Photosynthesis refers to green plants and certain bacteria containing chloroplasts. Under the irradiation of visible light, through photoreaction and carbon reaction (formerly known as dark reaction), photosynthetic pigments are used to convert carbon dioxide (or hydrogen sulfide) and water into organic matter and release. Biochemical process of oxygen (or hydrogen). There is also an energy conversion process that converts light energy into chemical energy in organic matter.
Photosynthesis is the sum of a series of complex metabolic reactions, the basis for the survival of the biological world, and an important medium for the earth's carbon-oxygen balance (ie, the balance between carbon dioxide and oxygen). Photosynthesis can be divided into oxygenic photosynthesis and anoxygenic photosynthesis. It is a biochemical process in which green plants and certain bacteria use chlorophyll to convert carbon dioxide and water into organic matter (mainly starch) under the irradiation of visible light and release oxygen. For almost all living things in the biological world, this process is the key to their survival, and the carbon and oxygen cycle on the planet, photosynthesis is essential.
The spectral component of solar radiation that is effective for plant photosynthesis is called photosynthetically active radiation, with a wavelength range of 380 to 710 nm, which is substantially coincident with visible light. The ratio of photosynthetically active radiation to direct solar radiation increases with increasing solar elevation, up to 45%. In the scattered radiation, the proportion of photosynthetically active radiation can reach as much as 60~70%, so the cloudy days increase the proportion of PAR. Photosynthetically active radiation accounts for about 50% of total solar radiation.
The part of the spectral energy of the absorbed solar radiation that causes the chlorophyll molecule to be excited during photosynthesis. The wavelength is represented by the symbol Qp in watts/meter 2. Photosynthetically active radiation is an energy source for plant life activities, organic matter synthesis, and yield formation.
And the turbidity of the atmosphere is reduced and increased. The ratio changes with time in sunny days, generally low in the morning and evening, high and stable around noon, high in summer and low in winter. In sunny winter, when the sun height increased from 10° to 45°, the photosynthetically effective emissivity increased from 0.35 to 0.45; in summer, it increased from 0.47 to 0.48. The photosynthetically effective emissivity in scattered radiation does not change substantially with the solar elevation angle, but there are certain changes in the different weather types of sunny and negative, and it is larger than the photosynthetically effective emissivity in direct radiation, between 0.50 and 0.60. between.
The light entering the crop group is divided into two parts: one is the direct light passing through the gap of the upper blade, which is a "spot"; the other is the transmitted light and partially scattered light transmitted through the blade, which is "shadow". The intensity and spectral composition of the two parts of the light are different, and the effects on photosynthesis are also different. The function mainly depends on the spot portion. Therefore, when studying crop group photosynthesis, it is recommended to divide the vegetation into three parts: the full-light area (the spot part), the whole-yin area (shaded part) and the semi-negative area (between the two).
The range of radiation wavelengths that contribute to the growth and development of green plants is wider than that of photosynthetically active radiation, which is roughly in the range of 300 to 800 nanometers. This part of the radiation is called physiological radiation. It acts in addition to photosynthesis, but also on others. Physiological activities have an impact.
Metering systems and instruments:
The part of the solar radiation that can be used by photosynthesis by green plants is called photosynthetically active radiation, or PAR for short. It is the basic energy source for biomass, directly affecting plant growth, development, yield and product quality.
PAR has three metering systems:
1. Optical system, which is based on the response characteristics of the human eye to brightness. The instrument has a illuminometer, etc. The observed physical quantity is the luminous flux density of the visible light band emitted by the radiation source, and is measured by illuminance (lx). measure. ;
2, the energy system, the thermocouple as the sensor, the instrument for measuring the radiation amount from the ability angle has a sky radiation meter, a direct radiation meter, a net radiation meter, a spectroradiometer, and the like. The radiance flux density in a certain characteristic wavelength range, ie, the photosynthetically active band, is also measured as irradiance (Wm -2 );
3, quantum system, using silicon, selenium photocells as sensors, measuring the amount of radiation from the optical quantum angle of the instrument, there are light quantum flux meter. It is measured by the quantum flux density (umol m -2 s -1 ).
Photosynthetically active radiation can be measured directly by the instrument or by direct solar radiation. In order to obtain the proportional coefficient between the direct solar radiation and the scattered radiation and the photosynthetically active radiation, the solar radiation meter or the sky radiation meter and the photosynthetically active radiometer can be simultaneously observed to calculate the coefficient values ​​of the day, month, season and year and their mutual values. relationship. The Soviet Union 莫. Mordau et al. studied the quantitative concern of direct solar radiation (S) and diffuse radiation (D) and photosynthetically active radiation (Qp), listed the calculation formula and pointed out that at mid-high latitudes from April to September. When the height of the sun at noon is not less than 20°, the calculation error of the total amount or total amount of photosynthetically active radiation does not exceed 5%. Its calculation formula is: Qp=0.43S+0.57D
Relationship between photosynthesis and photosynthetically active radiation Under the condition that it is not restricted by other environmental factors (such as temperature, moisture, etc.), the photosynthesis of vegetation canopy generally increases with the increase of PAR, but since two leaves obtain appropriate light, one blade obtains glare. The photosynthesis of a leaf is stronger in the shadow, so the uniform distribution of PAR in the canopy is important. In sunny conditions, the canopy of strong light is prone to light saturation, and the utilization of light energy is reduced. However, although the utilization of light energy in the canopy part of the shadow is higher, the PAR obtained is less, resulting in the entire crown. The photosynthesis of the layer is weakened; in cloudy conditions, the scattering PAR from all directions of the sky increases, which can penetrate deeper in the canopy, thereby reducing the photosynthetic saturation point of the entire canopy, enhancing the utilization of the canopy light energy, and thus enhancing Canopy carbon absorption.
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