Factors Influencing Photosynthesis
The factors influencing rate of photosynthesis can be classified into two categories,
A. Internal Factors
1. Chlorophyll
The amount of chlorophyll present has a direct relationship with the rate of photosynthesis, since, it is the pigment, which is photoreceptive
and is directly involved in trapping the light energy. 2. Photosynthetic Enzyme Systems
- The amount and nature of enzymes play a direct role on the rate of photosynthesis.
- Greater enzyme activity at higher light intensity increases the capacity of the leaf to absorb more light and thus increases the photosynthetic rate.
3. Leaf Resistance
- Photosynthesis shows close dependence upon leaf resistance.
- For C4 plants, leaf resistance (primarily controlled by stomatal aperture) appears to regulate photosynthesis, but in C3 plants, the internal resistances including carboxylation efficiency offer greater limitation to CO2 fixation than stomatal resistance.
- Environmental factors such as light intensity, photoperiod, CO2 concentration, humidity and soil moisture also affect photosynthesis via stomatal resistance.
4. Demand for Photosynthate
- Because of the greater demand, the rapidly growing plants show increased rate of photosynthesis in comparison to the mature plants.
- However, if the demand for photosynthesis is lowered by removal of meristem, then the photosynthetic rate declines.
5. Leaf Age
The Photosynthetic rate is higher in the newly expanding leaves and reaches a maximum as the leaves achieve full size.
The rate declines as the leaf ages due to reduced chloroplast functions and other anabolic reactions.
B. External Factors
1. Carbon dioxide
- CO2 is one of the raw materials for photosynthesis; therefore, its concentration affects the rate of photosynthesis markedly.
- Rate of photosynthesis increases with the increase in the atmospheric CO2 concentration up to a certain extent.
- Because of its very low concentration in atmosphere (current level of 350-360ppm), it acts as a limiting factor in natural photosynthesis.
- Rate of photosynthesis increases with increase in the atm. CO2 level of upto 1000ppm beyond which, there is a general decline in photosynthesis.
- At this enhanced level of CO2, the increase in the photosynthetic rate may be 10 to 30 times more than the normal CO2 level.
2. Light
Light affects the rate of photosynthesis in several ways. In general, photosynthesis can occur under artificial lights of sufficient intensity.
Role of light on photosynthesis can be discussed under the following sub-heads:
a. Light intensity
- Wolkoff (1966) found that the arte of photosynthesis is directly proportional to light intensity.
- But the extremely high light intensities do not favor for higher photosynthetic rates.
- The high light intensity which fails to accelerate photosynthesis is called light saturation intensity.
- Of the light falling on a leaf, about 80 per cent is absorbed, 10 per cent is reflected and 10 % is transmitted.
- The rate of photosynthesis is greater in intense light than in diffused light.
The plants are grouped into two types on the basis of light requirement.
i. Heliophytes (Sun plants)
ii. Sciophytes (Shade plants)
- At a specific light intensity, the amount of CO2 used in photosynthesis and the amount of CO2 released in respiration are volumetrically equal. This specific light intensity is known as light compensation point.
- At very high light intensity, beyond a certain point, the photosynthetic cells exhibit photo oxidation.
- This phenomenon is called solarisation and a result of this, inactivation of chlorophyll molecules, bleaching of chlorophyll molecules and even inactivation of some enzymes take place resulting in the destruction of whole photosynthetic apparatus.
- In general, low light intensity favours stomatal closure and in turn reduced rate of photosynthesis.
- With the increase in light intensity, the rate of photosynthesis increases, i.e., the rate of photosynthesis is directly proportional to light intensity.
- However, at stronger light intensity, increase in rate of photosynthesis is not proportional to light intensity.
- Except on cloudy days, light is never a limiting factor in nature.
- At certain light intensity, the amount of CO2 used in photosynthesis and the amount of CO2 produced in respiration are volumetrically equal.
- This point of light intensity is known as Light Compensation Point.
- Light compensation point is frequently in the order of 100 to 200 f.c. for sun loving leaves; while, the value is 100f.c. for shade-loving leaves.
- Thus, in shade-loving plants the compensation point lasts for a much shorter period than in sun loving plants.
b. Wavelength of light:
- For photosynthesis, the visible range of spectrum of light (PAR: 400 to 700 nm) is essential.
- Maximum photosynthesis is known to occur in the red part of the spectrum with the next peak in blue part and minimum in the green region (RED ,BLUE and GREEN). The region between 575 and 750nm (yellow to red) is quite congenial for photosynthesis.
- Ultra violet light has a lethal effect on plants if exposure is for a prolonged period.
c. Duration of light:
- Photosynthesis may be sustained for relatively long periods of time without any noticeable damaging effect on plants.
d. Photo-oxidation:
- When the light intensity for photosynthesizing tissue is increased beyond a certain limit, the cells become vulnerable to chlorophyll photo-oxidation; due to this, many more chlorophyll molecules become excited than can possibly be utilized. This causes damaging effect to the chloroplast membrane system.
- In presence of O2, the damaging effect of photo-oxidation is severe. It results in bleaching of chlorophyll and inactivation of some important enzyme involved in photosynthesis.
- Effect of temperature on photosynthesis is little than on other process.
- Very high and very low temperatures affect the photosynthetic rate adversely.
- The rate of photosynthesis increases with rise in temperature from 5 to 350C; beyond which, there is a rapid fall in photosynthesis.
- In the optimum range of temperature, the Temperature Quotient (Q10) is found to be 2.0 for the rate of photosynthesis (Q10=2.0).
3. Water
- Water is one of the raw materials in photosynthesis.
- It has an indirect effect on the rate of photosynthesis.
- Water availability affects the water relation of plant, thus affecting the rate of photosynthesis.
- In scarcity of water, cells become flaccid.
- Depending upon the availability of water, the rate of photosynthesis may be decreased from 10 to 90%.
4. Oxygen
- Oxygen is the by-product of photosynthesis.
- Accumulation of greater amount of oxygen molecules causes substantial inhibition of photosynthesis.
- Oxygen is also known to have a direct and competitive inhibition for RuBP carboxylase.
- As a result, glycolate synthesis is enhanced which leads to photorespiration.
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