The biochemistry of photoperception

Plant interactions with their light environment are complex and have evolved over hundreds of millions of years. Our time together is shorter than that, so I hope you will forgive me if I simplify a bit.

The starting point to understand plant responses, is to think like a plant. Plants have two essential prerogatives. To photosynthesize and to reproduce.

Plasticity Matters

Plants are sessile organisms. They are rooted in place, and don’t have the ability to change their environment, or to take a vacation if they don’t like the weather. For this reason, plants have evolved the ability to radically change their physical form to adapt to their ecological niche. This ability is known as plasticity.

PAR is defined as the portion of the electromagnetic spectrum between 400 and 700 nanometers in length. We perceive PAR as white light. Light spectra that fall outside of PAR can also have effects on plant health, and morphology.

Light wavelengths of 280-400 nanometers in length are known as ultraviolet, or UV wavelengths. Although these light qualities have limited photosynthetic potential, they have important effects on plants, most of which are detrimental.

Far-Red Light

At the other end of PAR, we have far-red light. Far-red light is classified as light spectra between 700 and 780 nanometers in length.

Far-red light has limited photosynthetic potential but has potent effects on plant morphology. In far-red enriched environments, which occur within dense canopies, plants will often exhibit Shade Avoidance Response. This response involves rapid shoot elongation to outgrow the surrounding canopy. This response is not based on light intensity, but on light quality, specifically the ratio of red to far-red light.

Phytochromes exist in two interconvertible states depending on the quality of light they absorb. Phytochrome red is the red-light absorbing state. When this pigment is saturated, it converts to phytochrome far-red. These states of phytochrome are constantly interconverting to mirror the light environment. This process is known as photo equilibrium.

This daily pattern of the interconversion of phytochrome red and phytochrome far-red creates a rhythm which enables photoperiod perception. This rhythm is known as the circadian clock and informs many biological processes.

Cryptochromes have a role in many aspects of plant development. In general, they oppose stem elongation, and favour leaf expansion. In many plants, cryptochromes also have a role in photoperiod perception, and entraining the circadian clock. Many plant responses that are triggered by blue light can be opposed by green light. This is similar to the red to far-red toggle effect of phytochrome response.

We have now examined Phytochromes, Cryptochromes, Phototropins, and UVR8.  These are the main signaling systems that plants use to determine light quality, quantity, duration, and direction.

Plants continuously measure the light environment through cross-talk between these various signaling systems. This information directs plant shape through hormone regulation and genetic expression, to enable the plant to optimize its form for Photosynthesis, and Reproduction.

Carotenoids transfer the energy that is harvested to the chlorophylls for further synthesis.  When chlorophylls are over saturated with light, carotenoids can dissipate the excess light energy as heat.

Chlorophylls and carotenoids will organize their locations within the chloroplast to optimize light capture. Under high intensity light, carotenoids primarily serve as photoprotective and photosynthetic support systems. Under lower light conditions within the canopy, carotenoids perform a primary role in photosynthesis.

Daily Light Integral

A plants photosynthetic potential is determined by the light intensity, or PPFD multiplied by the photoperiod duration. This calculation gives us the Daily Light Integral, or DLI which represents the total amount of light the plant receives during the photoperiod. The DLI is measured in mol, with 1 mol equaling some ridiculous number of photons.

Light amount alone does not determine photosynthesis. Other factors including water uptake and climate conditions will impact a plants ability to photosynthesize. Plants can only perform to their extent of their most limiting factor.

So…. What does this tell us about Horticulture Lights for Cannabis Growing?

An understanding of how plants interact with sunlight gives us important insights in creating an ideal spectrum for horticultural lights.  Historic LED manufacturers have focused on providing red and blue light only, as these represent the peak absorption spectra of chlorophyll.  This can result in plants inability to optimize their morphology for photosynthetic efficiency, as the light information they require is not available in the environment.

Provision of light in narrow bandwidth red and blue spectra can also lead to photobleaching under very high light intensities, as the supporting, and photoprotective roles of carotenoids are diminished by the limited spectra available.

Plants can adapt and perform under many different light conditions.  When we seek to optimize an artificial light environment for plant performance, we are learning that broad spectrum, high fluence lighting technologies provide the most complete plant response.