Plant Photosynthesis

Blackman’s law of limiting factors

Blackman (1905) proposed the law of limiting factors according to which when process is conditioned to its rapidity by a number of factors, the rate of process In limited by the pace of the slowest factor. Blackman’s law of limiting factor is modification of Leibig’s law of minimum, which states that rate of process controlled by several factors is only as rapid as the slowest factor permits. Theory of three cardinal points was given by Sachs in 1860. According to this concept, There is minimum, optimum and maximum for each factor For every factor, there is a minimum value when no photosynthesis occurs, an optimum value showing highest rate and a maximum value,above which photosynthesis fails to take place.

External factors

1. Light

In photosynthesis light is converted to chemical energy in the food formed. It can be studied under three headings (i) Light intensity (ii) Light quality and (iii) Light duration (i)

(i) Light intensity. Light intensity required to get the optimum value differs with different species. Usually with increase in light intensity, increase in rate is noticed Clouds, fog, dust and atmospheric humidity reduce the intensity. Submerged aquatic plants also get less light intensity. At very high light intensity the cells exhibit photo-oxidation by the process of solarization and if continues for few hours, the photosynthetic apparatus is destroyed. Plants which are able to grow in shades are called as sciophytes while those growing in intense light are called heliophytes. It also affects the opening and closing of stomata thereby affecting the gaseous exchange. The value of light saturation at which further increase is not accompanied by an increase in CO2 uptake is called light saturation point.

(ii) Light quality. Blue and red light of the spectrum is said to be the best light for the photosynthesis. The green light has inhibitory effect. On the other hand, plants growing in deep water absorb green light.

(iii) Light duration. Plants getting average light of 10-12 hours a day show higher rate of photosynthesis. Apple trees were found to carry on photosynthesis in continuous light for eighteen days (Bohning 1949).

2. Carbon dioxide

Carbon dioxide is present in low concentration and forms about 0.032% of the total atmosphere. Increased concentration of CO2 with other factors not becoming limiting rate of the process enhances. However, very high concentration of CO2 becomes toxic to the plants. It is doubtful whether CO2 is a limiting factor under field conditions.

3. Water

Water deficiency may decrease the rate as it is one of the raw materials for the process. Less availability of water may further check the rate by closing the stomata thereby affecting the entry of CO2.

4. 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 time factor which is due to 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.

5. Oxygen

Excess of O2 may become inhibitory for the process. Enhanced supply of O2 increases the rate of respiration simultaneously decreasing the rate of photosynthesis by the common intermediate substances. The concentration of oxygen in the atmosphere is about 21% by volume and it seldom fluctuates. An increase in oxygen concentration decreases photosynthesis and the phenomenon is called Warburg effect. The explanation of this problem lies in the phenomenon of photorespiration.
Oxygen may compete with CO2 for hydrogen and may be reduced in place of CO2

6. Osmotic relations

Availability of water is affected indirectly with respect to osmotic relations of the plants. Internal factors

1. Protoplasmic factors: There is some unknown protoplasmic factor which affects the rate of photosynthesis. It takes some time to initiate the process in seedlings even if the chlorophyll has appeared. Same is true, if the plant is shifted to light from prolonged darkness.

2. Chlorophyll contents: Quantity of chlorophyll seems to affect the process. In variegated leaves and green leaves, assimilation per unit leaf area has been found to be the same provided other factors are not limiting. The amount of CO2 fixed by a gram chlorophyll in an hour is called as photosynthetic number or assimilation number.

3. Accumulation of products: Accumulation of photosynthetic products, if not consumed or translocated results in stoppage of process gradually.

4. Structure of leaves: Characters like structure, position and distribution of stomata, intercellular spaces, vascular tissues have been noticed to affect the process directly.