as 1 µmol/m²), so it can only take a short flash of R light to during a scotoperiod to revert P_r back to P_fr. For example, flowering of a long-day plant may be induced by night interruption, using a series of short flashes of red light with photon flux levels as low as a few µmoles/m²s. Conversely, short-day plants may be induced to flower by a single flash with pure FR light at the very beginning of the dark photoperiod, after turning off all other lights. This effectively adds a couple of hours to the dark period for the purpose of flowering, which can be used to extend the light period for growth and optimize plant yields as a result. Switching the above methods for plants with opposite photoperiod requirements would delay flowering, which may also be desired sometimes (e.g. to provide the best quality flowers on schedule for certain holidays).

A good energy-saving (and thus, cost-saving) strategy is to use one set of lights for growth and another for photoperiod control when necessary. Since phytochrome response is in the low fluence range, the number of fixtures needed for photoperiod control may be much smaller than that of fixtures needed for growth. In addition, the operating time needed for photoperiod control can be much shorter, such as only minutes at a time. Since FR light is only partially photosynthetically active, its use in horticulture lighting is often limited for reasons of energy efficiency.

VYPRx PLUS with PhysioSpec Greenhouse compared to a typical “purple” LED grow light.

Another important R and FR photomorphogenic response important to horticultural lighting systems is called the shade avoidance response. Far-red light is transmitted through leaf tissue more so than red light, which causes an enrichment of far-red light, relative to red light, for plants grown under canopies. When a low R:FR ratio is perceived by phytochrome pigments, a shade avoidance response is activated to elongate hypocotyls or stems in an attempt to out-compete neighboring plants. This is very important when it comes to the spectral light quality of horticultural lighting systems. Photoperiodic lights that provide a low R:FR ratio to promote flowing may also induce a shade avoidance response in plants, which may result in an undesirable growth habit, especially if a compact growth habit is preferred.

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