Light intensity or light quantity refers to the total amount of light that plants receive. It is also described as the degree of brightness that a plant is exposed to. In contrast to light quality, the description of the intensity of light does not consider wavelength or color.
The intensity of light is usually measured by the units lux (lx) and footcandle (fc). One footcandle means the degree of illumination 1 foot away from a lighted standardized wax candle; 100 footcandles is 1 foot away from 100 candles that are lighted simultaneously. Lux (pl. luces) is the unit of illumination that a surface receives one meter away from a light source. One footcandle is equal to 10.76391 luces and 1 lux is approximately equal to 0.093 footcandle.However, the units footcandle and lux are merely based on visual sensitivity and do not provide information on the energy or photon content of light. According to Runkle (2006), the better unit of light intensity for studies involving plant responses is the μmol m-2s-1. It describes the number of photons of light within the photosynthetic waveband that an area of 1 sq meter receives per second. It can be measured using a light meter. To convert the intensity of light from the sun, for instance, 800 μmol m-2s-1 to footcandle, 800 is multiplied by 5 which results to 4,000 fc. Another unit of measurement is the mol m-2d-1 which describes the total number of photons received by an area of 1 sq meter in 24 hours.
The following intensity values in footcandle are given for some light conditions (Janick 1972): starlight- 0.0001, moonlight- 0.02, indoors near window- 100, overcast weather- 1000, direct sunlight- 10,000.
High light intensity means it is brighter compared to low light intensity. Some terms that are used with reference to light intensity are open or full sun, partial sun or partial shade, and closed or dense shade. Based on adaptation, crops can be classified as sun plants and shade plants with various intergrades in between.
Factors Affecting Light Intensity
The intensity of light can change with the time of the day, season, geographic location, distance from the equator, and weather. It gradually increases from sunrise to the middle of the day and then gradually decreases toward sunset; it is high during summer, moderate in spring and fall, and low during winter time.
Maximum intensity occurs at the equator, and gradually decreases with increasing distance from the equator to the south and north poles. Light intensity is also affected by dust particles and atmospheric water vapor, slope of the land, and elevation (Edmond et al. 1978).
Depending on the particular time of the year, the sun-to-earth distance varies; it is closest in January (about 147 million km) and farthest in early July (about 151 million km; Davis 1977). This causes a slight variation in the amount of light and heat that the earth receives. Likewise, many factors can affect indoor light.
According to Manaker (1981), the amount of natural light that may enter a building is affected by the location of windows or glass surface through which light enters, the presence of trees and shrubs, roof overhangs, window screens and awnings, and the tint and cleanliness of the glass. A gray glass allows 41% light transmission while clear glass allows 89%. Within a building, the amount of light, whether natural or artificial, will be further affected by curtains and blinds, surface textures, and reflectance from wall coverings, furniture, and other furnishings.
Further, leaves on a single plant differ in the amount of light that they receive. The amount of light incident on a leaf decreases as sunlight passes downward through the canopy. Leaves on the upper part of the canopy tend to shade and reflect light away from the lower leaves. Plants with somewhat vertical leaves (erectophyle plant type) allow more downward passage of light and tolerate high population planting than plants with drooping leaves (planophyle type) (Chapman and Carter 1976). Row planting and proper spacing can also minimize interplant shading.
Some Effects of Light Intensity on Plant Growth
Light is an absolute requirement for plant growth and development. However, different plants have optimum requirements and both deficient and excessive light intensities are injurious. Subject to physiological limits, an increase in the intensity of light will result to an increase in the rate of photosynthesis and will likewise reduce the number of hours that the plant must receive every day (Manaker 1981).
During summer when light supply is abundant and almost continuous in Alaska, potatoes and cabbages of enormous sizes have been produced (Janick 1972).
According to Chapman and Carter (1976), the minimum limit for the process of photosynthesis in most plants is between 100 and 200 fc. But light intensity of as low as 10 lux (0.93 fc), which occurs at twilight, can affect phototropic response (Vergara 1978).
Deficient light intensities tend to reduce plant growth, development and yield. This is because low amount of solar energy restricts the rate of photosynthesis. Below a minimum intensity, the plant falls below the compensation point. Photosynthesis significantly slows down or ceases while respiration continues. Compensation point is the metabolic point at which the rates of photosynthesis and respiration are equal so that leaves do not gain or lose dry matter.
Etiolation, a morphological manifestation of the adverse effect of inadequate light, is described by Chapman and Carter (1976) in the following manner: it develops white, spindly stems, elongated internodes, leaves that are not fully expanded, and a stunted root system.
Likewise, excessive light intensity should be avoided. It can scorch the leaves and reduce crop yields. Edmond et al. (1978) provided three explanations: (1) Chlorophyll content is reduced. This reduces the rate of light absorption and the rate of photosynthesis; (2) Excess light intensity is associated with increase in the temperature of leaves which in turn induces rapid transpiration and water loss. The guard cells lose turgor, the stomates partially or completely close, and the rate of diffusion of carbon dioxide into the leaves slows down. The rate of photosynthesis decreases while respiration continues, resulting to low availability of carbohydrates for growth and development; (3) High leaf temperature inactivates the enzyme system that changes sugars to starch. Sugars accumulate and the rate of photosynthesis slows down.
CHAPMAN SR, CARTER LP. 1976. Crop Production: Principles and Practices. San Francisco: W.H. Freeman and Company. p. 146-163.DAVIS TN. 1977. Sun-Earth distance. Alaska Science Forum (Feb. 16, 1977). Retrieved April 17, 2011 from http://www2.gi.alaska.edu/ScienceForum/ASF1/142.html.
VERGARA BS. 1978. Crop response to light variations. In: Gupta US, ed. Crop Physiology. New Delhi: Oxford & IB Publishing Co. p. 137-156.