Section 1: Introduction
Light is considered by many plant scientists as the single most important variable with respect to plant growth and development, and light is often the limiting factor in plant growth. Light essentially provides the energy required to run photosynthesis which is the process by which a plant utilizes carbon dioxide (CO2) and water (H2O) to form carbohydrates [(CH2O)n] such as sugars and starch. In fact, this process is the basis for almost all terrestrial life on Earth.
| LIGHT | ||||||
| CO2 (carbon dioxide) |
+ | H20 (water) |
——› | O2 (oxygen) |
+ | CH2O (carbohydrates) |
Light also affects plant growth and development by its involvement in functions other than photosynthesis. Changes in plant growth and development that are controlled by light, but not necessarily a function of photosynthesis are referred to as being photomorphogenic responses (photo = light, morpho = change and genic = growth). For example, a poinsettia plant will only form leaves (vegetative growth) if the length of the night is less than approximately 11.5 hours (long day). However, if the length of the night exceeds 11.5 hours (short days), physiological changes occur in the meristematic tissues and the plant forms floral structures. This process is an example of a photomorphogenic response and more specifically would be referred to as a photoperiodic response (photo = light and periodic = duration).
Before discussing greenhouse light control, a few basic concepts regarding light should be reviewed. Discussing light can be a little confusing because light displays both particle and wave properties. This simply means that light behaves as particles (photons) when it is measured in certain ways and as waves (wavelength) when measured in other ways. Photons are discrete particles of visible light with varying energy levels. The wavelength of light is indicative of the energy level of the light and may be expressed as a specific wavelength or its corresponding color (as perceived by the human eye). The shorter the wavelength, the higher the energy level of the corresponding photons.
