Limiting factors in photosynthesis

The light-independent stage of photosynthesis is influenced by several environmental factors that can limit its rate. A limiting factor is defined as a variable that can slow down the rate of a reaction because it is in the shortest supply or nearest its lowest value. For photosynthesis to occur as quickly as possible, all necessary factors must be at optimal levels; if any one factor is too low or too high, it will limit the overall rate.

• Light Intensity:

  • Light provides the energy for the light-dependent reaction, which in turn provides ATP and reduced NADP for the light-independent stage.

  • The higher the intensity of light, the more energy it provides, leading to a faster rate of the light-dependent stage.

  • At low light intensities, the rate of photosynthesis increases in proportion to the increasing light because light intensity is the limiting factor.

  • However, at higher light intensities, the rate of photosynthesis eventually reaches a plateau. This indicates that something else has become the limiting factor, such as carbon dioxide concentration or temperature.

  • Light also controls the opening of stomata to allow CO2 uptake, which indirectly affects the rate.

• Temperature:

  • Photosynthesis involves enzymes (e.g., ATP synthase, rubisco), and their activity is highly temperature-sensitive.

  • For most plants in temperate climates, the optimum temperature is around 25 °C. If the temperature falls below 10 °C, enzymes become inactive, and if it's more than 45 °C, they may start to denature, losing their 3D shape and function.

  • At high temperatures, stomata may close to avoid losing too much water, which reduces CO2 entry into the leaf and slows down photosynthesis.

  • The light-dependent reaction is largely unaffected by temperature because it is a photochemical process driven by light energy. However, the light-independent stage (Calvin cycle) is highly temperature-sensitive.

  • Therefore, at low light intensities, a rise in temperature has little effect on the rate of photosynthesis because light is the limiting factor. But at higher light intensities, an increase in temperature significantly increases the rate of photosynthesis.

• Carbon Dioxide Concentration:

  • CO2 is a raw material and reactant in photosynthesis, specifically combining with RuBP in the Calvin cycle.

  • Atmospheric CO2 concentration is typically about 0.04%.

  • Increasing the CO2 concentration to around 0.4% can increase the rate of photosynthesis. However, increasing it further can cause stomata to close, which then slows the rate down.

  • When CO2 concentration is low, it becomes the limiting factor, and the rate of photosynthesis increases with increasing CO2 until another factor becomes limiting.

  • A decrease in CO2 concentration leads to an increase in RuBP levels (as less CO2 combines with it) and a decrease in triose phosphate (TP) levels (as less GP is formed and converted to TP).

• Water:

  • Water is a reactant in the light-dependent stage (photolysis).

  • A constant supply of water is needed, as too little water will cause photosynthesis to stop.

  • Water deficiency can lead to a plant closing its stomata to conserve water, which indirectly limits photosynthesis by reducing CO2 intake.

  • However, water supply is usually not the primary limiting factor for photosynthesis, as other processes in the plant are affected before it directly limits photosynthesis. Too much water can also be detrimental by causing waterlogged soil, which reduces oxygen availability for roots and thus mineral uptake.

Optimum Conditions and Agricultural Applications: Agricultural growers understand these limiting factors and strive to create optimum conditions in controlled environments like glasshouses (greenhouses). This management includes adding carbon dioxide to the air (e.g., by burning propane), using lamps to provide light at night, and employing heaters or cooling systems to maintain optimal temperatures. These efforts aim to minimize the likelihood of any single factor limiting photosynthesis, thereby increasing crop yield.

Experimental Investigation: The effects of limiting factors can be investigated experimentally. For instance, the rate of photosynthesis in aquatic plants (like Elodea or Cabomba) can be measured by counting the gas bubbles produced in unit time. Alternatively, the Hill reaction can be used, where a redox indicator dye (e.g., DCPIP or methylene blue) is used with a chloroplast suspension to measure the rate of dehydrogenase activity as the dye changes color when reduced. The faster the color change, the faster the rate of the light-dependent stage.