Factors affecting photosynthesis

  1. External factors
    1. Light
      • It isthe most important factor of photosynthesis. Any kind of artificial light such as electric light can induce photosynthesis.
      • Out of the total solar energy, only 1-2 % is used for photosynthesis and the rest is used for other metabolic activities.
      • The effect of light on photosynthesis can be studied under three categories.
    1. Light intensity
      • Wolkoff (1966) found that the rate of photosynthesis is directly proportional to light intensity. But the extremely high light intensities do not favor for higher photosynthetic rates.
      • The high light intensity which fails to accelerate photosynthesis is called light saturation intensity.
      • Of the light falling on a leaf, about 80 per cent is absorbed, 10 per cent is reflected and 10 % is transmitted.
      • The rate of photosynthesis is greater in intense light than in diffused light.
      • The plants are grouped into two types on the basis of light requirement.
  1. Heliophytes (Sun plants)
  2. Sciophytes (Shade plants)
    • At a specific light intensity, the amount of CO2 used in photosynthesis and the amount of CO2 released in respiration are volumetrically equal.
    • This specific light intensity is known as light compensation point.
    • At very high light intensity, beyond a certain point, the photosynthetic cells exhibit photo oxidation. This phenomenon is called solarization and a result of this, inactivation of chlorophyll molecules, bleaching of chlorophyll molecules and even

      inactivation  of  some  enzymes  take  place  resulting  in  the destruction of whole photosynthetic apparatus.

    • In general, low light intensity favours stomatal closure and in turn reduced rate of photosynthesis.
    1. Light quality (wavelength)
    • Photosynthesis occurs only in the visible part of the light spectrum i.e., between 400 and 700 nm.
    • The maximum rate of photosynthesis occurs at red light followed by blue light.
    • The green light has minimum effect and photosynthesis cannot take place either in the infrared or in the ultraviolet light.
    1. Light duration
    • In general tropical plants get 10-12 hours of light per day and this longer period of light favours photosynthesis.
    1. Carbon dioxide
    • CO2 is one of the raw materials required for photosynthesis.
    • If the CO2 concentration is increased at optimum temperature and light intensity, the rate of photosynthesis increases.
    • But, it is also reported that very high concentration of CO2 is toxic to plants inhibiting photosynthesis.
    1. Temperature
    • The rate of photosynthesis increases by increase in temperature up to 40 ºC and after this, there is reduction in photosynthesis.
    • High temperature results in the denaturation of enzymes and thus, the dark reaction is affected.
    • The temperature requirement for optimum photosynthesis varies with the plant species.
    • For example, photosynthesis stops in many plants at 0 ºC but in some conifers, it can occur even at -35 ºC.
    • Similarly photosynthesis stops beyond 40-50 ºC in certain plants; but certain bacteria and blue green algae can perform photosynthesis even at 70 ºC.
    1. Water
    • Water has indirect effect on the rate of photosynthesis although it is one of the raw materials for the process.
    • The amount of water utilized in photosynthesis is quite small and even less than 1 per cent of the water absorbed by a plant.
    • Water rarely acts as a limiting factor for photosynthesis.
    • During water scarcity, the cells become flaccid and the rate of photosynthesis might go down.
    1. Oxygen
    • Oxygen is a byproduct of photosynthesis and an increase in the O2concentration in many plants results in a decrease in the rate of photosynthesis.
    • The phenomenon of inhibition of photosynthesis by o2 was first discovered by Warburg (1920) in green alga Chlorella and this effect is known as Warburg’s effect.
    • This is commonly observed in C3 plants. In plants, there is a close

    relationship between Warburg’s effect and photorespiration.

    • The substrate of photorespiration is glycolate and it is synthesized from some intermediates of Calvin’s cycle.
    • In plants that show Warburg’s effect, increased O2 concentration result in diversion of these intermediates of Calvin cycle into the synthesis of glycolate, thereby showing higher rate of photorespiration and lower photosynthetic productivity.
    1. Mineral elements
    • The elements like Mg, Fe, Cu, Cl, Mn, P etc are involved in the key reactions of photosynthesis and hence, the deficiency of any of these nutrients caused reduction in photosynthesis.
    1. Chlorophyll content
    • It is very much essential to tarp the light energy.
    • In 1929, Emerson found direct relationship between the chlorophyll content and rate of photosynthesis.
    • In general, the chlorophyll sufficient plants are green in colour showing efficient photosynthesis.
    • The chlorotic leaves due to irregular synthesis of chlorophyll or breakdown of chlorophyll pigment exhibit inefficient photosynthesis.
    1. Leaf
    • The leaf characters such as leaf size, chlorophyll content, number of stomata.
    • Leaf orientation and leaf age are some of the factors that are responsible for photosynthesis.
    • The maximum photosynthetic activity is usually seen in the physiologically functional and full size leaves (usually third/fourth leaf from the tip of the shoot system).
    1. Carbohydrates
    • If  the  accumulated  carbohydrates  are  not  translocated,  the photosynthetic rate is reduced and respiration is increased.
    • Sugar is converted into starch and gets accumulated in the chloroplasts.
    • This reduces the effective surface in the chloroplast and the rate of photosynthesis is decreased.
    1. Phytohormones
    • Treharne  (1970)  reported  first  that  photosynthesis  may  be regulated by plant hormone system.
    • He  found  that  gibberellic  acid  and  cytokinin  increase  the carboxylating activity and photosynthetic rates.
    • Meidner (1967) also reported that kinetin @ 3µm causes 12 per cent increase in photosynthesis within one hour of the treatment.
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