Most terrestrial plants grow through selective absorption of natural light from the sun. Sunshine is essential for plant growth, as it provides both the heat and light required for all growing plants through solar radiation. Solar radiation spans from infrared to ultraviolet, but not all of it reaches Earth’s surface.
The ultraviolet wavelengths, which are shorter, are absorbed by gases in the atmosphere, primarily 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 fall between 380 to 780 nm, with the peak of the visibility curve (photopic vision) at 555 nm.
While heat cannot fully replace light in this process, light can largely substitute for heat. The quality and quantity of sunlight reaching growing plants depend on atmospheric conditions and the season, varying across different locations and months.
Among the various weather elements, sunshine directly through radiation and indirectly through its effect on air temperatures influences crop distribution.
This is because it provides the energy needed for specific chemical activities within growing plants and promotes evaporation from the foliage.
Abundant sunshine is necessary for most plants. In this article, the concept of solar radiation, its energy content, energy balance, and effects on plant growth and development will be explored.
Read Also: Feijoas: History, Nutrition, Health Benefits and Growing Guide
The Importance of Solar Radiation as A Source of Energy
Radiant energy from the Sun is the primary source of energy for terrestrial life. Nearly all energy for the physical and biological processes occurring on Earth originates in the form of solar radiation.
Radiation is the ultimate factor driving changes and motion in the atmosphere and is the single most important control over climate. Solar radiation is a meteorological element of highest importance.
Solar radiation from the Sun comes as short-wave electromagnetic radiation, known as short-wave incoming radiation. The outgoing radiation from the soil is referred to as long-wave terrestrial radiation.
Energy Balance and Solar Radiation
Solar radiation is electromagnetic radiation emitted by the Sun. The Sun behaves almost like a black body emitting energy according to Planck’s law at a temperature of 6000 K. Solar radiation ranges from infrared to ultraviolet.
Not all radiation reaches the Earth’s surface, as the ultraviolet wavelengths (the shorter wavelengths) are absorbed by gases in the atmosphere, primarily by ozone.
The atmosphere acts as a filter for the solar spectrum, with different layers absorbing part of the radiation, reflecting and scattering some directly back into space, and redirecting others back to Earth.
These processes create a thermal balance that leads to a radiant equilibrium cycle. The net radiation is the difference between total incoming and outgoing radiations, measuring the energy available at the Earth’s surface.
This energy is available to drive processes like evaporation, air and soil heat fluxes, as well as smaller energy-consuming processes such as photosynthesis and respiration.
Energy Wavelengths and The Solar Constant
Depending on the type of radiation, it is known that 324 Wm-2 reaches Earth in the upper atmosphere (the solar constant is 1400 Wm-2). Of this, 236 Wm-2 is reissued into space as infrared radiation, 86 Wm-2 is reflected by clouds, and 20 Wm-2 is reflected by the ground as short-wave radiation.
Part of the re-emitted energy is absorbed by the atmosphere and returned to the Earth’s surface, contributing to the “greenhouse effect.”
The average energy reaching the outer edge of the atmosphere from the Sun is a fixed amount, called the solar constant. This energy ranges between 200 and 4000 nm wavelengths and is divided into ultraviolet radiation, visible light, and infrared radiation.
Read Also: Pawpaws: History, Nutrition, Health Benefits and Growing Guide
Types of Radiation
Solar radiation on Earth can be classified as follows:
1. Direct radiation: This radiation comes directly from the Sun without any change in direction. It is characterized by defined shadows projected onto objects it intersects.
2. Diffuse radiation: This radiation is scattered throughout the atmosphere due to reflection and scattering by clouds, particles, dust, mountains, trees, buildings, and the ground.
3. Global radiation: This is the total radiation and is the sum of direct and diffuse radiation. On a clear day with a cloudless sky, direct radiation predominates over diffuse radiation.
Effect of Clouds on Solar Radiation
Plants can only use a portion of the solar radiation spectrum, known as “photosynthetically active radiation (PAR),” which is estimated to be about 43% to 50% of total radiation. The amount of PAR available to crops decreases with increased cloud cover.
On cloudy days, PAR is reduced by 50%, while on partly cloudy days, it decreases by 25%, and on rainy days, by over 60%. Consequently, cloudy and rainy periods significantly impact crop development and yield.
Instruments Used to Measure Solar Radiation
Instruments used to measure solar radiation include:
1. Bellanis pyranometer or solarimeter
2. Sunshine recorder
3. Line spectrum sensor
4. Photometer
5. Lux meter, which measures light intensity
Effects of Solar Radiation on Crop Development
Crop production relies on solar radiation. One of the most important factors influencing plant development is the amount of solar radiation intercepted by the crop. Solar radiation provides energy for the plant’s metabolic processes.
The primary process involved is photosynthetic assimilation, where water, CO2, and light energy are utilized. Part of this energy is also used in the evaporation process within the plant’s organs and transpiration through the stomata.
Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, primarily sugars, using sunlight as the energy source. Depending on how carbon dioxide is fixed, plants are grouped into three types: C3, C4, and CAM.
1. C3 plants: Typically superior plants in temperate climates, such as wheat, barley, and sunflower.
2. C4 plants: Found in arid, hot, or tropical climates, such as corn, sugar, and sorghum. C4 plants are generally more productive than C3 plants due to lower photorespiration rates.
3. CAM plants: Adapted to extreme environments like deserts, where water conservation is crucial.
In temperate zones, the increased oxygen consumption due to photorespiration in C3 plants can limit productivity, particularly on hot days with low wind. Crops like rice critically require light 25 days before flowering, while barley requires it at the flowering period.
Radiant energy from the Sun is the primary energy source for all terrestrial life. The productivity of crops depends on their ability to intercept incident radiation.
Sunshine directly affects crop distribution by influencing radiation and air temperatures. This article underscores the vital role solar radiation plays in plant growth and crop production, highlighting its effects on various physiological processes.
Do you have any questions, suggestions, or contributions? If so, please feel free to use the comment box below to share your thoughts. We also encourage you to kindly share this information with others who might benefit from it. Since we can’t reach everyone at once, we truly appreciate your help in spreading the word. Thank you so much for your support and for sharing!