Factors Affecting Rate of Photosynthesis
Factors affecting photosynthesis can be divided into two broad categories - the internal and external (environmental) factors.
Internal Factors
1. Chlorophyll
The amount of chlorophyll present has a direct relationship with the rate of photosynthesis because this pigment is directly involved in trapping light energy responsible for the light reactions.
2. Leaf Age and Anatomy
Newly expanding leaves show gradual increase in rate of photosynthesis and the maximum is reached when the leaves achieve full size. Chloroplast functions decline as the leaves age. Rate of photosynthesis is influenced by variation in
- (i) number, structure and distribution of stomata
- (ii) size and distribution of intercellular spaces
- (iii) relative proportion of palisade and spongy tissues
- (iv) thickness of cuticle
3. Demand for Photosynthate
Rapidly growing plants show increased rate of photosynthesis in comparison to mature plants. When demand for photosynthesis is lowered due to poor meristematic activity, the photosynthetic rate declines.
External Factors
The major external factors which affect the rate of photosynthesis are temperature, light, carbon dioxide, water, and mineral elements.
Concept of Limiting Factors
When a process is affected by various factors, the rate of the process depends upon the pace of the slowest factor.
Light
The rate of photosynthesis increases with increase of intensity of light within physiological limits or rate of photosynthesis is directly proportional to light intensity. Except on a cloudy day and at nights, light is never a limiting factor in photosynthesis in nature.
At a certain light intensity the amount of CO2 used in photosynthesis and the amount of CO2 produced in respiration are the same. This point of light intensity is known as compensation point.
Wavelength of light absorbed by photosynthetic pigments affects rate of photosynthesis. Red light and to some extent blue light has an enhancing influence on photosynthesis.
The proportion of the total incident sunlight on earth, absorbed by green plants is generally a limiting factor. As per the estimates of the total incident light reaching the green plants, only about 1-2% is actually absorbed, because 70% is transmitted, and 28-29% is reflected back into the atmosphere.
Temperature
Very high and very low temperature affect the rate of photosynthesis adversely. Rate of photosynthesis will rise with temperature from 5°-37°C beyond which there is a rapid fall, as the enzymes involved in the process of the dark reaction are denatured at high temperature. Between 5°-35°C, with every 10°C rise in temperature rate of photosynthesis doubles or Q10 is 2 (Q = quotient), or slightly less than two.
Carbon dioxide
Since carbon dioxide being one of the raw materials for photosynthesis, its concentration affects the rate of photosynthesis markedly. Because of its very low concentration (0.03%) in the atmosphere, it acts as limiting factor in natural photosynthesis. At optimum temperature and light intensity, if carbon dioxide supply is increased the rate of photosynthesis increases markedly until CO2 concentration is as high as 3.0%. Thus, CO2 concentration in the atmosphere is always a limiting factor for photosynthesis.
Water
Water has an indirect effect on the rate of photosynthesis. Loss of water in the soil is immediately felt by the leaves, which get wilted and their stomata close down thus hampering the absorption of CO2 from the atmosphere. This causes decline in photosynthesis.
Oxygen
Concentration of oxygen as an external factor, is never a limiting factor for photosynthesis because it is a by-product of photosynthesis, and it easily diffuses into the atmosphere from the photosynthesizing organ, the leaf. However, excess of O2 surrounding a green plant, reduces photosynthetic rate by promoting the rate of aerobic respiration.
Mineral Elements
Some mineral elements like magnesium, copper, manganese and chloride ions, which are components of photosynthetic enzymes, and magnesium as a component of chlorophylls are important, and their deficiency would affect the rate of photosynthesis indirectly by affecting the synthesis of photosynthetic enzymes and chlorophyll, respectively.