The environmental factors affecting the process of photosynthesis follow the law of limiting factor as hypothesized by the British plant physiologist F. F. Blackman in 1905. According to this hypothesis, when a process is influenced by a number of factors, the rate of the process is limited by the slowest factor.
For example, photosynthesis can be influenced by both C02 concentration and light intensity. By increasing the CO, concentration at a fixed intensity of light, the rate of photosynthesis can be increased up to a particular concentration of CO after which no further increase can be noticed. At this point by increasing the light intensity to another fixed level, the rate of photosynthesis can be increased with increase in CO, concentration. In this example the intensity of light is the limiting factor for the rate of photosynthesis.
The implication of this hypothesis is that at any given time, photosynthesis can be limited either by light intensity or by C02 concentration, but not by both the factors.
There are several factors which affect the rate of photosynthesis as described below: Light intensity
The intensity of light influences the rate of photosynthesis. When other factors are not limiting, the rate of photosynthesis increases almost linearly with increase in light intensity. With further increase in light intensity, the rate of photosynthesis starts to level off and reaches
saturation indicating that factors other than light intensity have bcc. Limiting to photosynthesis at extremely high light intensity, when leaves are unable to utilize the absorbed light, the rate of photosynthesis declines by a phenomenon called photo inhibit! Photoinhibition is caused by the absorption of too much light which inactivates and dama reaction centre of PSII.
In the dark there is no photosynthetic CO, fixation (i.e. CO, uptake) taking place by the leaves; rather CO, is given off by the plant because of respiration. By convention, photosynthetic carbon assimilation is considered as negative at this situation. As the light intensity increases, photosynthetic C02 fixation increases until it equals CO, release by respiration. The light intensity at which the rate of photosynthetic C02 uptake equals the rate of respiratory CO, release is known as the light compensation point. Plants do not survive below the light compensation point.
Photosynthesis occurs only in the visible spectrum of light, the wavelength of which ranges from about 400 nm to 750 nm different wavelengths of light, (i.e., different qualities of light) are1 not equally effective in photosynthesis (see Box-I). Different plants show different rate of1 photosynthesis under different quality of light.
Carbon dioxide is the key inorganic molecule required for photosynthesis. Photosynthesis occurs at a very wide range of C02 concentration. At veiy low C02 concentration, photosynthesis is strongly limited by the low C02, while respiratory rate is unaffected. As a result, C02 fixed by photosynthesis is lower than the C02 released by respiration. So there is a net efflux of CO, from the plants. With the increase in C02 concentration, the rate of photosynthesis increases provided other factors are not limiting.
The COo concentration at which the rate of C02 fixation by photosynthesis equals the rate ofC02release by respiration is known as CO, compensation point. In C plants, increasing CO, concentration above the compensation point stimulates photosynthesis over a wide concentration range. On the other hand, photosynthesis in C. plants level off and reach saturation at a lower CO, concentration when compared with that of C3 plants.
Water is one of the raw materials used in photosynthesis. The amount of water utilized in photosynthetic reactions is quite small. Therefore, water rarely becomes a limiting factor for photosynthesis. However, water deficit stress reduces the rate of photosynthesis by affecting the process indirectly. One of the indirect cffccts is the stomata closure which reduces the entry of CO., in the leaf cells.