WHAT ARE THE FACTORS AFFECTING PHOTOSYNTHESIS ? EXPLAIN BLACKMAN’S LAW OF LIMITING FACTOR

Factors affecting Photosynthesis

Photosynthesis is affected by several external and internal factors.

External factors affecting Photosynthesis

1. Light

Three characteristics of light

(i) intensity (quantity)

(ii) quality (wave length) and ,

(iii) duration

significantly affect the rate of photosynthesis.

Intensity of Light

The effect of light intensity depends upon the photophilous (sun-loving plants) or sciophilous (shade-loving plants) nature of plant.  Photophilous plants responds favourably to higher light intensities than sciophilous plants.

At low light intensity, the rate of photosynthesis is reduced.  There is a point in light intensity where there is no gaseous exchange in photosynthesis.  It is called, “light compensation point”.

A plant cannot survive for long at compensation point because there is a net loss of organic matter due to respiration of non green organs and respiration in dark.

As the light increases, the rate of photosynthesis also increases.  The light intensity at which a plant can achieve maximum amount of photosynthesis is called “saturation point.”  Beyond saturation point, the rate of photosynthesis begins to decline.  The phenomenon is called “solarisation.”  It is due to two reasons (a) photo inhibition due to reduction in hydration and closure of stomata.  (b) photo-oxidation or oxidation of photosynthetic pigments and enzymes.

Quality of light

Photosynthesis takes place only in the wave length of visible spectrum (390-760 nm).  The maximum absorption occurs in blue and red regions of the spectrum.  However, the maximum rate of photosynthesis is observed in orange-red light (600-700 nm) and second maximum in the blue-violet light (430-470 nm).  As green light is reflected by the chlorophyll molecules, it is not used in photosynthesis, hence a very low rate of photosynthesis is observed in yellow and green light (470-600 nm).

Duration of Light

Photosynthesis can occur in continuous illumination without any harm to the plant.

2. Temperature

Temperature in the range of 10-35 °C is optimum for photosynthesis. According to Vant Hoff’s law, the rate of photosynthesis increases with increase in temperature within this range.  When temperature increases from minimum to optimum, the rate of photosynthesis doubles for every 10°C rise in temperature.  Above the optimum temperature, the rate of photosynthesis shows an initial increase for short duration but later declines. This decline with time is called time factor.

The effect of temperature varies with the habitat of plants .The plants of cold climate carry on photosynthesis at much lower temperature than those of warm climates.

In the evergreen species of cold regions , photosynthesis is known to occur below 0°C.  On the other hand, algae in hot water springs may carry on photosynthesis at a temperature of 75 °C.

High temperature affects the activity of enzyme, and therefore, the rate of photosynthesis decreases.  If the temperature is too high, the enzyme becomes denatured and it stops photosynthesis.

 

3. Carbon Dioxide

Increase in carbon dioxide increases the rate of photosynthesis in most C3 plants. When, carbon dioxide concentration is reduced, there comes a point at which illuminated plant parts stop absorbing carbon dioxide from their environment.  It is known as “carbon dioxide compensation point.”

A very high carbon dioxide concentration causes the stomata to close.  This inhibits exchange of gases and as a result, the photosynthetic rate decreases.

4. Oxygen

C3 plants show optimum photosynthesis at low oxygen concentration. At a very high oxygen concentration, the rate of photosynthesis begins to decline in all the plants.  The phenomenon is called “Warburg effect.” 

5. Water

The rate of photosynthesis in wilted leaves is very less. 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.  The rate of photosynthesis decreases drastically if water supply is withheld for some time.  The inhibitory effect is due to dehydration of protoplasm, closure of stomata, change in organisation of enzyme systems and inhibition of NADPH2.

Internal Factors

(1)  chlorophyll –It is the most important internal factor for photosynthesis.  Light energy is trapped by chlorophyll.  No photosynthesis can take place in the absence of chlorophyll.  Emerson (1929) observed a direct relationship between chlorophyll content of the leaf and the rate of photosynthesis.

(2) protoplasmic factor Certain factors which affect the rate of photosynthesis are known to be present in the protoplasm.

(3)  photosynthetic products –When sugar is manufactured at a more rapid rate, it accumulates, in the mesophyll cells.  If the accumulation of products takes place beyond a limit, photosynthesis stops.

(4)  internal structure of leaf – Diffusion of carbon dioxide in the leaf is affected by its structural features.  These include size of stomata, their position, amount of intercellular spaces, thickness of epidermis and cuticles.  Leaves with Kranz anatomy are more efficient in photosynthesis.

 

BLACKMAN’S LAW OF LIMITING FACTOR

F. F. Blackman (1905) stated that “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.” This law is known Blackman’s law of limiting factor. It can be explained by the following example.

Suppose a leaf is subjected to a light intensity, sufficient for the assimilation of 5 mg carbon dioxide per hour.  Under these conditions, if a plant is supplied with only one 1 mg of carbon dioxide, the rate of photosynthesis would be limited by carbon dioxide (because concentration of carbon dioxide is less than what can be assimilated in the available intensity of light).

Now if the carbon dioxide concentration is gradually increased, the rate of photosynthesis will also increase correspondingly until the assimilation rate of 5 mg carbon dioxide per hour has been reached.

However, further increase in carbon dioxide concentration, i.e., more than 5 mg per hour will not increase the rate of photosynthesis.  This is because the light now becomes the limiting factor.  Under these circumstances, any further increase in the rate of photosynthesis can be brought about only if the light intensity is increased.

Figure showing Blackman’s law of liming factor

The above experiment shows that of all the factors, the photosynthetic rate depends upon the factor which is present in relatively minimum concentration.  This factor is known as “limiting factor.”  All other factors present in optimum or maximum concentration will not be able to increase the rate of photosynthesis until the amount of limiting factor is increased correspondingly.