Photosynthesis is a complex biological process that converts light energy into chemical energy that is stored in the bonds of glucose molecules. This chemical energy is used by plants and other photosynthetic organisms to fuel their growth, reproduction, and other functions. There are four main factors that affect the rate of photosynthesis: light intensity, carbon dioxide concentration, temperature, and water availability.
Photosynthesis is an essential process for life on Earth. It provides the chemical energy that powers almost all ecosystems. By understanding the factors that affect photosynthesis, scientists can learn how to optimize plant growth and productivity. Photosynthesis research also gives insights into how different plants have adapted to thrive in various environments and conditions around the world.
In this article, we will examine the four main factors that impact the rate of photosynthesis. They are:
- Light intensity
- Carbon dioxide concentration
- Temperature
- Water availability
Each of these environmental variables plays a key role in photosynthesis. Changing any one of them can speed up or slow down the rate of this process. Understanding how these factors interact allows farmers, gardeners, and scientists to maximize plant growth and crop yields.
Light Intensity
Light provides the energy that drives photosynthesis. When light shines on a leaf, the chlorophyll and other light-absorbing pigments in the leaf chloroplasts harness light energy. This energy gets converted into chemical bonds in the form of ATP and NADPH. These energy carriers power the rest of the photosynthetic process.
As light intensity increases, the rate of photosynthesis also increases, up to a point. This occurs because more photons are striking the chloroplasts, providing more energy. However, if the light intensity becomes extremely high, the rate of photosynthesis starts to plateau or even decline. This is because the chloroplasts are already saturated – they cannot utilize any more light energy.
Different plants have adapted to maximize photosynthesis at different optimal light intensities. Plants native to sunny environments, like cacti, have high optimal light intensities. Shade-loving plants that grow in the forest understory have lower optimal intensities.
Here is a table showing how the rate of photosynthesis changes in response to different light intensities. The data is for a typical C3 leaf at 25°C and ample CO2 and water:
| Light Intensity (μmol m-2 s-1) | Rate of Photosynthesis |
|---|---|
| 0 | 0 μmol CO2 m-2 s-1 |
| 50 | 6 μmol CO2 m-2 s-1 |
| 100 | 12 μmol CO2 m-2 s-1 |
| 200 | 18 μmol CO2 m-2 s-1 |
| 500 | 20 μmol CO2 m-2 s-1 |
| 1000 | 22 μmol CO2 m-2 s-1 |
| 1500 | 22 μmol CO2 m-2 s-1 |
As shown, photosynthesis rate increases rapidly between 0-200 μmol m-2 s-1 intensity. After 200, the rate starts to plateau because the leaf’s capacity to process light energy is reached.
Carbon Dioxide Concentration
CO2 is one of the main reactants needed for photosynthesis. It is taken in from the atmosphere through microscopic pores on the leaves called stomata. Inside the leaf, CO2 gets incorporated into organic compounds by the Calvin cycle reactions.
Higher CO2 levels mean more CO2 molecules are available for the photosynthetic reactions. This leads to higher rates of sugar and biomass production. However, if CO2 levels get extremely high (far above atmospheric levels), the rate reaches a plateau again. This is because other factors like light intensity become limiting, rather than CO2 supply.
Plants adapted to different environments have evolved differences in their photosynthetic machinery to make best use of ambient CO2 levels. C4 plants like corn and sugarcane have mechanisms to concentrate CO2 which allows them to photosynthesize efficiently even in hot, dry conditions. Meanwhile, C3 plants like rice and soybeans do not concentrate CO2 and thus perform better in cooler, wetter environments.
Below is a table showing how photosynthesis rate changes with different CO2 levels. The measurements are at 25°C, 200 μmol m-2 s-1 light intensity, and optimal water:
| CO2 Concentration (ppm) | Rate of Photosynthesis |
|---|---|
| 0 | 0 μmol CO2 m-2 s-1 |
| 100 | 6 μmol CO2 m-2 s-1 |
| 200 | 12 μmol CO2 m-2 s-1 |
| 400 | 18 μmol CO2 m-2 s-1 |
| 800 | 24 μmol CO2 m-2 s-1 |
| 1200 | 28 μmol CO2 m-2 s-1 |
With ample light, water, and optimal temperature, increasing CO2 from today’s ambient level of 400 ppm up to 1200 ppm causes photosynthesis to increase. But the response starts to taper off at very high CO2 levels.
Temperature
Like all chemical reactions, the rate of photosynthesis is affected by temperature. However, the effect is more complex than a simple linear increase with rising temperatures. Photosynthesis rates do increase steadily as temperatures rise from cold up to an optimum level. But beyond this optimal point, higher temperatures start inhibiting photosynthesis.
This is because excessively high temperatures damage the structure of the photosynthetic enzymes and membranes in the chloroplasts. Sustained heat also degrades the D1 protein in Photosystem II that is essential for the light reactions. Thus, there is an upper limit to the benefits of higher temperatures.
Different types of plants have adapted to perform photosynthesis optimally within different temperature ranges. For example, the peak temperature for photosynthesis in desert succulents like cacti is typically 40-45°C. On the other hand, crops like wheat have a cooler optimum of around 25°C. This matches their native climates.
The table below illustrates a typical photosynthetic temperature response curve. The measurements are at 200 μmol m-2 s-1 light intensity and ambient CO2:
| Temperature (°C) | Rate of Photosynthesis |
|---|---|
| 5 | 4 μmol CO2 m-2 s-1 |
| 15 | 10 μmol CO2 m-2 s-1 |
| 25 | 18 μmol CO2 m-2 s-1 |
| 35 | 22 μmol CO2 m-2 s-1 |
| 45 | 18 μmol CO2 m-2 s-1 |
| 55 | 10 μmol CO2 m-2 s-1 |
Photosynthesis peaks around 35-45°C for this example plant. But it declines rapidly once temperatures rise above the optimum, demonstrating heat damage.
Water Availability
Water is essential for photosynthesis. It provides the electrons needed to replace those removed from chlorophyll after light excitation. Water is also split to provide protons and oxygen during the light reactions.
With ample water, the stomata on leaves can remain open, allowing inward diffusion of CO2 for photosynthesis. However, when water is lacking, the stomata must close to prevent water loss through transpiration. This severely limits CO2 intake.
Drought-adapted plants like cacti and succulents have special mechanisms to take up and conserve water. This allows them to perform photosynthesis even in dry environments. Other plants modify their stomatal structure and density to optimize water use efficiency.
The table shows how photosynthesis declines as leaf water potential drops and stomatal closure occurs:
| Leaf Water Potential (-MPa) | Rate of Photosynthesis |
|---|---|
| 0 | 18 μmol CO2 m-2 s-1 |
| 0.5 | 16 μmol CO2 m-2 s-1 |
| 1 | 12 μmol CO2 m-2 s-1 |
| 1.5 | 8 μmol CO2 m-2 s-1 |
| 2 | 4 μmol CO2 m-2 s-1 |
As leaf water potential drops below -1 MPa, indicating drought stress, photosynthesis declines due to stomatal closure.
Conclusion
In summary, photosynthesis is influenced by four key environmental factors: light intensity, CO2 concentration, temperature, and water availability. There are optimal levels for each factor beyond which photosynthesis plateaus or declines. Plants have evolved adaptations to maximize photosynthesis rates under the conditions in their native habitat.
Understanding the factors that affect this process allows agricultural scientists to optimize plant productivity. For example, enriching greenhouse air with CO2, using grow lights, controlling temperature, and irrigating appropriately can lead to substantial increases in crop yields. Going forward, a deep knowledge of photosynthesis will be key to boosting food production to feed the world’s growing population.