Factors affecting Photosynthesis

Photosynthesis occurs under the influence of many factors, both internal and external.

When several factors affect any (bio)chemical process, Blackman's (1905) law of limiting factors comes into effect.

Blackman's Law of Limiting Factors

According to this law "When a process is conditioned as to its rapidity by a number of separate factors, the rate of the process is limited by the pace of the slowest factor."

In other words. "If more than one factors affect a chemical process then its rate will be determined by the factor which is nearest to its minimal value".

It is the factor which directly affects the rate if its quantity is changed.

A limiting factor is that factor which is deficient to such an extent that increase in its value directly increases the rate of the process.

A. External factors affecting photosynthesis

(1) Light :

It is an essential factor for photosynthesis. It affects the rate of photosynthesis in three ways.

I. Light Intensity: It affects the rate of photosynthesis. Optimum value of light intensity is different for different plants.

There are two categories of plants :

(a) Sciophytes : They grow under diffuse light, e.g., Oxalis etc.

(b) Heliophytes: They grow in direct sunlight, e.g., Dalbergia etc.

(i) The optimum light intensity for sciophytes is 10% of full summer sunlight.

(ii) The-optimum light intensity for heliophytes is 50-70% of full summer sunlight.

(iii) Optimum for C4 plants is more than full summer sunlight. e.g., Maize, sugarcane.Optimum light in ensity is also called saturation point.

Figure: Effect of light intensity on photosynthesis of C4 and C3 plants
Figure: Graph of light intensity on the rate of photosynthesis

(iv) As the light intensity decreases rate of photosynthesis also decreases. The point in light intensity where there is no apparent exchange of gases between the environment and the photosynthetic organ is called light compensation point. Increase in light intensity from compensation point to saturation point increases the rate of photosynthesis. Light intensity above the saturation point decreases the rate of photosynthesis, this effect is called solarisation. This includes photoinhibition and photo-oxidation.

II. Light quality : Light between 400-700nm wavelength constitute the photosynthetically active radiation or PAR. Maximum photosynthesis takes place when whole white light is available to the plant. Maximum photosynthesis takes place in red and blue light and minimum photosynthesis takes place in green light.

III. Duration of light: Light duration does not affect the rate of photosynthesis, but it affects the total photosynthesis.

(2) Carbon dioxide:

Carbon dioxide concentration is the major factor influencing the rate of photosynthesis.

The CO2 concentration is very low in the atmosphere (between 0.03 and 0.04 percent).

This level of CO2 is far below the requirement for optimum photosynthesis.

Thus, the rate of photosynthesis could be increased several times by increasing CO2 concentration to about 0.05%.

At current level of CO2 concentration the rate of photosynthesis is optimum for C4 plants and lower for C3 plants.

If CO2 concentration increases then only C3 plant would be benefitted. A stage in CO2 concentration when there is no net absorption of CO2 by illuminated plant organ is called CO2 compensation point. At this point, photosynthesis rate is equal to respiration rate. CO2 compensation point for C3 plant is 25-100 ppm and for C4 plant is 0 -10 ppm.

Figure: Photosynthetic response of C3 and C4 plants to CO concentration

C4 plants show saturation at about 360 ppm, while C3 plants shows this beyond 450 ppm. Thus current CO2 concentration is limiting for C3 plants.

Very high CO2 concentration causes the stomata to close. This inhibits exchange of gases and as a result the photosynthetic rate decreases.

(3) Temperature:

The dark reactions are controlled enzymatically in stroma, hence are temperature regulated.

Optimum value of temperature for photosynthesis in C3 plant is 20-25°C and for C3 plants is 30º–45ºC) The minimum temperature at which most plant starts photosynthesis is 0-5°C. It is however as low as –35°C for some gymnosperms.

(i) Maximum temperature at which photosynthesis can occur is 50-55°C for desert plants (Opuntia) and 70-75°C for hot spring algae.

(ii) Tropical plants have a higher temperature optimum than the plants of temperate climate.

(iii) In C4 plants, pyruvate phosphate dikinase enzyme is sensitive to low temperature, while in C3 plants affinity of RuBisCO for CO2 decreases at high temperature.

(iv) Q10 values for photosynthesis is 2.

(4) Water:

Photosynthetic process utilizes less than 1% of the water absorbed by a plant, hence, it is rarely a limiting factor in photosynthesis.

But water scarcity affects photosynthesis indirectly.

It has been observed in several experiments that the rate of photosynthesis decreases drastically if water supply is withheld for sometime.

Water stress can cause stomatal closing, wilting of leaves and reduced metabolic activities in leave.(5) Oxygen.

Increase in O2 concentration decreases the rate of photosynthesis in C3 plants.

This phenomenon of the reduction of photosynthesis by O2 was first discovered by Warburg using an alga called Chlorella and is called as Warburg's effect.Oxygen is inhibitory because:

(i) It favours photorespiration.

(ii) It destroys the activated state of chlorophyll.

(6) Mineral nutrient.

Few mineral elements are directly involved in photosynthesis. Mg is a component of chlorophyll. Mn, Ca and Cl are essential for photolysis of water.

B. Internal factors affecting photosynthesis

Photosynthesis is under the influence of several internal (plant) factors.

The plant factors include the number, size, age and orientation of leaves, mesophyll cells and chloroplasts, internal CO2 concentration and the amount of chlorophyll.

The plant or internal factors are dependent on the genetic predisposition and the growth of the plant.

(1) Chlorophyll.

Of the internal factors, chlorophyll is the most important because light energy is trapped by only this substance.

There is no photosynthesis in the absence of chlorophyll.

The non green parts of variegated leaves (e.g., Croton), therefore, do not have starch.

Photosynthetic number or assimilation number shows a relationship between the chlorophyll and photosynthesis.

It is the amount of carbon dioxide (in gms) assimilated by one gram of chlorophyll in an hour.

Emerson (1929) observed a direct relationship between the chlorophyll content of a leaf and the rate of photosynthesis.

If all other factors are favourable, increased chlorophyll leads to an increase in photosynthesis.

(2) Photosynthetic products.

With the accumulation of the end products of photosynthesis in mesophyll cells, there is decrease in their photosynthetic rate because concentration of these products in the cells increases the rate of respiration.

Concept Builder

1.'Photosynthesis is an anabolic, endergonic; oxidation-reduction process.

2.Destruction of chlorophyll due to, high light intensity is called solarisation.

3.In C3 plants, more CO2 is released in light than in dark due to photoresprration.

4.Maximum absorption peak of, chlorophyll a and chlorophyll b is in blue light.

5.Chloride ions stimulate quick release of electrons from water.

6.Pheophytin is chlorophyll a ,in which Mg++ is replaced by two hydrogens.

7.Atriplex hastata shows Calvin cycle whereas Atriplex rose us shows Hatch and Slack cycle.

8.The reaction centre in bacteria is B-890 and reducing agent is NADH2, in bacterial photosynthesis, no photolysis of H2O occur, therefore phothesynthesis is an oxygenic and cyclic ETS dominates.

9.Moll's half leaf experiment is used to show that carbon dioxide is essential for photosynthesis.

10.Wilmott's bubbler is used to determine the rate of photosynthesis by counting the O2 bubbles.