Light and Temperature Influence Plant Growth

The Role of Light Intensity and Temperature in Plant Development

Jul 11, 2009

While plant growth is regulated internally, it is greatly influenced by external conditions such as the intensity and duration of light and the temperature.

Many external environmental factors exert great influence on the growth and development of plants. Two of the most important are the intensity and duration of the light received by the plant and the temperature of the air and soil around the plant.

Light Intensity and Duration

Aside from its effect through photosynthesis, light influences the growth of individual organs or of the entire plant in less direct ways. The most striking effect can be seen between a plant grown in normal light and the same kind of plant grown in total darkness. The plant grown in the dark will have a tall and spindling stem. The leaves fail to expand, and both leaves and stem, lacking chlorophyll, are pale yellow. Such a plant is said to be etiolated.

Plants grown in shade instead of darkness show a different response. Moderate shading tends to reduce transpiration more than it does photosynthesis. Hence, shaded plants may be taller and have larger leaves because the water supply within the growing tissues is better. With heavier shading, photosynthesis is reduced to an even greater degree and small, weak plants result.

Both the intensity and duration (length) of light may have different and characteristic effects upon plant growth and development. It has been found that the length of the daylight period may have a striking effect upon vegetative growth and reproductive activities of plants. The reaction of plants in relation to the length of the day is called photoperiodism.

Plants of the temperate regions may be divided into three categories according to the effect photoperiod (length of exposure to daylight) has upon the vegetative growth and reproduction of the plant:

Short – Day plants (SDP) flower when the day length is short. Short – day plants grow vegetatively during the long days of summer and do not produce flowers until the days become shorter in late summer and early fall. Among short-day plants are poinsettias, most aster and goldenrods, the ragweeds, chrysanthemums, sorghum, and many others. Many short – day plants are extremely sensitive to light exposure during the night. In some types, even vehicle headlights or the light from a flashlight can prevent flowering.

Long – Day plants (LDP) come into flower only under extended periods of illumination and produce only vegetative growth when the photoperiod is long. Among long – day plants are hollyhock, radish, garden beet, spinach, iris, and clover.

Day – Neutral plants (DNP) on the other hand are not particularly sensitive to the length of day. Included in this group are the bean, tomato, vetch, cyclamen, nasturtium, roses, snapdragon, carnation, and many common weeds.

Who has not marveled at the glorious riot of blooms to be seen in a florist shop in midwinter? By cleverly manipulating light conditions, horticulturists and florists can bring flowers into bloom in seasons when they should not be reproductively active.

In general, growth is promoted when temperature rises and inhibited if the temperature falls. However, the growth rate does not continue to increase indefinitely with temperature rise. High – temperature injury due to desiccation (drying) and a runaway metabolic rate eventually occurs.

Temperature affects growth though its effect on metabolic activities: digestion, translocation of materials, respiration, and the building of new cells. Also, high temperatures increase transpiration and thus reduces turgor and growth, especially during the day.

Each species has a minimum temperature, below which it fails to grow; an optimum at which the growth rate is highest; and a maximum, above which, growth comes to an end. The optimum temperature may vary with each stage of development and with the length of time the temperature prevails.

Temperature affects not only the rate but also the type of growth. When photoperiodism was first discovered, the photoperiod seemed the most important, perhaps the only, environmental factor causing flower formation. It is now known that the light response for many plants is modified by temperature. The suitable photoperiod alone may be insufficient to bring about flowering unless it is accompanied by suitable temperatures.

Temperature also plays a major role in the cycle of activity and inactivity known as dormancy in plants of temperate climates. Dormancy is especially prominent in woody plants; the leaves drop in autumn, the tree is inactive during the winter, and with the coming of spring, activity and growth are renewed.

The length of the dormant period varies, and for many species a period of low temperatures is required to break the dormancy and permit growth to resume. Most deciduous fruit trees, such as the apple, peach, and cherry, require extended winter rest periods and therefore can be grown only in temperate climates.

While some plants require freezing temperatures to break the rest period, others need only low temperatures above freezing. Most bulbs, tubers, and other underground stems require at least a short rest period.

The copyright of the article Light and Temperature Influence Plant Growth in Botany is owned by Dennis Holley. Permission to republish Light and Temperature Influence Plant Growth in print or online must be granted by the author in writing.