Factors Affecting Photosynthesis
Some of the factors such as light, water, carbondioxide, temperature and chlorophyll control the process of photosynthesis. In the absence of any one of these factors operation of the process is not at all possible. Hence these are absolute requirements and they greatly influence the photosynthetic rate. F.F, Blackman in 1905 proposed the ”Law of Limiting Factors” to explain the nature of operation of these factors. Following are the external and internal factors affecting Photosynthesis.
Three characteristics of light (i) intensity (quantity). (ii) quality (wave length) and (iii) duration, significantly affect the rate of photosynthesis.
(i) 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.
There is a reduction in the rate of photosynthesis at low light intensity . There is a point in light intensity where there is no gaseous exchange in photosynthesis. It is termed as, “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 termed as“saturation point.” Beyond saturation point, the rate of photosynthesis begins to decline. The phenomenon is termed as “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.
(ii) 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 in orange-red light (600-700 nm) and second maximum in the blue-violet light (430-470 nm). As the chlorophyll molecules reflect green light , it is not used in photosynthesis, hence a very low rate of photosynthesis is observed in yellow and green light (470-600 nm).
(iii) Duration of light: Photosynthesis can occur in continuous illumination without any harm to the plant.
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 termed as 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 occurs 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.
Increase in carbon dioxide increases the rate of photosynthesis in most C3 plants. When there is a reduction in the carbon dioxide concentration, there comes a point at which illuminated plant parts stop absorbing carbon dioxide from their environment. It is termed 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.
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 termed as “Warburg effect.”
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.
It is the most important internal factor for photosynthesis. Chlorophyll trap the light energy. 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. Chlorophyll plays a vital role in the light reaction as it is capable of absorbing radiant energy of sunlight and converting it into chemical energy.
This chemical energy in the presence of enzymes help to carry out various reactions of dark phase of photosynthesis leading to the synthesis of carbohydrates. Quality and proportion of chlorophyll a and chlorophyll b present in leaves also influence the photosynthetic efficiency of plants.
Age of leaf:
As a leaf develops, the rate of photosynthesis rises with the age till it becomes maximum at full maturity. Afterwards the rate of photosynthesis begins to decline . This is because ageing or senescence brings about deactivation of enzymes and degeneration of chlorophyll.
Certain protoplasmic factors such as quantity of photosynthetic enzymes also influence the rate of photosynthesis. Such factors mostly affect the dark reaction. Plants lose the capacity for photosynthesis at temperature above 30 degree centigrade or at strong light intensities even though all other conditions are normal.
When sugar is manufactured at a more rapid rate, it accumulates, in the mesophyll cells. Concentration of these products in the cells increases the rate of respiration. If the accumulation of products takes place beyond a limit, photosynthesis stops.
Internal structure of leaf
Structural features of the leaf affect the diffusion of carbon dioxide. 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. 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 termed as 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 there is an increase in the light intensity.
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.