4.2 - Rate of Photosynthesis

Limiting factors in photosynthesis

• The rate of photosynthesis isn't fixed - it rises and falls with the conditions around the plant.
• A limiting factor is the factor in shortest supply at that moment, so it's the one holding the rate back.
• Increasing the limiting factor raises the rate; increasing any other factor makes no difference until the limiting one is raised.
• Which factor is limiting depends on the surroundings:
  • At night - Light is limiting
  • A cold winter day - Temperature is often limiting
  • Warm and bright - Carbon dioxide is usually limiting

The three environmental factors

1. Light intensity - Provides the energy for photosynthesis. Raising it from low increases the rate steadily, but only up to a point. Beyond that, more light makes no difference because temperature or carbon dioxide has become limiting instead. The rate-against-light-intensity graph climbs, then levels off at a plateau.
2. Carbon dioxide concentration - A raw material of photosynthesis. Increasing it raises the rate up to a point, then the graph flattens - showing light or temperature is now limiting. Same rise-then-plateau shape as the light graph.
3. Temperature - Affects the enzymes that control photosynthesis:
   • In the cold, enzymes work slowly, so the rate is slow - this is usually why temperature is limiting.
   • As temperature rises, the rate increases up to an optimum at around 25°C.
   • Above the optimum the rate falls sharply, and by around 45°C the enzymes are denatured (their shape is destroyed) and the rate drops to zero.
   • The rate-against-temperature graph therefore rises to a peak then drops steeply - unlike the light and carbon dioxide graphs, which plateau.

The amount of chlorophyll can also limit the rate

• Chlorophyll is the pigment that absorbs the light, so less chlorophyll means a reduced rate.
• Things that can damage chloroplasts or stop the plant making enough chlorophyll include:
  • Disease - e.g. infection with tobacco mosaic virus
  • Environmental stress - e.g. a shortage of nutrients
• With less chlorophyll, the plant can't absorb as much light, slowing photosynthesis.

Interpreting graphs with more than one limiting factor

• A single graph can show more than one factor limiting photosynthesis at once.
• Reading it tells you which factor is holding the rate back in each region of the graph.

Reading one line

• Where a line is still rising, the factor on the horizontal axis is the limiting factor there.
• Where a line has levelled off (plateaued), some other factor has become limiting instead.

Comparing two lines - Temperature

• Plot rate of photosynthesis against light intensity as two lines at two different temperatures, 15°C and 25°C.
• At low light intensity both lines rise together as light is increased - light is the limiting factor for both.
• Each line then levels off when light stops being limiting.
• The 25°C line levels off at a higher rate than the 15°C line.
• This shows temperature was the limiting factor for the cooler line: once light was plentiful, the 15°C plant couldn't go as fast because its lower temperature held it back.

Comparing two lines - Carbon dioxide

• Plot rate against light intensity as two lines at the same temperature but two different carbon dioxide concentrations, 0.4% and 0.04%.
• Both lines rise, then level off when light is no longer limiting.
• The line at the higher CO₂ concentration (0.4%) levels off higher, so carbon dioxide was the limiting factor for the 0.04% line.
• Here the limiting factor can't be temperature, because both lines were at the same temperature.

The general method

1. Find where a line is still rising - the factor on the horizontal axis is limiting there.
2. Find where it has levelled off - another factor has become limiting.
3. Compare the heights of the plateaus to spot which factor differs between the lines - that is the one limiting the lower line.

The inverse-square law for light intensity

• When investigating light intensity by moving a lamp closer to or further from a plant, the amount of light reaching the plant changes with distance.
• The further away the lamp, the less light arrives - so distance and light intensity are inversely related.
• It isn't a simple inverse, though: light intensity falls in proportion to the square of the distance from the source.
• This is the inverse-square law: light intensity is proportional to $\frac{1}{d^2}$.

The effect of changing distance is large

Because of the square, small changes in distance have a big effect on intensity:

• Halving the distance - Light intensity becomes four times greater (2² = 4).
• Reducing distance to a third - Intensity becomes nine times greater (3² = 9).
• Doubling the distance - Intensity becomes four times smaller.
• Trebling the distance - Intensity becomes nine times smaller.

Using 1/d² as a measure of light intensity

• The quantity $\frac{1}{d^2}$ can be used directly as a measure of light intensity, in arbitrary units.
• For example, with the lamp 10 cm from the plant: light intensity = $\frac{1}{{10}^2}$ = $\frac{1}{100}$ = 0.01 arbitrary units.
• This lets the rate of photosynthesis be plotted against a proper measure of light intensity, rather than against raw distance - which would give a misleadingly shaped graph.

Controlling greenhouse conditions for maximum yield

• A greenhouse lets a grower control the factors that limit photosynthesis, so none of them holds the rate back.
• The aim is to keep every limiting factor at or near its ideal level at the same time.
• Crops then grow faster and can be harvested more often.

Controlling each factor

• Temperature - The greenhouse traps the Sun's heat so it doesn't fall low enough to limit the rate:
  • In winter, a heater keeps it at the ideal level.
  • In summer, when it might climb past the optimum and start to denature the enzymes, shades and ventilation cool it down.
• Light - Light is always needed, so growers often add artificial lighting after sunset to give plants more time photosynthesising.
• Carbon dioxide - Can be raised above the level in ordinary air. A paraffin heater raises both temperature and carbon dioxide, because burning paraffin gives off carbon dioxide as a by-product.
• Pests and disease - Keeping plants enclosed makes them easier to protect.
• Minerals - Fertiliser can be added to the soil for healthy growth.

Striking the economic balance

• All of this costs money, so there is a balance to strike.
• A grower aims to supply just enough heat, light, and carbon dioxide to keep conditions ideal for a fast rate - but no more.
• Spending on extra heat or light beyond what raises the crop is simply wasted.
• Get the balance right and the faster-grown, more frequent crop earns more than the cost of maintaining the conditions - so the grower makes a profit.