Factors affecting Photosynthesis
Factors
affecting Photosynthesis
“When a process is conditioned as to its rapidity by a
number of separate factors, the rate of the process is limited by the pace of
the lowest factor”
“at any given point
of time the lowest factor among essentials will limit the rate of
photosynthesis”.
Factors affecting photosynthesis are further grouped into
External or Environmental factors
Internal factors.
I. External factors:
Light, carbon dioxide, temperature,
water, mineral and pollutants.
II. Internal factors:
Pigments, protoplasmic factor,
accumulation of carbohydrates, anatomy of leaf and hormones.
External factors:
1. Light
Energy for photosynthesis comes only from light.
Photooxidation of water and excitation of pigment molecules are directly
controlled by light. Stomatal movement leading to diffusion of CO2 is indirectly
controlled by light.
a. Intensity of Light:
Intensity of light plays a direct role in the rate of
photosynthesis. Under low intensity the photosynthetic rate is low and at
higher intensity photosynthetic rate is higher. It also depends on the nature
of plants. Heliophytes (Bean Plant) require higher intensity than Sciophytes
(Oxalis).
b. Quantity of Light:
In plants which are exposed to light for longer duration
(Long day Plants) photosynthetic rate is higher.
c. Quality of light:
Different wavelengths of light affect the rate of photosynthesis
because pigment system does not absorb all the rays equally. Photosynthetic rate
is maximum in blue and red light. Photosynthetically Active Radiation (PAR) is
between 400 to 700 nm. Red light induces highest rate of photosynthesis and
green light induces lowest rate of photosynthesis.
2. Carbon dioxide
CO2 is found only 0.3 % in the atmosphere but plays a vital
role. Increase in concentration of CO2 increases the rate of photosynthesis
(CO2 concentration in the atmosphere is 330 ppm). If concentration is increased
beyond 500ppm, rate of photosynthesis will be affected showing the inhibitory
effect.
3. Oxygen
The rate of photosynthesis decreases when there is an
increase of oxygen concentration. This Inhibitory effect of oxygen was first
discovered by Warburg (1920) using green algae Chlorella.
4. Temperature
The optimum temperature for photosynthesis varies from plant
to plant. Temperature is not uniform in all places. In general, the optimum
temperature for photosynthesis is 25oC to 35oC. This is not applicable for all
plants. The ideal temperature for plants like Opuntia is 55oC, Lichens 20oC and
Algae growing in hot spring photosynthesis is 75oC. Whether high temperature or
low temperature it will close the stomata as well as inactivate the enzymes
responsible for photosynthesis.
5. Water
Photolysis of water provides electrons and protons for the
reduction of NADP, directly. Indirect roles are stomatal movement and hydration
of protoplasm. During water stress, supply of NADPH 1 H1 is affected.
6. Minerals
Deficiency of certain minerals affect photosynthesis e.g.
mineral involved in the synthesis of chlorophyll (Mg, Fe and N), Phosphorylation
reactions (P), Photolysis of water (Mn and Cl), formation of plastocyanin (Cu).
7. Air pollutants
Pollutants like SO2, NO2, O3 (Ozone) and Smog affects rate of photosynthesis.
Internal Factors:
1. Photosynthetic Pigments
It is an essential factor and even a small quantity is
enough to carry out photosynthesis.
2. Protoplasmic factor
Hydrated protoplasm is essential for photosynthesis. It also
includes enzymes responsible for Photosynthesis.
3. Accumulation of Carbohydrates
Photosynthetic end products like carbohydrates are
accumulated in cells and if translocation of carbohydrates is slow then this
will affect the rate of photosynthesis.
4. Anatomy of leaf
Thickness of cuticle and epidermis, distribution of stomata,
presence or absence of Kranz anatomy and relative proportion of photosynthetic
cells affect photosynthesis.
5. Hormones
Hormones like gibberellins and cytokinin increase the rate of photosynthesis.
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