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.

External Factors for Blackman’s law

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.

(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

5. Oxygen

Excess of O2 may become inhibitory for the process. 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.

6. CO2 Concentration

• CO2 is essential for the reduction of ribulose bisphosphate during the Calvin cycle for the production of carbohydrates.
• As long as there are no other limiting factors an increase in CO2 concentration up to about 0.05% will increase photosynthetic rate.
• At this level photosynthetic rate reaches plateau.
• Concentrations above 0.1% can damage leaves.
• Optimum concentration of CO2 is therefore just below 0.1%
• In dense, warm, well lit vegetation areas low levels of CO2 limit photosynthesis.

7. 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.