Horticulture is the branch of agriculture that deals with science, art, technology and business of plant cultivation. The role of light illumination in plant growth is an important factor that needs to be considered. The Photosynthesis process in plants uses water, carbon dioxide intake and light as the source of energy to produce glucose, an essential nutrient for the plant, and oxygen as shown in figure 1.
In the past, plant cultivators in green house environments always used either natural sunlight, High Pressure Sodium (HPS) or Fluorescent lamps to illuminate crops. There were certain disadvantages in using these light sources because natural sunlight is obviously only available during daytime and fluorescent lighting consumes energy, has a high temperature which prevents it from being placed close to the plant and contains toxic material such as mercury, making proper disposal costly.
The development of Light Emitting Diodes (LED) in the last few decades has introduced a new source of lighting to horticulturalist with many superior advantages.
Foremost, every green plant does need all wavelengths in the visible region (400-700 nm) in equal proportion. The absorption spectrum of plants can be matched by using tunable LED’s as shown in Figure 3 below. This illumination source is much more suitable than an HSP source whose peak emissions widely differ from the absorption spectrum of green plants.
This illumination source is much more suitable than an HSP source whose peak emissions widely differ from the absorption spectrum of green plants as shown in Figure 4.
Some wavelengths of interests for LED’s as applicable to plant growth are:
LEDs provide the unique opportunity for horticulture industry to use a narrow bandwidth of illumination. Several LEDs at different wavelengths can be combined to provide an illumination source which follows the plant sensitivity curve. Aside from this, there are several other advantages of using LED’s in horticulture which include:
Figure 5 shows LED illumination is a green house.
The wavelength selection of illumination is observed in Figure 5. Several researchers have experimented with effects of different intensity and wavelengths on growth of different crops. It is important to understand that different crops may behave differently under different illumination levels and a different “light recipe” may be needed for each crop.
Photosynthetic Photon Flux (PPF) designates the range of visible spectral radiation which plants use in photosynthesis process. PPF is measured in units of mmol/m2/s (One mole is 6.023x 1023 photons) and crop growers have experimented with different levels of light intensity. Increased PPF has caused in increased growth of the plant. Although Red light is sufficient for plant growth, Blue light is important for increased leaf thickness and number of chloroplasts/cell. Rice plants grown in a combination of blue and red LED’s showed a higher photosynthetic rate than those grown under Red illumination alone (Reference 1). Although a combination of Red and Blue LED illumination is very useful for better crop growth, since it appears to be purplish grey, it makes it difficult to observe the disease in plants visually. Addition of green light, although not important for plant growth makes it possible to assess the damage by human eye.
Another important issue is development of metrics for quantifying PPF and light absorption by crops. Crop growers need to calibrate their LED light sources and find the optimum light recipe as far as flux efficacy, appropriate wavelengths for different crops and optimum geometry of illumination is a concern.
Allied Scientific Pro has recently introduced the SGAL App which allows the photometric measurements of LED light sources applicable to horticulture. The PPF measurement is simply done by pointing the device at the light source and pressing a button. The software also has features which allows recording of data on a day to day basis and monitoring the growth of the plant. These diaries will help the grower to closely monitor the best course of action. Figure 6 shows highlights of this new interesting device.