Factors affecting rate of Photosynthesis

Factors affecting rate of Photosynthesis:

It is the process of synthesis of food by green plants in presence of sunlight. This is the only process by which solar energy is trapped in food molecules and during the process oxygen is also evolved. The survival of all types of organisms depends upon this process only.

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It is the most important anabolic process by which green plants synthesize complex carbohydrates from simple substances like carbon dioxide and water with the help of light energy and purify the atmosphere by consuming CO₂ and evolving O₂.

The rate of photosynthesis is controlled by several external and internal factors. These are as follows-

External Factors affecting rate of photosynthesis:

(a) Light-Light comes in the form of solar radiations from the sun and works as a source of energy. Out of the total light reaching the earth, only 2% is used in photosynthesis. The effect of light on photosynthesis can be studied under the following headings-

• Light Intensity- Under low light intensity the rate of photosynthesis is less. An increase in light intensity causes an increase in the rate of photosynthesis. Very high light intensities are not good for photosynthesis as there takes place photooxidation of photosynthesising cells. This phenomenon is known as polarization. The requirement of light intensities by the plants varies. For example- certain plants require low light intensity for optimum photosynthesis and low shady habits. Thes plants are called sciophytes. On the other hand, certain plants love intense light and require high light intensity for optimum photosynthesis. Such plants are called heliophytes.
• Light Quality-Maximum photosynthesis occurs when whole light is available to the plant. Most plants achieve a higher rate of photosynthesis in blue and red wavelengths of light. Minimum photosynthesis takes place in a green part of the spectrum. However, shade plants mostly depend upon green light and a small amount of other wavelengths for their photosynthesis. Red light stimulates the synthesis of more carbohydrates while blue light is favourable to protein synthesis.
• Duration of Light- It is assumed that a greater quantity of photosynthesis will take place in a plant exposed to longer periods of light. In general, a plant carries on much photosynthesis if it is provided with light for 10 to 12 hours a day. Bohning (1949) found that the leaves on young apple trees exposed to a continuous illumination and the usual atmospheric concentration of CO₂ photosynthesize at an undiminished rate of periods of at least 18 days without any noticeable damaging effect on the plant.

(b) Concentration of CO₂- It acts as a natural limiting factor as its concentration remains low (about 0.03% by volume). With an optimum increase in the concentration of CO₂, the rate of photosynthesis increases subsequently but beyond this, the excess amount of CO₂ becomes toxic for plants and lowers the rate of photosynthesis. The CO₂ is usually the limiting factor in photosynthesis on clear summer days when plants are provided with adequate water. 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 an increase in the rates of photosynthesis.

(c) Temperature- The optimum temperature for photosynthesis is 20 to 35°C. If the temperature is increased too high, the rate of photosynthesis is also reduced by the time factor which is due to the denaturation of enzymes involved in the process. Photosynthesis occurs in conifers at high altitudes at 35°C. Some algae in hot springs can undergo photosynthesis even at 75°C.

(d) Oxygen- In most plants, an increase in the oxygen concentration results in a decrease in the rate of photosynthesis. The normal atmospheric concentration of oxygen is high enough to induce a lower rate of photosynthesis than obtains at a lower concentration of oxygen. This is demonstrated by the work of McAlister and Myers (1940) showing the effect of high and low concentrations of oxygen on the rate of photosynthesis of wheat plants. The explanation of this seems to be that oxygen actually exerts as direct inhibitory effect on photosynthesis, and the greater the concentration of oxygen the greater this effect.

Contrary to the above statement the energy product in oxygen respiration is necessary for photosynthesis. Even a small amount of this energy may supplement light energy.

(e) Water- Water keeps the mesophyll cells in turgid conditions and plays a significant role in the opening and closing of stomata. In addition to this, a decrease in the availability of water leads to dehydration of protoplasm and ultimately affects the efficiency of photosynthetic enzymes.

(f) Mineral Elements- A number of mineral elements are essential for photosynthesis. They are Mg (a component of chlorophyll), Mn, Cl and Ca (for photolysis of water), Fe and Cu (for the synthesis of chlorophyll), P (ATP synthesis), K, S etc. (activities of enzymes). Reduced availability of minerals will naturally decrease the rate of photosynthesis. Minerals also become toxic when present in excess.

Internal Factors affecting rate of photosynthesis:

These include chlorophyll content, protoplasmic factor and accumulation of carbohydrates. The chlorophyll is an important internal factor for photosynthesis. There is a direct relationship between the chlorophyll content and photosynthesis. Photosynthesis depends not only on chlorophyll content but also on certain protoplasmic factors. Accumulation of carbohydrates in plant cells inhibits the rate of photosynthesis. As the leaf grows in age its rate of photosynthesis falls down. As the leaf grows in size its photosynthesis increases till it becomes maximum in size. The C₄ plants having kranz anatomy are photosynthetically more efficient than C₃ plants. The number of stomata also influence the rate of photosynthesis.

Law of Limiting Factors:

F.F.Blackman proposed the principle of limiting factors. It states that “when a process is conditioned as to its rapidly by a number of separate factors, the rate of the process is limited by the pace of the slowest factor”. The slowest factor means a factor, which is present in the quantity or intensity that is lesser than what is required for the process.