The importance of light in plant life
Green plants are the only living organisms on Earth capable of converting the light energy of sunlight into chemical energy of organic compounds. This process occurs due to chlorophyll contained in the leaves (and other green parts) of plants and is called photosynthesis.
K.A. Timiryazev established: photosynthesis is mainly a process of binding and storing solar energy. According to calculations, 16752 kJ of energy must be accumulated in 1 kg of dry organic matter. Given the amount of plant biomass on Earth, the amount of bound and stored energy in the form of organic matter is enormous.
The process of photosynthesis is the interaction of carbon dioxide and water under the influence of chlorophyll molecule and light quanta to form glucose and oxygen molecules. The chemical equation for the reaction can be represented as follows:
6CO2 + 6H2O + 2822 kJ -(light, chlorophyll)→ C6H12O6 + 6O2
Participating in other biochemical reactions, glucose is transformed into more complex sugars (starch, cellulose, lignin, etc.) and organic compounds. The rate of the process is equal to the formation of 1 gram of organic matter on the total leaf surface of 1 m2 in 1 hour. In this case, the amount of air containing carbon dioxide and passed through the stomata of leaves should be 2 m3. For estimation: per 1 m2 of winter wheat crops leaf surface area is 17-20 m2, corn, beets, potatoes – 3-8; clover and alfalfa – 24-37 m2.
Light has a significant impact on product quality. In hay and grain, the lack of light reduces the amount of protein, sugar beet reduces sugar content, potato reduces starchiness, and sunflower reduces oil content.
Lack of light leads to pale coloring of leaves and their weak development, thinning and stretching of stems, flowering and fruiting are delayed or do not occur. Grain crops show reduction of bushiness, leaves become narrow, bush node is formed near the very surface, stems are elongated, internodes are weakened, grain does not mature or is of low quality, lodging of plants is possible.
Photoperiodism is a property of plants to respond to the duration of light during the day. Photoperiodic response determines the onset of growth and development phases. Depending on the light duration, plants are divided into long-day plants with light duration of at least 12 hours, short-day plants with light duration of less than 12 hours, and neutral-day plants.
Short-day plants include plants from the subtropics and tropics: corn, rice, millet, soybeans, beans, cotton, and others. Lengthening the daylight period lengthens the growing season.
Long-day plants mostly come from temperate latitudes: wheat, rye, oats, barley, long-fibred flax, peas, vetch, mustard. They require 16 to 18 hours of daylight for normal vegetation.
Solar energy affects soil fertility. It has been experimentally established that barley yields on soil irradiated by sunlight are higher than on soil in the dark, which is due to the large amount of nutrients in the upper layers.
Under the influence of sunlight changes the activity of microbiological, biological, enzyme, nitrifying processes and enhances humus oxidation. Sunlight is a factor of soil fertility, but there is not enough scientific data in this direction.
Regularities of the light regime
K.A. Timiryazev said that a ray of light that does not fall on a plant is forever lost to mankind.
The Physiologically Active Radiation Utilization Ratio (PAR) is the ratio of light energy converted into chemical energy by the plant during photosynthesis to the light energy delivered to the leaf surface.
For example, PAR for grain crops is 2.6%, for flax – 3.6, for potatoes – 2.4, for root crops – 1.9, for lupine – 4.8%. On average, the share of accumulated light energy for cultivated plants is 1.5-2.5% (0.5-3%).
The amount of solar energy received by plants depends on the duration of daylight hours, the latitude of the area, the exposure of slopes, weather conditions and atmospheric dustiness.
The daily cycle of photosynthetic activity is directly related to the intensity of sunlight: it begins in the morning hours, increases by 12-14 p.m., and then weakens. No photosynthesis occurs in the dark hours of the day.
The efficiency of photosynthesis depends on the spectral composition, size and shape of the leaf surface.
Regulating the light regime
As a rule, the regulation of light regime is closely related to the regulation of the thermal regime of the soil and can be aimed at increasing, for example, in the northern regions, or at reducing in the southern.
Regulation of the light regime of agricultural plants is carried out by means of agricultural techniques:
- Seeding rate – affects the density of stems, and therefore the availability of light energy to the plants.
- The direction of sowing depending on the sides of the light. Cereals planted from north to south showed a yield by 0.2-0.3 t/ha higher than those planted from east to west, due to better lighting in the morning and evening hours and mutual shading of each other in the afternoon.
- Seeding method. A more even distribution of plants over an area allows the plants to be optimally supplied with light.
- Place of sowing – placement of light-loving plants on the southern slopes.
- The timing of sowing – early sowing, as a rule, allows increasing photosynthetic activity and increasing the yield. Delayed sowing leads to a decrease in the accumulation of organic matter and a decrease in yield.
- Weed control.
- Mixed crops of shade-tolerant and light-loving plants. Allows a more efficient use of solar energy per unit of soil area.
The use of agrotechnical methods allows increasing the use of physiologically active radiation by 2 times.
The use of intermediate crops of winter crops, single-cut (after mowing the grass), stubbles and subordinate (undersowing) after harvesting the main crop allows you to receive additional yields of grain or green mass from the same area, which, among other benefits, allows additional use of light energy.
A promising direction of agricultural science is the development of varieties with a higher solar energy utilization factor.
Agrochemistry. Textbook / V.G. Mineev, V.G. Sychev, G.P. Gamzikov et al. – M.: Publishing house of the All-Russian Scientific and Research Institute named after D.N. Pryanishnikov, 2017. – 854 с.
Plant breeding/P.P. Vavilov, V.V. Gritsenko, V.S. Kuznetsov et al; Edited by P.P. Vavilov. – M.: Agropromizdat, 1986. – 512 p.: ill. – (Textbook and textbooks for higher education institutions).