EFFECT OF RADIATION ON CROP PRODUCTION
Solar energy provides two essential needs to plants-
a. light, required for photosynthesis and for many other functions of the plant- including seed germination, leaf expansion, growth of stem and shoot, flowering, fruiting and even dormancy, and
b. thermal conditions required for the normal physiological functions of the plant.
Light is indispensable for the synthesis of the most important pigment of the plant, i.e. the green chlorophyll. The chlorophyll is capable of absorbing radiant energy and converting it into potential chemical energy of carbohydrates.
6 CO2 + 12 H2O
CCH12O6 + 6 H2O + 6O2
The carbohydrates manufactured by the plants during photosynthesis is the only link between the solar energy and the living world.
Light affects the plants in four ways :
a. Light intensity
b. Light quality
c. Light duration
d. Light direction
a. Light intensity :
Although about one percent of the light energy is converted into potential chemical energy, very low intensity reduces photosynthesis, and may even results in the closing of the somata, and consequently reduced vegetative growth of the plants. Very high light intensity is detrimental to plants in many ways. It increases the rate of respiration and thus disrupts the photosynthesis-respiration balance. The most harmful effect of high light intensity is the phenomenon of solarization in which all the cell contents are oxidized by atmospheric oxygen. However, the rate of photosynthesis is directly proportional to light intensity up to certain limit.
Fig 1. Effect of increasing light intensity on the rate of photosynthesis is expressed in terms of rate of oxygen evaluation.
Plants have been divided into sun (heliophytes) and shade (sciophytes) loving species on the basis of their marked differences in the saturation high intensity as shown in Fig.2. At a certain light intensity the amount of carbon dioxide used in photosynthesis and amount of carbon dioxide produced in respiration are volumetrically equal. This point is known as compensation point.
2. Light Quality :
For photosynthesis the visible of spectrum (between 350 to 750 mm) is essential. The maximum photosynthesis is shown to occur in the red part of the spectrum with the next peak in blue part and minimum in the green region. The region between 570 to 750 mm (yellow to red) is quite congenial for photosynthesis (Fig.3.) The red light promotes germination and far-red light inhibits germination. The pigment (phytochrome) occurs in two reversible forms-Per and Prf which develop under red and far-red light. The germination depends upon the balance between two forms:
There are three broad classes of germination response which are :
- Seed germination promoted by white light e.g. Tobacco, Latuce
- Seed germination inhibited by white light e.g. Allium Sp. and
- Seed germination not influenced by white light e.g. Rice, Wheat.
3. Light duration :
Duration of light has consideration importance from the farmers point of view in selecting the crop species or variety of a crop, as it in fluencies the flowering of crops. The response of plants to the relative lengths, and alternations of light and dark periods with regard to the initiation of flowering is called photoperiodism.
Based on flowering in response to different photoperiod, Garner and Allard divided plants into three groups.
a. Long Day Plants :
Plants which flower in response to days longer than critical day lengths. In other word, long day plants flower when the dark period is shorter than the critical dark period.
Examples: Gram, Barley, Wheat, Pea, Sugar beet.
4. Direct of Light :
Shoots, roots and leaves show different orientation to the direction of light. Shoots elongate towards the source of light (positively phototrophic) while roots towards soil (negative phototrophic). Again, in temperate regions, the southern slopes show better growth of crops than the northern slops due to direction of light.