Greenhouses or indoor gardening systems are designed to provide non-stressful environmental conditions, optimal temperature, sufficient water supply, nutrients and carbon dioxide. Under such conditions, light often becomes the main limitation to crop growth. In this article, you will find the importance of light and issues pertaining to the use of supplemental lighting in greenhouses or indoor gardening systems.
What is Light? Technically, light refers to an electromagnetic radiation with a wavelength that is visible to the eye (otherwise, it is “dark”). Light is a complex factor, ultimately influencing all aspects of plant performance. It is the primary resource for photosynthesis.
When considering plant responses to light, plant growth is influenced by three different properties: intensity, duration and quality of light.
In the context of plants, intensity of light refers to the amount of light received per unit area and is usually expressed either in terms of radiant energy flux (Wm-2, or Jm-2s-1) or quantum flux (micromol m-2s-1). Duration refers to the length of the light period. This is important for determining the total amount of light received. On this basis, plants can be categorized to short-day plants, long-day plants and day neutral plants. Quality of light refers to the type of radiation received by the plant and is usually expressed in wavelength.
Visible light belongs to the wavelengths between 380nm to 750nm. Wavelengths lower than 380nm are called ultra-violet (UV) wavelengths and contain high amounts of energy that can be harmful to plants. Only about 5% of sunlight is in the UV range, and greenhouse coverings tend to reduce that fraction further. It should be noted, that some artificial light sources generate UV.
Photosynthesis is driven by wavelengths in the range of 400nm to 700nm, where roughly 50% of solar radiation occurs. Below this range, photosynthetic pigments poorly absorb light, and at the higher wavelengths, the energy is insufficient to support the requirements of photosynthesis. Plants utilize the light environment as an important developmental cue. Plants use their pigments, mainly by phytochrome to detect receiving light. Phytochrome participates in many aspects of plant development, including the regulation of stem elongation, leaf enlargement, and sometimes flowering.
Leaves absorb wavelengths effectively up to just above 700nm, mainly due to photosynthetic pigments, but do not absorb well in the 700nm to 860nm ranges. In addition to plant pigments, water within plant tissues absorb infra-red (IR) region of receiving light. Only a small fraction of the absorbed energy from light is dissipated by plant metabolism. The great majority is dissipated by evapotranspiration, convective heat loss, and long-wavelength re-radiation (>3000nm).
In midsummer, near noon on a clear day, full sunlight approaches light intensity of 1000 Wm-2. Since about 45% of sunlight is in the 400nm to 700 nm range used for photosynthesis, plants can only use about 450 Wm-2 or about 2000 micromol m-2 s-1. Similar or higher intensities can be achieved using artificial light sources, but this requires considerable costs for both power and lamps.
If we consider a plant, for individual leaves oriented at right angles to incoming radiation, it needs roughly 1/3 of full sunlight (about 150 Wm-2) for photosynthesis. These values would be typical for sun-adapted species such as greenhouse vegetable crops. In shade-adapted species, including many foliage plants need lower intensities of receiving light. This might lead to a misconception that maximum crop growth can be achieved at only 1/3 full sunlight. For many several reasons, however this is not the case. These are:
Whole plants are not composed of single leaves exposed to direct sunlight coming in at right angles to their surfaces. In a real canopy, leaves are often positioned oblique to the incoming beam lowering the amount of available light.
In a shoot canopy the leaves overlap, and the incoming light is absorbed before it reaches the lower leaves.
Differences in light absorbing pigment contents in different tissues cause variation in absorbing light.
For these reasons, well developed leaf canopies require higher intensities to reach the light compensation point than do single leaves. These considerations help us to understand a relationship accepted by many greenhouse vegetable producers, the “1% Rule”: for every 1% increase in light intensity reaching the crop there is a 1% increase in productivity.
Workplace illumination is a normal requirement for greenhouses, and flower growers need to be aware that low light intensities of light can modify plant development (e.g. flowering, dormancy). The use of supplemental lighting is an issue with respect to “light pollution”. Supplemental lighting is used for several purposes within greenhouse systems:
Lamps are turned on before dawn and/or in the evening in order to extend the photoperiod and achieve developmental control (conversely, blackout curtains are used during daylight hours to shorten the photoperiod). Less commonly, the equivalent to daytime extension can be achieved by providing a crop with one or two hours of illumination during the middle of the night (night interruption lighting). The intensity requirement for photoperiodic regulation of plant development is low. A traditional procedure is to use 100W incandescent lamps suspended 3 feet above a crop on 4.5 foot centers. High intensity discharge lamps, more widely spaced, are also used for this purpose.
Low intensity supplemental light can sometimes benefit low-light requiring plants. (e.g. a little supplemental light can improve results with African violets)
High intensity lighting, sufficient to significantly increase photosynthesis, is expensive in terms of capital and operating costs. High intensity treatments during the main phase of growth are only used with high-value crops (e.g. roses). It is used during the early establishment of greenhouse vegetable crops and some flower crops. Furthermore, it is sometimes used during both asexual and sexual plant propagation.
Commonly used greenhouse lamps include ordinary incandescent and fluorescent lamps, high-pressure sodium lamps and halide lamps. The lamps differ in several important respects:
Incandescent bulbs emit large amounts of radiation in the infra-red, and can be a significant heat load. They are inexpensive, but have poor energy efficiency and are not as durable as some other lamps. They are useful in promoting the far-red functions of phytochrome.
Fluorescent lamps are more energy efficient and have longer bulb lifetimes. Bulb output declines with age, and emission of common fluorescent lamps is largely confined to the visible spectrum. Fluorescent lamps cost more than incandescent lamps, mainly because of the ballast requirements. Fluorescent bulbs are not hot, due to their low infra-red output, and are commonly used in plant propagation.
High-pressure sodium lamps and metal halide lamps are high intensity discharge lamps. These lamps are more costly but provide higher radiant outputs than the previous types, plus they are energy efficient.
The high-pressure sodium lamp output is dominated by the major emission lines of sodium. It is more widely used, but some plant species does not respond well to it. Metal halide lamps provide high intensity and a broader representation of wavelengths, which can sometimes justify their high cost.
Houseplants placed in the “wrong” lighting situation often show one or more of the following symptoms:
Other factors, such as watering practices, also can cause these symptoms. If plants are performing poorly in one location, try moving them to another location with more or less light for a few weeks. Avoid compensating with additional fertilizer as this can increase the problem.
Cast Iron Plant (Aspidistra)
Corn Plant, Dragon Tree (Dracaena)
Kentia Palm (Howea)
Parlor Palm, Neanthe Bella (Chamaedorea)
Peace Lily (Spathiphyllum)
Pothos, Devil’s Ivy (Epipremnum)
Snake Plant (Sansevieria)
Plants for medium light
African Violet (Saintpaulia)
Boston Fern (Nephrolepis)
Chinese Evergreen (Aglaonema)
Dumb Cane (Dieffenbachia)
English Ivy (Hedera)
False Aralia (Dizygotheca)
Grape Ivy (Cissus)
Prayer Plant (Maranta)
Rex Begonia (Begonia)
Spider Plant (Chlorophytum)
Swedish Ivy (Plectranthus)
Weeping Fig (Ficus)
Jade Plant (Crassula)
Norfolk Island Pine (Araucaria)
Ponytail Palm (Beaucarnea)
Rubber Plant (Ficus)
Unguentine Plant, Aloe (Aloe)
Ranil Waliwitiya has ten years of experience in plant related nutrients, plant growth regulators and pesticides including their formulations and plant physiological responses.