Photosynthetically active radiation refers to the solar radiation energy that can be absorbed and transformed by light and pigments in the process of photosynthesis of green plants.
The wavelength range of photosynthetically active radiation is 380-710 nm, which basically coincides with visible light. The proportion of photosynthetically active radiation in direct solar radiation increases with the increase of the sun’s altitude angle, up to 45%. In the scattered radiation, the proportion of photosynthetically active radiation can reach as much as 60~70%, so cloudy days increase the proportion of PAR. Photosynthetically active radiation accounts for about 50% of the total solar radiation on average.
Why monitor photosynthetically active radiation?
Why monitor photosynthetically active radiation?
Photosynthetically active radiation is a very important environmental factor for plant growth. In facility agriculture, different crop varieties and different growth and development stages require different light intensity and duration. In order to achieve a good planting effect, it is necessary to adjust the appropriate light intensity and time according to the growth and development of the plant. Therefore, reasonable regulation of photosynthetically active radiation can not only effectively save light and electricity, but also further improve crop yield and quality.
How to monitor photosynthetically active radiation?
There are three measurement systems for photosynthetically active radiation:
1. Optical system. This system is based on the response characteristics of the human eye to brightness. The instruments include illuminometers, etc. The observed physical quantity is the luminous flux density of the visible light band emitted by the radiation source, which is measured by illuminance (lx).
2. Energetic system. Using thermocouples as sensors, there are sky pyranometers, direct pyranometers, net pyranometers, spectroscopic pyranometers, etc. The radiant flux density in a certain characteristic wavelength range, that is, in the photosynthetically effective band, is also measured as irradiance (Wm-2).
3. Quantum systems. Using silicon, selenium photocells, etc. as sensors, instruments that measure the amount of radiation from the angle of photons include photon flux meters. It is measured by the photon flux density (umol m-2 s-1).
Photosynthetically active radiation can be measured directly by instruments. For example, using the principle of photoelectric induction, the measurement is carried out by a photosynthetically active radiation sensor equipped with a high-precision photoelectric induction element. When there is light, the light is focused by the lens by the light projector, transmitted to the light receiver lens, and then to the receiving sensor. The sensor converts the received light signal into an electrical signal and transmits it to the platform, which can effectively measure the environment. The amount of photosynthetically active radiation.
Photosynthetically active radiation can also be estimated from direct solar radiation. In order to obtain the proportionality coefficient between the direct solar radiation and scattered radiation and the photosynthetically active radiation, the pyranometer or sky pyranometer and the photosynthetically active radiation can be observed synchronously, and the coefficient values of the day, month, season and year and their interaction can be calculated. The Soviet Union X. Mordau et al studied the quantitative relationship between direct solar radiation (S) and diffuse radiation (D) and photosynthetically active radiation (Qp), listed the calculation formula and pointed out that in the middle and high latitudes 4 When the height of the sun is not lower than 20° at noon in September, the calculation error of this formula for the total daily or monthly total photosynthetic active radiation does not exceed 5%. Its calculation formula is: Qp=0.43S+0.57D.
in conclusion
With the continuous development of my country’s agriculture, how to carry out agricultural production more efficiently and energy-saving has become a problem that needs to be considered on the road of my country’s agricultural development. By grasping the influence law of photosynthetically active radiation intensity on crop growth and development, crop yield and quality can be further improved.
