Factors affecting Photosynthesis:

The term rate always involve time, so the rate of photosynthesis can be considered to be how fast photosynthesis takes place. This can be measured by the amount of glucose produced by a plant over a given time. Many external and internal factors affect the rate of photosynthesis. There are following factors which affect the process of photosynthesis. Intensity of light, CO2 concentration in the air, Temperature and water are the important external factors that influence photosynthesis. Internal factors include chlorophyll content and the accumulation of the products of photosynthesis.

There are three main factors like light, temperature and CO2 are called limiting factors. In a process like photosynthesis which is affected by more than one factor, its rate is limited by the factor which is closest to its minimum value. So, at any point of time if one of the three factors is in low supply, this factor will be the limiting factor. Blackmann (1905) proposed law of limiting factor. Only a change to the limiting factor will increase or decrease the rate of photosynthesis.


         i.            Light: Light is essential for photosynthesis. Sun is the ultimate source of light energy on earth. There is a linear relationship between incident light and CO2 fixation rates at low intensities. At higher light intensities theret is no further increases as other factors become limiting. Hence, except for plants in shade or in dense forests, light is rarely a limiting factor in nature. Both quality and intensity of light are important for photosynthesis.

a)      Light Quality:  The light consists of rays of different wave lengths. Only red and blue light are effective for photosynthesis process. Light of wave length longer than 700nm is not effective for photosynthesis for green plants. According to Engelmann maximum photosynthesis occur in the red and blue part of the spectrum.

b)      Light Intensity: Photosynthesis begins at very low intensity. It becomes maximum at bright day light, but it decreases in strong light. Different plants require different intensity of light. Most of the light is reflected by the green leaves of plant only a small part of light is absorbed. Thus only about 0.5- 1.5% of light used in  photosynthesis. Light is not a limiting factor at high intensity but it is a limiting factor at high intensity. The rate of photosynthesis increases with increases in light intensity. High light intensity increases the temperature of leaves, therefore rate of transpiration increases. The stomata closed, it stops the entry of CO2. Thus photosynthesis gets stopped, it reduces yield.

       ii.            Carbon- Dioxide: Atmosphere contains 0.03%-0.04%  of the gas by volume. It is a very small amount, so CO2 remain a limiting factor. The increase in the amount of CO2 increases the photosynthesis. This increases is more rapid up to 1% of CO2 concentration, but it slows down beyond this point. Higher concentration of CO2 have an inhibitory effect on photosynthesis. With increase in CO2 concentration the yield of plant increases.

The C3 and C4 plants respond differently to CO2 concentrations. At low light conditions neither group responds to high CO2 conditions. At high light intensities, both C3 and C4 plants show increase in the rate of photosynthesis. The fact that C3 plants respond to higher CO2 concentration by showing increased rates of photosynthesis leading to higher productivity has been used for some green-house crops such as tomatoes and bell pepper. They are allowed to grow in CO2 enriched atmosphere that leads to higher yields.

      iii.            Temperature: A suitable temperature is necessary for photosynthesis. The dark reaction being enzymatic are temperature controlled. The C4 plants respond to higher temperature and show higher rate of photosynthesis while C3 plants have a much lower temperature optimums. There are three cardinals of temperature of photosynthesis.

a)      Minimum: It is minimum temperature at which the photosynthesis starts.  The plants of cold and temperate regions have lower values of these cardinals. But trophic plants have higher stability for these cardinals. Minimum temperature for many lichens is -20°C. It is -35°C for some conifers. Photosynthesis hardly starts at about 5°C in tropical plants. Desert plants like cactus can carry out photosynthesis even at 55°C.

b)      Optimum: Maximum photosynthesis occurs at optimum temperature. Photosynthesis increases with increase in temperature up to 25°C. This increases follow Vant Hoff's  Law. According to this law the rate of chemical reaction doubles for every rise of 10°C.

c)       Maximum: It is the highest temperature at which photosynthesis can occur. There is initial increase in the rate of photosynthesis at this temperature but this is soon followed by a decline. Higher the temperature more rapid is the decline. The decline is due to the following reason:

·         Accumulation of the end products of photosynthesis.

·         Inhibitory effect of high temperature on the activity of enzyme.

·         Failure of CO2 to diffuse rapidly.

·         Increased consumption of the photosynthate in photorespiration.

·         Destructive effect of high temperature on chlorophyll pigment.

     iv.            Water: It is one of the raw material for photosynthesis. The amount of water used in photosynthesis is very small. Less than 1% of water is absorbed by the plant. Therefore, it cannot be a limiting factor directly. But the water content of the leaf often act as a limiting factor indirectly, it maintain the turgor pressure of assimilatory cells. The rate of photosynthesis decreases in the cells which have lost their turgor. The loss of turgor of guard cells closed the stomata which reduces the rate of photosynthesis further.

Water stress causes the stomata to close hence reducing the CO2 availability. Besides, water stress also makes leaves wilt, thus reducing the surface area of leaves and their metabolic activity as well.

       v.            Oxygen: Photosynthesis does not take place in cells  which lack oxygen because energy produced in O2 respiration is necessary for photosynthesis and oxygen is required for the production and maintenance of some substances which are essential for photosynthesis process. High level of O2 inhibit the rate of photosynthesis and promotes photorespiration.

     vi.            Leaf Anatomy: The factors of leaf anatomy influences the diffusion of CO2 through the stomata. They also affect on the amount of light reaching the chlorenchyma. These factors are:

·         Different leaves have different thickness of cuticle and epidermis.

·         They develop palisade. They have different sizes and distribution of the intercellular spaces.

·         They have different sizes, positions and distribution of the stomata.

·         They develop different types of sclerenchyma and vascular tissues.


         i.            Chlorophyll content: Chlorophyll is essential pigment for the process of photosynthesis. Etiolated plants and non- green tissue do not show photosynthesis. Only the green cells produced starch in variegated leaves. There are two views about the effect of chlorophyll on photosynthesis process.

a)      Willslatter and Stroll: According to them the rate of photosynthesis is not proportional to the amount of chlorophyll content. The rate of photosynthesis depends on the concentration of enzymes.

b)      Emerson: In 1927, he found a direct relationship between the amount of food formed and the chlorophyll content.

       ii.            Protoplasmic Factor: There are some internal factors are present in the protoplasm of cells. These factors are called as unknown factors or protoplasmic factors. They are enzymatic in nature. Arnon in1954 has demonstrated that cells free chloroplasts are capable of carrying out photosynthesis. This indicates that protoplasm can do photosynthesis also.


 Importance of above factors can be illustrated with the help of following experiment:

        I.            Experiments to demonstrate that starch is formed during photosynthesis:

Pluck a healthy green leaf of a plant which was in  the sunlight. Place it in a beaker containing boiling water for about two minutes. Then transfer the leaf to a beaker containing alcohol. Warm it over a water bath for a few minute.

We will observe that the leaf turns to white, indicating that the chlorophyll has been removed. Now wash the leaf carefully in water without damaging it. Place the leaf to a dilute solution of iodine. This will turn the leaf bluish black. The changing of the colour of leaf to bluish black. The changing of the leaf's colour to bluish black after it has been treated with iodine solution shows that the leaf contains starch.

      II.            Experiment to demonstrate that CO2 as essential for photosynthesis:

Get two healthy potted plants of almost the same size and place them in the dark for about 24 hours to de-starch the leaves. Now place them on glass plates. Cover the plants with separate bell jar. Keep some crystals of KOH in a petri-dish and place it under one of the jars. Keep  the plants in sunlight for photosynthesis to take place. After 3-4 hour pluck a leaf from each plant. Boil the leaves in water and subsequently in alcohol, using a water bath to remove chlorophyll. Now use a few drops of iodine to test for starch in each leaf. Only one leaf i.e. blue black shows the presence of starch. This happens because KOH absorbs the CO2  present inside one bell jar. As a result, the leaves do not get CO2 for photosynthesis. Thus the process of photosynthesis is inhibited and starch is not synthesized.

    III.            Experiment to show that sunlight is essential for photosynthesis:

Keep a potted plant in the dark for about 24 hours. On one of the leaves, stick black paper strips (one below and one above the leaf). Now, place the plant in sunlight for a few hours. Pluck the leaf to remove black strips. Boil the leaf, first in water and then in alcohol, to remove chlorophyll.  After washing the leaf with water, keep it in a petridish. Add a few drops of iodine solution. The leaf turns blue black except in the region that has been converted. This region did not receive light and hence no starch was formed. The uncovered region received light and starch was formed due to photosynthesis. Plants take up different nutrients like N2, P, Fe, Mg etc. along with water through the root. These nutrients contribute not only to the process of photosynthesis but also to the general development of the plants. For example, nitrogen is need for the synthesis of protein and other compounds.

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