11.10 Factors affecting Photosynthesis

An understanding of the factors that affect photosynthesis is necessary. The rate of photosynthesis is very important in determining the yield of plants including crop plants. Photosynthesis is under the influence of several factors, both internal (plant) and external. The plant factors include the number, size, age and orientation of leaves, mesophyll cells and chloroplasts, internal \(CO _2\) concentration and the amount of chlorophyll. The plant or internal factors are dependent on the genetic predisposition and the growth of the plant.

The external factors would include the availability of sunlight, temperature, \(CO _2\) concentration and water. As a plant photosynthesises, all these factors will simultaneously affect its rate. Hence, though several factors interact and simultaneously affect photosynthesis or \(CO _2\) fixation, usually one factor is the major cause or is the one that limits the rate. Hence, at any point the rate will be determined by the factor available at sub-optimal levels.

When several factors affect any [bio] chemical process, Blackman’s (1905) Law of Limiting Factors comes into effect. This states the following:

If a chemical process is affected by more than one factor, then its rate will be determined by the factor which is nearest to its minimal value: it is the factor which directly affects the process if its quantity is changed.

For example, despite the presence of a green leaf and optimal light and \(CO _2\) conditions, the plant may not photosynthesise if the temperature is very low. This leaf, if given the optimal temperature, will start photosynthesising.

Light

We need to distinguish between light quality, light intensity and the duration of exposure to light, while discussing light as a factor that affects photosynthesis. There is a linear relationship between incident light and \(CO _2\) fixation rates at low light intensities. At higher light intensities, gradually the rate does not show further increase as other factors become limiting (Figure 11.10). What is interesting to note is that light saturation occurs at 10 per cent of the full sunlight. Hence, except for plants in shade or in dense forests, light is rarely a limiting factor in nature. Increase in incident light beyond a point causes the breakdown of chlorophyll and a decrease in photosynthesis.

Carbon dioxide Concentration

Carbon dioxide is the major limiting factor for photosynthesis. The concentration of \(CO _2\) is very low in the atmosphere (between 0.03 and 0.04 per cent). Increase in concentration upto 0.05 per cent can cause an increase in \(CO _2\) fixation rates; beyond this the levels can become damaging over longer periods.

The \(C _3\) and \(C _4\) plants respond differently to \(CO _2\) concentrations. At low light conditions neither group responds to high \(CO _2\) conditions. At high light intensities, both \(C _3\) and \(C _4\) plants show increase in the rates of photosynthesis. What is important to note is that the \(C _4\) plants show saturation at about \(360 ~\mu lL ^{-1}\) while \(C _3\) responds to increased \(CO _2\) concentration and saturation is seen only beyond \(450 ~\mu lL ^{-1}\). Thus, current availability of \(CO _2\) levels is limiting to the \(C _3\) plants.

The fact that \(C _3\) plants respond to higher \(CO _2\) concentration by showing increased rates of photosynthesis leading to higher productivity has been used for some greenhouse crops such as tomatoes and bell pepper. They are allowed to grow in carbon dioxide enriched atmosphere that leads to higher yields.

Temperature

The dark reactions being enzymatic are temperature controlled. Though the light reactions are also temperature sensitive they are affected to a much lesser extent. The \(C _4\) plants respond to higher temperatures and show higher rate of photosynthesis while \(C _3\) plants have a much lower temperature optimum.

The temperature optimum for photosynthesis of different plants also depends on the habitat that they are adapted to. Tropical plants have a higher temperature optimum than the plants adapted to temperate climates.

Water

Even though water is one of the reactants in the light reaction, the effect of water as a factor is more through its effect on the plant, rather than directly on photosynthesis. Water stress causes the stomata to close hence reducing the \(CO _2\) availability. Besides, water stress also makes leaves wilt, thus, reducing the surface area of the leaves and their metabolic activity as well.