Factors affecting photosynthesis

The factors that affect the rate of photosynthesis are light availability, water availability, temperature and carbon dioxide concentration.

These factors can either positively or negatively impact the rate of photosynthesis. These factors can be plotted on graphs and their effects visually depicted as to how they affect the rate of photosynthesis.

An image depicting the process of photosynthesis showing a plant in profile with its roots embedded and branching out in soil. The roots draw up water and minerals in the soil. The stem and leaves of the plant are depicted above the soil and, on the left-hand side, an image of a cloud raining is positioned above the plant, and on the right-hand side is an image of the sun shining rays onto the plant. Carbon dioxide enters through the stomata, plants use the sun’s energy to turn water into sugar, and oxygen is released into the air.

Use this page to revise the following factors affecting photosynthesis:

Effect of light availability

A picture of a tree, looking up from the base of the trunk through the crowning branches, with the sun and sunlight streaming through the branches visible.

Ample amounts of light are essential for photosynthesis to occur, as light splits water in the light dependent stage.

As the light intensity increases, chlorophyll in plant cells can absorb more light. The rate of photosynthesis increases.

The rate of photosynthesis will reach a maximal point called the light saturation point. This is the point in which no matter how much increase there is in light, the rate of photosynthesis will not increase. This can be seen on a graph with a plateauing of a trend line.

Limiting factors in relation to light are conditions that limit or restrict the rate of photosynthesis to reach its optimal rate. For example:

  • low light intensity in winter months
  • shorter days during winter.

Click on parts of the graph depicting the effect of light availability on photosynthesis, to find out more about that process.

Two graphs depicting, along the x-axis the wavelength of light in nm ranging from 400nm on the left of the x-axis through 500nm, 600nm, with 700nm at the right of the x-axis. In the first graph the y-axis is labeled “absorption of light by chloroplast pigments” and the graph depicts 3 coloured lines, corresponding, respectively to chlorophyll a, chlorophyll b, and carotenoids. In the second graph the y-axis is labeled “rate of photosynthesis” and depicts one coloured line.

Plants can have a variety of different light absorbing pigments that absorb different wavelengths of light.

For plants that have chlorophyll as their light trapping pigment their most useful wavelengths of light are red and violet, whilst green light is least useful (reflects the colour and is not absorbed).

Effect of water availability

Cross-section photograph of plants with roots visible in soil at the bottom of the photograph, and the stems and leaves of the plants above the soil.

Plants need water for survival.

Adequate water availability is essential for photosynthesis to occur, as water is split in the light dependent stage, producing oxygen molecules and energy for the Calvin cycle.

Plants take in water through their root system and water is lost through transpiration via the stomata in the leaf.

An image depicting a plant in cross section, with roots embedded and spreading out in soil at the bottom of the image, and stem and leaves branching into the air/space above the soil. Small circles representing water are depicted in the soil, while raindrops are depicted in the air in the top left corner of the image. A number one is placed near the roots and its caption reads “roots take up water from the soil”. A number two is placed near the base of the stem, just above the soil line and its caption reads “water travels up through the plant”. A number three is placed at the top of the image, where the leaves of the plant are depicted and its caption reads “water lost by transpiration”. All three numbers sit along an arrow depicting movement from the soil up into the air, demonstrating that water travels up the plant through the roots, stem and leaves before transpiring into the air.

The rate of photosynthesis will decline when soils dry out and water supply lessens. This is known as a plant being in water deficit. The plant will also close its stomata to prevent further water loss and therefore prevent the uptake of carbon dioxide.  With low levels of carbon dioxide, the Calvin cycle will stop functioning and, therefore, reduce the rate of photosynthesis.

The rate of photosynthesis will also decrease if there is too much water in the soil, known as water logging. Water fills spaces in the soil and reduces the available oxygen in the soil, which prevents root cells from undergoing cellular respiration.

Effect of temperature

a picture of a thermometer showing temperature in degrees celsius

Ideal temperatures are essential for photosynthesis to occur at optimum levels.

At low temperatures, enzymes, like RuBisCo , have low rates of collisions to catalyse the reaction between RuBP and carbon dioxide. Therefore the rate of photosynthesis is low and less glucose is produced.

As temperatures increase, Rubisco and its substrates have higher rates of collisions to catalyse the reaction. Therefore, the rate of photosynthesis increases and more glucose is produced.

However, if temperatures exceed the enzyme’s optimal working conditions, the rate of photosynthesis decreases rapidly as the enzyme begins to denature.

Click on parts of the graph depicting the effect of temperature on photosynthesis to find out more about the process.

Effect of carbon dioxide concentration

picture of a daisy flower amongst green grass. A stylised circle surrounds the flower and at the top right-hand side of the circle there is an image of a cloud with the periodic symbol for carbon dioxide at the bottom of the cloud with an arrow alongside pointing down to the flower.

Adequate carbon dioxide availability is essential for photosynthesis to occur, as it is a key input in the light independent stage.

As the rate of carbon dioxide increases, the rate of photosynthesis will also increase.

The rate of photosynthesis will reach a maximal point.  This is the point at which further increases in carbon dioxide concentrations will not increase the rate of photosynthesis as enzymes are saturated. This can be seen on a graph with a plateauing of a trend line.

Other limiting factors in relation to the rate of photosynthesis can include:

  • too few enzymes to catalyse reactions in the Calvin cycle for a given CO2 concentration
  • limited coenzymes, such as NADPH, restricting the Calvin cycle.

Click on parts of the graph depicting the effect of carbon dioxide concentration on photosynthesis to find out more about that process.