BOTANY Higher Secondary Second Year - Textbooks Online

orientation. Photosynthesis is not affected by all environmental factors at a
given time.
According to Blackmann who postulated Law of Limiting factor in
1905, photosynthesis is limited by slowest step of the most limiting factor
in the pathway. This means that at a given time, only the factor that is
most limiting among all will determine the rate of photosynthesis. For
example, if CO 2 is available in plenty but light is limiting due to cloudy
weather, the rate of photosynthesis under such situation is controlled by
the light. Further, if both CO 2 and light are limiting, then the factor which
is the most limiting of the two will control the rate of photosynthesis.
Both quality and intensity of light influence photosynthesis. Light
between the wavelength of 400nm to 700nm is most effective for
photosynthesis and this light is called photosynthetically active radiation.
As the intensity of light increases the rate of photosynthesis increases.
However, if the light intensifies, the rate of photosynthesis decreases.
This is because of higher intensity of light destruction of chlorophyll occurs.
Photochemical reactions and dark reactions of photosynthesis respond
differently to temperature. Photochemical reactions in the thylakoid remain
unharmed by temperature, whereas the enzymatic dark reactions get
influenced adversely. At higher temperature, the enzymes become inactive.
Low temperature also inactivates the enzymes.
The current level of CO 2 is about 0.036 per cent or 360 ppm (parts
per million), which is very low as compared to the concentration of other
gases in the atmosphere such as O 2 and N 2. The rate of photosynthesis in
all plants increases with increase in the concentration of CO 2 upto 500
ppm, when other factors are not limiting.
Availability of water in soil has a prominent effect on photosynthesis.
If the soil water becomes limiting factor, the rate of photosynthesis declines.
Among various nutrients, nitrogen has a direct relationship with
photosynthesis. Since, nitrogen is a basic constituent of chlorophyll and all
enzymes involved in dark reactions, any reduction in nitrogen supply to
plants has an adverse effect on photosynthesis. In general all essential
elements affect the rate of photosynthesis.
190