Lesson 14 Effect of Different Spectrum of solar Radiation on Plant Growth


The light received by plants in a greenhouse has three qualities that growers are concerned with intensity (brightness), quality (colour whiteness) and the ability for light (amount of light received) to reach the plants. Plants respond to all three qualities. Light transmission into the greenhouse is the most important factor affecting plant growth and crop production [2].

The importance of light in the growth of plants is a well-established phenomenon.  A common observation is that plants grown in the dark are yellow (chlorotic), taller (etiolated), have thinner stems, and in general, are not so healthy looking.

Greenhouse plant producers also know the importance of light for proper plant production.  They often grow plants with artificial lights, if sunshine is inadequate; or in a shaded area, if sunshine appears to be too plentiful (and hot) or if the plant producers are trying to slow the growth of the plants.

In this lesson the discussion will focus on (1) basic principles of light and plant development, (2) principles on light regulation of plant development in the emerging field of photo-morphogenesis, and (3) alternative methods of regulating plant development that modifies the wavelengths of light that surrounds the plant.

14.2 Light – Radiant Energy

All light is made up of energy.  Light available to plants includes the different  wavelengths of the electromagnetic spectrum viz.the wavelengths that humans can see (visible light) and the wavelengths that humans can't see (such as microwaves and infrared light). Light for the plant is used for producing food through the process of photosynthesis.  The characteristics of direction and spectral composition of light in the plant's environment is transferred to the plant through the interception and activation of pigment systems (coloured cells of the plant). This information affects the morphological development (size/proportion of root and shoots) of the plant.

14.3 Colour - The Wavelength Distribution of Radiant Energy

According to the Random House Webster's College Dictionary (1992 edition), colour is "the quality of an object or object with respect to light reflected by it, usually determined visually by measurement of hue, saturation, and brightness of the reflected light".  Note that this definition is based on human vision.  For our purposes, a more appropriate definition of colour would be the distribution of wavelengths coming from a radiation source, or reflected from a reflective object.

14.4 Plant Uses of Radiant Energy and Plant “Vision”

Plants utilize specialized pigments to intercept and capture radiant energy.  For example, plants capture the energy in light during the process of photosynthesis.  Photosynthetic wavelengths (400-700 nm) activate the chlorophyll pigments, which transform light energy into chemical energy for production of carbon molecules (sugars) that are then used to construct more complex compounds, and ultimately plant cells and organs (root, leaf, stem, flower, fruit).

Plants also monitor radiant energy within their environment for the purpose of adjusting their growth appearance.  This monitoring of the light environment (“plant vision”) and subsequent response is termed photo-morphogenesis. Photo-morphogenesis is more properly defined as the ability of light to regulate plant growth and development, independent of photosynthesis.  Plant processes that appear to be photomorphogenic include internode elongation (distance between leaves on stem), chlorophyll development, flowering, abscission (deleafing), lateral bud outgrowth, and root and shoot growth.

If photosynthesis is the “engine” providing the energy for plant growth, photomorphogenesis is the “steering wheel” to influence the direction and final plant appearance.

Photomorphogenesis involves the activation of several photoreceptor (pigment) systems (Senger and Schmidt, 1994).  These systems include phytochrome, which absorbs red (R) light (wavelengths 660-680 nm) and far-red (FR) light (730-740 nm), “cryptochrome”, which absorbs ultra-violet (UV-A) (320-380 nm) and blue light (400-500 nm), and a UV-B receptor (290 nm).  These receptors detect the light environment and subsequently influence plant growth and development.

Plants monitor the environment by sensing changes in the quality (wavelength(s)), quantity (intensity), duration (length of exposure), and direction of light.  Light perception in plants is a sequential process. Light must first be absorbed by the photoreceptor, and then the photoreceptor is transformed to either its Red (Pr) or Far Red (Pfr) form. Depending on the distribution of the wavelengths of the light, a specific proportion (ratio) of the two forms of the photoreceptor (Pr and Pfr) is established within the plant. This ratio becomes a “message” to the plant, and causes the production of plant growth regulators to stimulate a plant growth response.

14.5 Light Energy Capture by Plants - Photosynthesis

One of the main roles of light in the life of plants is to serve as an energy source through the process of photosynthesis.  Using water and carbon dioxide, plants produce the “foodstuffs” necessary for growth and survival.  Carbohydrates (starches and sugar) for plant components and stored chemical energy are produced during this biochemical process in plants.

Plants capture the energy in light using a green photoreceptor pigment called chlorophyll.  In the research laboratory, chlorophyll can easily be extracted from plant tissue using chemical solvents. Chlorophyll can also be extracted by abrasion, as anyone who has ever pruned tomato plants by hand, or has gotten grass stains on their clothes, can attest.

14.6 Photosynthetic Radiation

Photosynthetically active radiation is well established as the primary measurement for quantifying radiation of the plant light environment. The following guidelines put forth by LI-COR (1979) and generally accepted by most plant science journals should be followed in the reporting of photosynthetically active radiation (PAR):

14.6.1 Units

The mole is the unit for a very large number of photons. It equals 6.02 x 1023 photons and it is also designated as an Avogadro's number of photons.

14.6.2 Terminology

Photosynthetically Active Radiation (PAR) is defined as the photons of radiation in the 400 to 700 nm waveband.  PAR is a general term that can describe either the photosynthetic photon flux density (PPF), or the photosynthetic irradiance (PI).

Photosynthetic Photon Flux Density (PPF, or sometimes written as PPFD) is defined as the photon flux density of PAR.  This is the number of photons in the 400 to 700 nm (PAR) waveband contacting a unit surface area over a given time period. The appropriate unit is (micro-mol per square meter per second), or abbreviated as µmol m-2 s-1.

Photosynthetic Irradiance (PI) is defined as the radiant energy flux density of PAR.  This is the energy of the radiation in the 400-700 nm waveband, which is contacting a unit surface area over a given period of time.  The appropriate unit is Watts per square meter, or abbreviated as Wm-2.

14.7 Light Regulated Plant Development - Photomorphogenesis

Photomorphogenesis is defined as the ability of light to regulate plant growth and development, independent of photosynthesis.  Plant processes that appear to be photomorphogenic include internode elongation, chlorophyll development, lowering, abscission, lateral bud outgrowth, and root and shoot growth.

Photomorphogenesis differs from photosynthesis in several major ways.  The plant pigment responsible for light-regulated growth responses is phytochrome, not chlorophyll.  Phytochrome is a colourless pigment that is in plants in very small amounts.  Only the red (600 to 660 nm) and far red (700 to 740 nm) wavelengths of the electromagnetic spectrum appear to be important to influence the phytochrome pigments.  The wavelengths, which affect photosynthesis, are broader (400 to 700 nm) and less specific.

Photomorphogenesis requires very little light energy (light intensity) to get a growth-regulating response.  Plants generally require a greater amount of energy for photosynthesis to occur.


14.8.1 Phytochrome Wavelengths (Red and Far-red Light Responses)

The second most discussed effect of radiation, after photosynthesis and its subsequent effect on plant growth rates, is photomorphogenesis and its specific effects on plant development.  The wavelengths specific for phytochrome responses are Red and Far-red light. The plant light environment must be characterized according to the absorption spectra or action spectra of phytochrome, since phytochrome is the pigment involved in the regulation of plant development.  The action or response spectrum is indicated by the wavelengths that will cause a plant response.

Phytochrome is found in both active (Pfr) and inactive (Pr) forms.  The relative proportion of each form is beneficial to know, since it is this proportion, which determines the type of plant response.  Unfortunately it is not easy to measure the proportion of active and inactive forms of phytochrome directly. However, separately measuring photon flux densities at 660 nm and 730 nm offers an indication of the proportion of Pfr to Ptotal in the plant (Smith, 1994), and provides (Pfr/Ptot) which is the proportion of Far-red “active” form to the total phytochrome in green leaves.

Reporting specific wavelength ratios for the quantification of the wavelengths of light important to phytochrome is consistent with McCree's (1979) recommendations on spectral measuring and reporting.  He suggested that certain parts of the radiation spectrum were identified with specific physiological plant responses, and that simplified measures of the quantity of radiation available to plants in those spectral regions should be reported.

It is unrealistic to expect complete spectroradiometric data (the intensity of light at each wavelength) for all experiments, and specifically for those, which are not photo biological in nature.  Even if such data were available, the data would be hard to use to interpret the plant response results of an experiment, because the action spectra for various plant responses are not universally known.

14.8.2 “Cryptochrome” Wavelengths (Blue Light Responses)

There are a series of well-documented plant responses that have been attributed to radiation in the blue portion (400 to 500 nm) of the electromagnetic spectrum.  Unfortunately, our knowledge on the action or even the location of this hypothesized plant pigment (“cryptochrome”) is not known.  In addition some of the plant’s responsiveness to blue light may be attributed to perception and activation of phytochrome in these wavelengths (Mohr et al., 1984).


14.9.1 Using Light to Regulate Plant Growth in the Greenhouse

  • Exposing the Plants to Red and Far-red Light

  • Supplementing the Greenhouse Light Environment with Fluorescent Light

  • Filtering out FR Light Using the Greenhouse Covering


1. Dennis Decoteau, 1998. “Plant Physiology: Manipulating Plant Growth with Solar Radiation.” Greenhouse Glazing & Solar radiation Transmission Workshop, Centre for Controlled Environment Agriculture, Rutger University, Cook College. PP.1 to 8

2.  Nadia sahib and Gene Giacomelli. “The primary factor affecting light level and quality inside a greenhouse include the structure and environment.”  http.//ag.arizona.edu/ceac

Last modified: Wednesday, 5 March 2014, 6:04 AM