Role of light is an important role in this world. As Light factor is the most essential abiotic factor without the life cannot exist. The chief natural source of light is sunlight and has great ecological significance. It plays important roles in the life of plants, such as
- It provides energy to maintain life on earth.
- It is absorbed to chlorophyll in green plants and converted into chemical energy contained in simple sugar molecules.
- It influences morphology and anatomy of plants.
- It exerts many stimulation effects upon plants especially upon the differentiation of tissues and organs.
- It also effects the physiologic processes such as transpiration, photosynthesis, photoperiodism, phototropism, etc. and chemical composition of plants.
Types of Radiations
Light has different wavelengths which are as follow:
All the radiant energy the earth receives from the sun is in the form of electromagnetic waves varying in length from 290 to 5000 millimicron. The wavelengths between approximately 400 to 750 millimicron pass through earth’s atmosphere and illuminates it. About 40-60% of the solar radiation reaching earth lies within this range.
When sunlight is passed through a prism, it is dispersed into a sense of wavelengths exhibiting different colors as follows. Red (750-626 millimicrons), orange (626-595 millimicrons), yellow (595-574 millimicrons), green (574-490 millimicrons), blue (490-435 millimicrons) and violet (435-400 millimicrons). All these colors make up the visible spectrum of light which affect, most physiological processes of plants such as photosynthesis, etc.
The wavelengths longer than the 750 millimicrons are called infrared. These radiations produced heat and can be detected by animals because of sensation of heat these produce near infrared radiations (less than 750 millimicrons) are important biologically as these influence hormones governing germination and responses to day length (photoperiodism).
Wavelengths less than 390 millimicrons are too short to be seen and known as ultraviolet. They are very active in certain chemical reactions; however, these are not required by the plants.
Light can be measured with the help of photometers or light meters. The instruments used to measure other wavelengths of radiant energy in addition to light are called radiometers.
Units of Measurement
Light can be expressed in terms of gram-calories unit of energy, if suitable filters are used to exclude infrared and ultraviolet. It can also be expressed in terms of intensity with reference to the illumination produced by a standard candle. The amount of light received at a distance of 1 m from a standard candle is called a lux (L) or meter candle (M.C). The light intensity at 1 foot from a standard candle is called a foot-candle (F.C). which equals 10.764 L. By common agreement of world scientists the lux has been accepted as the standard international unit for expressing light intensity. Light energy can be expressed as lux-seconds or lux-hours.
Methods of Measurement
Methods of measurement of wavelengths are as follow
A simple radiometer consists of two thermometers sealed in an evacuated glass tube. The bulb of one is covered with black paint and that of the other is left naked. The meter is kept in a dark box until a measurement is to be made. It is held in a horizontal position over the area where radiation is to be measured. The degree of difference registered by two thermometers after 5 minutes is the measure of radiant energy.
Radiometers are more sensitive to the longer wavelengths; therefore, these are used to light measurement for studying the process of photosynthesis. These are only light-measuring instruments that integrate all wavelengths of radiant energy.
Spectrophotometer used to measure light energy of different wavelengths of the spectrum (quality of light). These are portable instruments which give readings directly. These are quite expensive and light readings in 1 m are not closely comparable among such instruments.
These are sensitive portable units that can –read light intensity directly. Photoelectric cells provide most accurate and absolute value of light intensities quickly and conveniently. These can be used under water when encased in a suitable water tight cell. However, the sensitivity of the cells is affected by light intensity and temperature. Similarly, it is not possible to get these cells accurately standardized. Also, photoelectric cells are sensitive chiefly to blue, violet, and ultraviolet rays. Therefore, photoelectric cells are used to measure effect of light intensity on structure and function other than photosynthesis.
This method is suitable for measuring the effect of light over a long period Solutions that slowly decompose in the presence of light are exposed continuously and analyzed time to time. For example, a concentrated solution of anthracene in benzene is used which slowly polymerizes when exposed to light. The amount of polymer accumulated during the stipulated period is taken as a measure of light energy received. An important disadvantage of this technique is that they are more sensitive to ultra violet light than to light.
The Secchi Disc
This method is used to measure the penetration of light through water. A white disc of about 20 cm in diameter is lowered horizontally during midday when the sky is clear until it just disappears from sight. This depth is recorded. Then the disc is raised until it reappears and the depth is again recorded. The mean of these readings is taken as measurement of light penetration.