Category : 11th Class
Blackmann's law of limiting factors
F.F. Blackmann (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 is limited by the pace of the slowest factor'. is usually a limiting factor in photosynthesis under field conditions particularly on clear summer days under adequate water supply.
Blackmann's law of limiting factor is modification of Liebig'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 photosynthesis starts, an optimum value showing highest rate and a maximum value, above which photosynthesis fails to take place.
Factors : The rate of photosynthetic process is affected by several external (Environmental) and internal factors.
External factors
(1) Light : The ultimate source of light for photosynthesis in green plants is solar radiation, which moves in the form of electromagnetic waves. Out of the total solar energy reaching to the earth about 2% is used in photosynthesis and about 10% is used in other metabolic activities. Light varies in intensity, quality (Wavelength) and duration. The effect of light on photosynthesis can be studied under these three headings.
(i) Light intensity : The total light perceived by a plant depends on its general form (viz., height, size of leaves, etc.) and arrangement of leaves. Of the total light falling on a leaf, about 80% is absorbed, 10% is reflected and 10% is transmitted.
In general, rate of photosynthesis is more in intense light than diffused light. (Upto 10% light is utilized in sugarcane, i.e., Most efficient converter).
Another photosynthetic superstar of field growing plants is Oenothera claviformis (Winter evening-primrose), which utilizes about 8% light.
However, this light intensity varies from plant to plant, e.g., more in heliophytes (sun loving plants) and less in sciophytes (shade loving plants). For a complete plant, rate of photosynthesis increases with increase in light intensity, except very high light intensity where 'Solarization' phenomenon occurs, i.e., photo-oxidation of different cellular components including chlorophyll occurs.
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 uptake is called light saturation point.
(ii) Light quality : Photosynthetic pigments absorb visible part of the radiation i.e., to For example, chlorophyll absorbs blue and red light. Usually plants show high rate of photosynthesis in the blue and red light. Maximum photosynthesis has been observed in red light than in blue light. The green light has minimum effect. On the other hand, red algae shows maximum photosynthesis in green light and brown algae in blue light.
(iii) Duration of light : Longer duration of light period favours photosynthesis. Generally, if the plants get 10 to 12hrs light per day it favours good photosynthesis. Plants can actively exhibit photosynthesis under continuous light without being damaged. Rate of photosynthesis is independent of duration of light.
(2) 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 some conifers at high altitudes at – 35°C. Some algae in hot springs can undergo photosynthesis even at 75°C.
(3) Carbon dioxide : Carbon dioxide present in the atmosphere is about 0.032% by volume and it is really a low concentration which acts as limiting factor in nature. If we increase the amount of under laboratory conditions and if the light and temperature are not the limiting factors, the rate of photosynthesis increases. This increase is observed upto 1% of concentration. At the same time very high concentration of becomes toxic to plants and inhibits photosynthesis.
(4) Water : Water is an essential raw material in photosynthesis. This rarely, acts as a limiting factor because less than 1% of the water absorbed by a plant is used in photosynthesis. However, lowering of photosynthesis has been observed if the plants are inadequately supplied with water.
(5) Oxygen : Excess of may become inhibitory for the process. Enhanced supply of increases the rate of respiration simultaneously decreasing the rate of photosynthesis by the common intermediate substances. The concentration for oxygen in the atmosphere is about 21% by volume and it seldom fluctuates. O2 is not a limiting factor of photosynthesis. An increase in oxygen concentration decreases photosynthesis and the phenomenon is called Warburg effect. (Reported by German scientist Warburg (1920) in Chlorella algae).
This is due to competitive inhibition of RuBP-carboxylase by increased levels, i.e., competes for active sites of RuBP-carboxylase enzyme with CO2. The explanation of this problem lies in the phenomenon of photorespiration. If the amount of oxygen in the atmosphere decreases then photosynthesis will increase in cycle and no change in cycle.
(6) Pollutants and Inhibitors : The oxides of nitrogen and hydrocarbons present in smoke react to form peroxyacetyl nitrate (PAN) and ozone. PAN is known to inhibit Hill reaction. Diquat and Paraquat (Commonly called as Viologens) block the transfer of electrons between Q and PQ in PS. II. Other inhibitors of photosynthesis are monouron or CMU (Chlorophenyl dimethyl urea) diuron or DCMU (Dichlorophenyl dimethyl urea), bromocil and atrazine etc. Which have the same mechanism of action as that of viologens.
At low light intensities potassium cyanide appears to have no inhibiting effect on photosynthesis.
(7) Minerals : Presence of and is essential for smooth operation of light reactions (Photolysis of water/evolution of oxygen) and ions are important for synthesis of chlorophyll.
Internal factors
(1) Protoplasmic factors : There is some unknown factor which affects the rate of photosynthesis.
These factors affect the dark reactions. The decline in the rate of photosynthesis at temperature above 30°C or at strong light intensities in many plants suggests the enzymatic nature of this unknown factor.
(2) Chlorophyll content : Chlorophyll is an essential internal factor for photosynthesis. The amount of fixed by a gram of chlorophyll in an hour is called photosynthetic number or assimilation number. It is usually constant for a plant species but rarely it varies. The assimilation number of variegated variety of a species was found to be higher than the green leaves variety.
(3) Accumulation of end products : Accumulation of food in the chloroplasts reduces the rate of photosynthesis.
(4) Structure of leaves : The amount of that reaches the chloroplast depends on structural features of the leaves like the size, position and behaviour of the stomata and the amount of intercellular spaces. Some other characters like thickness of cuticle, epidermis, presence of epidermal hairs, amount of mesophyll tissue, etc., influence the intensity and quality of light reaching in the chloroplast.
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