In the last part of photosynthesis series , we shall look into factors governing the process of photosynthesis in plants and also importance of photosynthesis to both plants and other organisms.
Earlier blogs can be read :
Part I of Photosynthesis : Introduction
Part VI – Differences between Light and Dark reactions or Calvin Cycle
Part V – Photorespiration, C4 Pathway and CAM metabolism pathway
Part VI – C3, C4 and CAM types of photosynthesis comparison
Factors affecting Photosynthesis :
- The rate of photosynthesis is crucial in determining the yield of plants. So it becomes important to learn about the factors which has an influence over the process of photosynthesis.
- Photosynthesis is governed by – environmental and genetic (internal) factors.
- Environmental factors – light, CO2, temperature, soil, water, nutrients etc.
- Genetic factors – Related to leaves such as number, age size and orientation, mesophyll cells and chloroplast, internal CO2 concentration and amount of chlorophyll.
Law of limiting factors or Blackman’s Law of Limiting Factors :
- As plants photosynthesizes, all the above factors will influence simultaneously affects its rate. When so many factors controls a biochemical process, usually one factor is the one that limits the rate of that process. Hence at any point the rate of photosynthesis is determined by the factor available at sub-optimal levels.
- In a situation ( as observed in photosynthesis) when more than one factor affects the outcome of a biochemical process, Blackmann’s law of limiting factors comes into effect.
- According to Law of limiting factors or Blackman’s Law of Limiting Factors : If a biochemical process is influences by several factors at once, then its rate is determined by factor which is nearest to its minimal value. It is the factor which directly affects the process when its quantity is changed.
- For example, despite the presence of adequate light, CO2, good leaf conditions, but plants don’t properly perform photosynthesis under cloudy conditions (less temperature). When cloud cover is reduced and temperature increases, photosynthesis rate tends to increase.
1) Light :
- Solar radiation happens to the ultimate source for light for the process of photosynthesis.
- The intensity of light is directly proportional to the rate of light-dependent reactions of photosynthesis and in turn, the rate of photosynthesis increases. In other words, with increasing in light intensity, the number of photons falling on the leaves increases and that helps more chlorophylls to excite electron to generate ATP and NADPH from light reaction.
- It is estimated that, 2% of total solar energy is used by plants for photosynthesis.
- Light varies in intensity, quality (wavelength) and duration, so its important to study effect of light on photosynthesis on the basis of three light variations.
- Intensity of light: There is a linear relationship between light at low intensities and CO2 fixation. If the intensity of light is high then, photosynthesis doesn’t increases after some time as other factors becomes limiting.
- The plants reach a level of light saturation at 10% of the full sunlight. Light saturation is a point where further increase in light intensity is not accompanied by an increase in CO2 uptake.
- Under the influence of very high light intensity Solarization’ occurs – a phenomenon where photo-oxidation of different cellular components including chlorophyll and photosynthesis decreases.
- Intensity of light also controls the opening and closing of stomata thereby affecting the gaseous exchange.
- The value of light saturation is called light saturation point.
- Quality of light : Photosynthetic pigments absorb visible part of the radiation i.e., 380 mμ, to 760 mμ. Chlorophyll a absorbs blue and red light and usually plants show high rate of photosynthesis in the blue and red light.
- Duration of light : Longer duration of light period is good for photosynthesis.
2) Temperature : The dark reactions are enzymatic and hence are more temperature sensitive when compared to light reaction of photosynthesis. If the temperature exceeds more than 30°C, the rate of photosynthesis abruptly falls due to thermal inactivation of enzymes.
The effect of temperature also depends on the conditions the plants are growing (habitat). The C4 plants can tolerate high temperature and shows enhanced photosynthesis rate in hat climatic conditions. However C3 plants have lower temperature optimum.
3) Carbon di oxide concentration:
CO2 happens to be the most limiting factor for photosynthesis. The atmosphere normally contains 0.03 to 0.04 per cent by volume of carbon dioxide. An increase in CO2 concentration to 0.05% can help in enhanced CO2 fixation rates ( provided light is not limiting). However beyond this levels, its damaging to plant for longer periods.
4) Water :
Water is one of the reactants of light reaction. The effect of water on photosynthesis is more of indirect effect. Since lack of water puts more stress on plants, causing stomata to close and hence reduces CO2 intake. Lack of water also makes leaves to wilt and this reduces the surface area of the leaves, eventually affecting photosynthesis.
5) Oxygen and minerals:
- Excess of oxygen inhibits photosynthesis as oxygen tends to compete with CO2 for active sites of enzyme ribulose 1,5-biphosphate carboxylase, also called RuBisCO. This leads to photorespiration and decrease in photosynthesis rates. This phenomenon is called Warburg effect.
- A decrease in atmospheric oxygen, increases the rate of photosynthesis in C3 plants but shows no change in C4 plants.
- Presence of minerals such as , Mn++ and CI– are very crucial for photolysis of water and release of oxygen, during light reactions. Mg++, Cu++ and Fe++ ions are important for synthesis of chlorophyll.
Internal factors :
- The condition, age, and the arrangement of leaves are important internal factors for regulating photosynthesis. Healthy leaves will have good number of mesophyll cell and increased chlorophyll molecules, hence more efficient light and dark reactions leading to increased photosynthesis. The amount of CO2 that reaches the chloroplasts also depends on the above factors of leaves.
- The amount of CO2 fixed by a gram of chlorophyll in an hour is called photosynthetic number or assimilation number.
- The rate of photosynthesis decreases when there is more food ( in the form of starch) accumulated in chloroplasts.