Factors Affecting Photosynthesis

_{Factors Affecting Photosynthesis}

Light And Photosynthesis

Light is an important factor as photosynthesis does not take place in dark. When considering effect of light on photosynthesis it is important to distinguish between the effects of light intensity and light quality.

1. Light Quality

The light consists of rays of different wavelengths. The study of absorption spectrum revealed that only red and blue light are effective in photosynthesis and green light is mostly reflected transmitted, therefore, not used in photosynthesis. Light of wavelength longer than 700 nm is not effective in photosynthesis for green plants.

1. Light Intensity

Photosynthesis begins at illuminations of very low intensity, reaches at maximum bright diffuse daylight and decreases in strong light. However, plants differ in their need for intensity of light., ferns or shade plants require less intensity of light as compared to trees.

At low light intensities, the light is a limiting factor and the rate of photosynthesis increases with an increase in light intensity until a maximum is reached. High light intensity affect the rate of photosynthesis adversely because temperature of the leaves is increased resulting in Transpiration, shortage of water in mesophyll cells and temporary closure of stomata.

Solarisation or Photoinhibition: Very strong light (high irradiance) damage the leaves. It is called solarisation or photo-inhibition. Actually, certain cell constituents, especially chlorophyll and cell enzymes are oxidized and carbon dioxide is released. Certain internal factors also contribute to solarisation.

Carbon Dioxide (CO₂) & Photosynthesis

Carbon dioxide is needed in the dark reactions where it is fixed into organic compounds. The chief source of carbon dioxide in land plants is the atmosphere where it is constituent the 0.03% of the atmosphere. This concentration is to be used by green plants in photosynthesis under normal conditions of light and temperature. Therefore, generally the carbon dioxide concentration act as a limiting factor in the photosynthesis. However, CO₂ concentration is atmosphere is maintained by carbon dioxide reservoirs: ocean water which stores CO₂ as carbonate and bicarbonate ions, combustion of fossil fuels and by bacteria found in soil and water which oxidizes organic matter to release carbon dioxide.

Other factors also limit the release of carbon dioxide into plants. These include the boundary layer surrounding the leaf, Stomata resistance and resistance offered by mesophyll cells to diffuse of liquid carbon dioxide. Similarly, shortage of water result in closure of stomata and intercellular carbon dioxide concentration decreases.

An increase in carbon dioxide concentration causes an increase in rate of photosynthesis provided the light into limiting factor. At room temperature and in bright light the carbon dioxide concentration can be increased in ten to thirty times form normal concentration, i.e., from 0.3% to 1%. However, beyond this limit CO₂ concentration becomes injurious. This had led to some greenhouse crops such as tomatoes, being grown in carbon dioxide rich atmospheres. Also, C-4 plants inhibit photo-respiration when carbon dioxide concentration is higher and stimulate photosynthesis, thus increasing the yield.

Temperature And photosynthesis

The dark reactions and to a certain extend to light reactions, are enzyme-controlled and affected by temperature. The temperature at which photosynthesis starts (minimum temperature) differ from species to species, for example plants growing in cold and temperate regions it may be -20°C (lichens) or 35°C (conifers) and those growing in tropical regions it is 5°C. the optimum temperature for tropical plants is usually about 25°C. the rate doubles for every 10°C rise up to 35°C. higher temperature denatures the enzymes or destroy the photosynthesis. In C-3 plants increase in temperature results in increase in rate of respiration that favors photo-respiration.

Water And Photosynthesis

Water is reactant (raw material) in photosynthesis but it affects so many other cell processes as well, therefore, it is impossible to measure the direct effect of the water on photosynthesis. However, by studying the yields of water-deficient plants, it can be shown that periods of temporary wilting can lead to severe yield losses. Even slight water deficiency might significantly reduce crop yields. The may be because plants usually close their stomata in response to wilting and entry of CO₂ necessary for photosynthesis is stopped. Also, it has been shown that abscisic acid (ABA) develop in response to water shortage that closes the stomata. Water deficiency, i.e., when water becomes limiting, causes retardation of leaf expansion, therefore, reduction in growth.

Mineral Nutrients

Mineral nutrients are the limiting factors. Many organisms settle to the bottom when they die, taking their mineral with them. Thus, the surface waters where light and CO₂ are most abundant become short of phosphates, nitrates and other essential nutrients affecting and decreasing the rate of photosynthesis. When special conditions bring the nutrients to the surfaces, increase in the rate of photosynthesis can be noticed in the form of profuse bloom of phytoplankton.

Leaf Age

As leaf grows, their ability to photosynthesize increases for a time and then, often even before maturity, begins slowly to decrease. Old leaves become yellow and are unable to photosynthesize due to breakdown of chlorophyll.