When it comes to photomorphogenesis, the most understood developmental processes are those controlled by the red and far red light spectra. For the purposes of this discussion, we will refer to red (R) light as the spectral region around 660 nm and far red (FR) light around 730 nm. In order to better understand the influence that these two spectral regions have on plant development, you need to first understand the significance of the pigment known as phytochrome, which is responsible for R and FR light mediated responses.

Phytochrome is a pigment protein which exists in two interconvertible forms – a red light absorbing form (P_r) and a far red absorbing form (P_fr). Phytochrome converts from one form to another upon absorbing the corresponding light until an equilibrium is established, with the relative amount of each phytochrome form depending on the ratio of R to FR light in the light spectrum. To put it another way, when P_r absorbs R light it is converted into P_fr, and when P_fr absorbs FR light it is converted into P_r. There is some overlap in in the spectra of both forms, and phytochrome does absorb some blue light as well, but for the sake of this guide, this will not be discussed.

The prevalence of one form or the other (which depends on the R/FR spectral ratio) can stimulate or inhibit a number of developmental processes such as: seed germination, leaf unrolling, chlorophyll formation, and stem elongation. Additionally, phytochrome is the controlling factor of promoting (or suppressing) flowering in photoperiodic plant species. For the sake of brevity, and to discuss important applications related to horticulture lighting systems, we will focus on the influence that phytochrome has on flowering and stem elongation.

Two important blue light photoreceptors are cryptochromes and phototropins. Blue light is important for a variety of plant responses such as: suppression of stem elongation, phototropism, chloroplast movement within cells, stomatal opening, and activation of gene expression, some morphogenic genes and others not. Stomatal opening and height control are of particular relevance to horticultural lighting systems. A low overall blue light content (e.g. less than 10% of the total photon flux) can lead to leaf edema (swelling of the leaves) and developmental problems in several plant species. The absolute content of blue light has a progressively stronger effect on plant height reduction. This may be desirable in some cases (e.g. to produce more compact seedlings and reduce transportation costs) but generally leads to a lower photosynthetic efficiency. A high relative content of blue light reduces the plant leaf area and may be undesirable for that reason. Near UV light has an effect similar to blue light, with further reduced photosynthetic efficiency, especially below 400 nm (although the other effects may be stronger by comparison). It also affects the biosynthesis of compounds responsible for the flavor of certain fruits, increased anthocyanin concentration, as well as that of other compounds which are not directly produced by photosynthesis alone. Whenever the use of near UV light is necessary to control a corresponding sensory mechanism or the production of a specific molecule of interest by the plant, a trade-off may have to be reached, similarly to that for far red light.

The least understood spectrum related to photomorphogenic responses in plants is green light (500 – 600 nm). The control effects of green light are generally opposite those of red and blue light. For example, green light has been shown to reverse blue light induced plant height reduction and anthocyanin accumulation. The phytochrome and cryptochrome photoreceptors mentioned earlier are also responsive to green light, though to a significantly lesser extent than to red or blue light. So far, all efforts by researchers to find photoreceptors responding primarily to green light have given no definitive results. However, it should be mentioned that the addition of green light into the spectrum of horticultural lighting systems has demonstrated to be beneficial to the growth of several plant species. Similar to far-red light, green light penetrates deeper into leaves and canopies than red or blue light, and can significantly increase the rate of photosynthesis. The addition of green light also significantly improves the color rendering index (CRI) of a horticultural lighting systems, which allows growers to effectively monitor crops for disease or nutrient deficiency/toxicity symptoms, without the use of specialized glasses.

In horticultural lighting systems, there are a number of choices – especially when it comes to using LED lights -- which can range from narrow-band spectral composition (i.e. pink or purple) to broad spectrum, otherwise known as white light. Depending on the crop you are producing, selecting an LED horticultural lighting system with the appropriate light quality is critical, not only to drive photosynthesis, but to achieve the desired morphological responses.

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