Question
Hi. I having working on a science fair project and I really need your help in this. I am writing a term paper which is due aroung Feb. 28,2007. My problem is Do plants grow best in sunlight or artificial light. I am trying to write about the effects of light. Can you give more in depth info. about the effects of light please?!

Answer
The sun, a torch, and a light bulb all emit energy in the form of particles called photons. The photons from the sun are a product of thermonuclear fusion. A torch uses a chemical reaction to burn. A light bulb converts electricity to photons. But a photon is a photon ?and light is light ?whether it comes from the sun or a flashlight.

Vastly more energy comes from the sun than from any artificial light. But the light from the sun is different from a street lamp another way: most artificial light doesn抰 emit as much energy in the red and blue region of the light spectrum as sunlight does. In other words, different ratios of reds, yellows and blues all combine together to make up white sunlight.

Researchers can successfully grow plants using only artificial light in growth chambers. But sunlight is best for most plants. It抯 generally more intense than artificial light, and it抯 pretty equally distributed among the different wavelengths that earthly plants have evolved to like best.

And there抯 another difference between lamps ?even 揼row lamps??and sunlight. Grow lamps need energy to light up. Sunlight is unlimited and free.

General Lighting Requirements For Plants
To determine how much light a plant will require, consider where and how it grows best in its natural environment. Most vegetables, for instance, grow best in full sunlight, which means as much light as possible must be supplied to grow vegetables indoors. Foliage houseplants like the Philodendron grow in full shade and therefore can grow normally with relatively little artificial light. Exotic plants, such as Bromeliads and Orchids, grow in varying conditions depending on the species. Some grow in deep shade in the jungle, while others grow in bright sunlight. The lighting level required for growth indoors depends upon the characteristics of the particular plant being grown.

Plants NEED Darkness
Plants need dark periods. Periods of light (called photo-periods) and dark periods and their relative lengths have an effect on plant maturity. Recent studies have conclusively proven that it is not just the length of the day which affects growth, but the duration of the dark period which follows. The dark period of each day affects flowering and seeding of most plants. Although many plants can grow under continuous light, nearly all plants prefer a dark period each day for normal growth. All plants need some darkness to grow well or to trigger flowering. The ideal photoperiods of plants vary, some preferring long days and short nights; others the reverse; and some do best when the length of the night and day periods are equal.
 
Color Temperature
In scientific terms, Kelvin temperature is a measure of the color of a light source relative to a black body at a particular temperature expressed in degrees Kelvin (癒).

In simpler terms, it is the degree of warmth or coolness of a light source, not with regards to the physical temperature, rather to the visual temperature of the light. The higher the degree K, the more blue, or "cooler" the lamp appears. The lower the degree K, the more "warm", or red the light appears.

Incandescent lights have a low color temperature (approximately 2700癒) and have a red-yellowish tone; natural daylight  
 
has a high color temperature (approximately 6000癒) and appears bluish. Today, the phosphors used in fluorescent and high intensity (HID) lights can be blended to provide any desired color temperature in the range from 2800癒 to 6000癒.


Light is probably the most essential factor for house plant growth. The growth of plants and the length of time they remain active depend on the amount of light they receive. Light is necessary for all plants because they use this energy source to photosynthesize. When examining light levels for tropicals, consider 3 aspects of light: (1) intensity, (2) duration and (3) quality.

Light intensity influences the manufacture of plant food, stem length, leaf color, and flowering. A geranium grown in low light tends to be spindly and the leaves light green in color. A similar plant grown in very bright light would tend to be shorter, better branched, and have larger, dark green leaves. House plants can be classified according to their light needs, such as high, medium and low light requirements. The intensity of light a plant receives indoors depends upon the nearness of the light source to the plant (light intensity decreases rapidly as you move away from the source of light). The direction the windows in your home face will affect the intensity of natural sunlight that plants receive. Southern exposures have the most intense light, eastern and western exposures receive about 60% of the intensity of southern exposures, and northern exposures receive 20% of a southern exposure. A southern exposure is the warmest, eastern and western are less warm and a northern exposure is the coolest. Other factors which can influence the intensity of light penetrating a window are the presence of curtains, trees outside the window, weather, seasons of the year, shade from other buildings and the cleanliness of the window. Reflective (light colored) surfaces inside the home/office will increase the intensity of light available to plants. Dark surfaces will decrease light intensity.

Day-length or duration of light received by plants is also of some importance, but generally only to those house plants which are photosensitive. Poinsettia, kalanchoe, and Christmas cactus bud and flower only when day-length is short (11 hours of daylight or less). Most flowering house plants are indifferent to day-length.

Low light intensity can be compensated by increasing the time (duration) the plant is exposed to light, as long as the plant is not sensitive to day-length in its flowering response. Increased hours of lighting allow the plant to make sufficient food to survive and/or grow. However, plants require some period of darkness to develop properly and thus should be illuminated for no more than 16 hours. Excessive light is as harmful as too little light. When a plant gets too much direct light, the leaves become pale, sometimes sunburn, turn brown, and die. Therefore, during the summer months, protect plants from too much direct sunlight.

Additional lighting may be supplied by either incandescent or fluorescent lights. Incandescent lights produce a great deal of heat and are not very efficient users of electricity. If artificial lights are to be used as the only source of light for growing plants, the quality of light (wavelength) must be considered. For photosynthesis, plants require mostly blues and reds but for flowering, infrared light is also needed. Incandescent lights produce mostly red, and some infrared light, but are very low in blues. Fluorescent lights vary according to the phosphorus used by the manufacturer. Cool white lights produce mostly blue light and are low in red light. Foliage plants grow well under cool white fluorescent lights and these lights are cool enough to position quite close to plants. Blooming plants require extra infrared which can be supplied by incandescent lights, or special horticultural type fluorescent lights.

Light effects on plants are separated into four main categories: photosynthesis,
photomorphogenesis, photoperiodism and phototropism.

Photosynthesis is the essential process in which plants convert light energy
into chemical energy in the form of organic molecules, some of which also serve
as building blocks for plant structure.

Photomorphogenesis is the effect of light intensity and quality on plant growth
and development. For example, some seeds require light to germinate. Germination
of other seeds is prevented by light. Seedlings are etiolated (tall thin  
lacking chlorophyll) when grown without light. Leaf morphology is very different
in high light and low light resulting in sun leaves and shade leaves.
Light also has a role in circadian rhythms such as diurnal movements of leaves.

Photoperiodism is the effect of the relative length of the daily light/dark
period mainly on flowering. Some plants flower only when the daily light period
is above a certain length (long day plants), some when the daily light period is
below a certain length (short day plants), and some flower regardless of the
length of the daily light period (day neutral plants).

Phototropism is the directional growth of plant parts (leaves, stems, roots) in
which the direction of bending is determined by the direction of the light
source.