Most terrestrial plants grow by selective absorption of natural light from the sun. Sunshine is important in plant growth because the heat and the light required by all growing plants are supplied by solar radiation.

The solar radiation ranges from infrared to ultraviolet. Not all the radiation reaches earth’s surface, because the ultraviolet wavelengths, that are the shorter wavelengths, are absorbed by gases in the atmosphere, primarily by ozone.

Direct solar radiation has wavelengths ranging from 300 to 3,000 nm, and is divided into three bands: ultraviolet radiation, visible radiation and infrared radiation.

The wavelengths of visible radiation for humans are in the range from 380 to 780 nm, and the peak of the visibility curve (photopic vision) is at 555 nm. While heat cannot entirely replace light in this process, light can in large measure replace heat.

The quality and the quantity of the sun-light transmitted to growing plants are both dependent upon atmospheric conditions, as well as upon the season of the year. They vary from place to place and from month to month.

Of the various weather elements, sunshine, directly through radiation, and indirectly through its effect upon air temperatures, influences the distribution of crops.

This is because it furnishes the required energy for certain chemical activities within growing plants, as well as promotes evaporation from the foliage.

Abundant sunshine is required of most plants. In this unit you will be made to understand solar radiation and the energy content, energy balance and effects of solar radiation on plant growth and development.

Radiant energy from the Sun is the major source of energy for terrestrial life. Practically all the energy for all the physical and biological processes occurring on the Earth arise in the form of solar radiation.

Radiation is the ultimate of all the changes and motion of the atmosphere and it is the single most important control of climate. It is a meteorological element of highest important.

Radiation from the Sun comes in forms of short-wave electromagnetic radiation. The shortwave radiation is referred to as short-wave incoming radiation. The outgoing radiation from the soil is called the long-wave terrestrial radiation.

Energy Balance

Light and Solar Radiation to Plant Growth and Development

Solar radiation is the set of electromagnetic radiation emitted by the Sun. The Sun behaves almost like a black body which emits energy according to Planck’s law at a temperature of 6000 K.

Solar radiation ranges goes infrared to ultraviolet. Not all the radiation reaches Earth’s surface, because the ultraviolet wavelengths, that are the shorter wavelengths, are absorbed by gases in the atmosphere, primarily by ozone.

The atmosphere acts as a filter to the bands of solar spectrum, and at its different layers as solar radiation passes through it to the Earth’s surface, so that only a fraction of it reaches the surface.

The atmosphere absorbs part of the radiation, reflects and scatters the rest some directly back to space, and some to the Earth, and then it is irradiated. All of these produce a thermal balance, resulting in radiant equilibrium cycle.

The net radiation is the difference between total incoming and outgoing radiations and is a measure of the energy available at the ground surface.

It is the energy available at the Earth’s surface to drive the processes of evaporation, air and soil heat fluxes as well as other smaller energy consuming processes such as photosynthesis and respiration.

Energy Wave lengths

Depending on the type of radiation, it is known that the 324 Wm-2 reaching the Earth in the upper atmosphere (1400 Wm-2 is the solar constant), 236 Wm-2 are reissued into space infrared radiation, 86 Wm- 2 are reflected by the clouds and 20 Wm-2 are reflected by the ground as short-wave radiation. But part of the re-emitted energy is absorbed by the atmosphere and returned to the earth surface, causing the “greenhouse effect”.

The average energy that reaches the outside edge of the atmosphere from the sun is a fixed amount, called solar constant. This energy contains between 200 and 4000 nm wavelengths and it is divided into ultraviolet radiation, visible light and infrared radiation.

Ultraviolet Radiation

Ultraviolet radiation consists of shorter wavelengths band (360 nm). It has a lot of energy and interacts with the molecular bonds. These waves are absorbed by the upper atmosphere, especially by the ozone layer.

Visible Light

This radiation band corresponds to the visible area with wavelengths between 360 nm (violet) and 760 nm (red), it has a great influence on living beings.

Infrared Radiation

This consists of wavelengths between 760 and 4000 nm. It corresponds to the longer wavelengths and it has little energy associated with it. Its absorption increases molecular agitation, causing increase in temperature.

Types of Radiation

Solar radiation on the earth can be classified as:

1. Direct Radiation

This radiation comes directly from the sun without any change in its direction. This type of radiation is characterized by projecting defined shadow onto the objects that intersect.

2. Diffuse Radiation

This radiation comes from all over the atmosphere as a result of reflection and scattering by clouds, particles in the atmosphere, dust, mountains, trees, buildings, the ground itself, and so on.

3. Global radiation

This is the total radiation. It is the sum of the two radiations above. On a clear day with a clear sky, the direct radiation is predominant above the diffuse radiation.

Effects of cloud on Solar Radiation

Plants are able to use only a portion of the solar radiation spectrum. This portion is known as “photosynthetically active radiation (PAR)” and is estimated to be about 43% to 50% of total radiation.

Amount of PAR available to a crop is reduced proportionately to cloud cover.

On a cloudy and partly cloudy day, PAR will be reduced by 50% and 25% respectively and by over 60% on rainy days.

It is not surprising, then, that cloudy, rainy periods have significant effect on crop development and yield.

Instruments Used to Measure Solar Radiation

The instruments used to measure solar radiation include:

Bellanis pyranometer or solarimeter
Sunshine recorder
Line spectrum sensor
Photometer
Lux meter measures the light intensity

Effects of Solar Radiation on Crops

Crop production is exploitation of solar radiation. One of the most important factors that influence plants development is the solar radiation intercepted by the crop. Solar radiation brings energy to the metabolic process of the plants.

The principal process is the photosynthetic ^{assimilation using water, CO}2 ^{and light energy.}^{A part of this, energy is}used in the evaporation process inside the different organs of the plants, and also in the transpiration through the stomata.

Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Depending on how carbon dioxide is fixed the plants can be grouped into three types: C3, C4, and CAM.

The C3 plants are usually the more superior plants, which are the temperate weather crops (wheat, barley and sunflower, etc.).

The C4 category are species from arid weather or hotter or tropical weathers (corn, sugar or sorghum). The C3 types are generally considered less productive than C4. One difference lies in the fact that photorespiration is very active in C3 plants.

The photorespiration makes plants increase the oxygen consumption when they are illuminated by the sun, and this is very important for agriculture in temperate zones.

In a hot day with no wind, the CO2 concentration in the plant decreases considerably for photosynthesis consumption. Therefore, the relationship between carbon and oxygen decreases, and the CO2 fixation increases the photorespiration. Crops like rice critically requires light 25 days prior to flowering while for barley, it is at flowering period.

In summary, radiant energy from the Sun is the major source of energy for terrestrial life. Practically all the energy for all the physical and biological processes occurring on the Earth arises in the form of solar radiation.

The productivity of a crop depends on the ability of plant cover to intercept the incident radiation. Of the various weather elements, sunshine, directly through radiation, and indirectly through its effect upon air temperatures, influences the distribution of crops.