Light
Light is the energy source of all plants. A complex series of chemical reactions
combined with light energy, carbon dioxide, and water produces sugar molecules. Light
also generates heat and often needs to be controlled to prevent plant scald or dehydration.
Light varies in intensity (brightness), duration (day length), and quality (color). The
brightness of light is measured in either foot-candles or in Photosynthetic Photon Flux
(PPF). Because the process of photosynthesis depends on the number of photons
intercepted (PPF) and not just the light given off by a point source, professionals more
often use PPF measurements than foot-candles. The most importance light is measured in
the 400 to 700 nm waveband. A quantum sensor measures PPF energy in micromoles of
photons per unit area per time (1μ mole m-2 s-1), a pyranameter sensor conveys
measurements in watts per meter squared (W/m2), and a photometric sensor states light
measurements in foot-candles (f-c). Because a photometric sensor is partially insensitive
to red and blue waves, the pyranameter sensor is the preferred sensor when measuring
light quality and intensity.
The light needs of plants are not all the same. Some plants require or can withstand very
high light intensities, while others grow best at medium or low light intensity levels.
There is a point at which the amount of sugars produced by photosynthesis equals the
amount used up in respiration. This point is called the light compensation point. The
light compensation point for each species of plant. It is low for plant that grow in shade
and high for plants that normally need full sunlight.
As light intensity increases in the early afternoon, there is a second point where growth is
limited. The light saturation point is the intensity at which there is no further increase in
photosynthesis with an increase in light intensity. A continuing increase in light intensity
damages plants such as African violets and schefflera. If the light intensity is too high,
leaves kept in this condition will develop a light, yellowish appearance. During the
flowering period of chrysanthemums, hydrangeas, and geraniums, very bright light
together with high temperatures will cause petal blight. The heat raises the rate of water
loss from the petals to the point where enough water cannot be supplied from the roots.
When light intensity is too low, the rate of growth and development are very low. Also
the distance between the leaves stretches, bud and flower formation is delayed.
Photoperiod is a term applied on the length of time that light shines on a plant each day.
Day length changes with the seasons of the year and with the latitude of the location. On
March and September 21 is the equinox when the sun is directly over the equator and the
day and night lengths are 12 hours long. June 21 is the summer solstice when the sun
reaches its most northern point above the equator, resulting in the longest days in the
summer. December 21 is the shortest day and the longest night.
Research by Garner and Allard has shown that the length of light and dark periods has an
effect on the growth of plants. In some plants, the beginning of flower buds depends
upon the duration of light. In the early 1920s, Garner and Allard were working on
tobacco and soybean plants and showed that tobacco failed to flower when grown
outdoors in the summer, yet when grown in a greenhouse in the winter it flowered. The
authors learned that the short day caused the plants to bloom. The response to day length
is called photoperiodism and based on the results of their findings; plants identified three
classes of plants:
1.
Short-day plants flower when the day length is shorter than a critical number of
hours. Chrysanthemums and poinsettias are both short-day plants.
2.
Long-day plants flower only if the day length is longer than a critical number of
hours. Tuberous begonias and spinach are long-day plants.
3.
Day neutral plants (indeterminant) are those plants for which the day length does
not influence flowering directly. African violets and roses are day-neutral plants.
The quality of light is called the action spectrum. Visible light consists of many
wavelengths ranging from the short ultraviolet waves to the long infrared waves. Each
color has a specific wavelength in the action spectrum. Each wavelength of light causes
chemical and thermal responses in plants that influence various phases of growth. White
light is a blend of violet, indigo, blue, green, yellow, orange, and red. The wavelengths
of these colors range from 400 to 700 nm. USDA researchers found that red light with a
wavelength of 660 nm prevents the flowering response, and that far-red light with a
wavelength of 735 nm stimulates flowering in plants.