C3 Photosynthesis

• C3, C4, and CAM plants use different photosynthetic mechanisms.
• These mechanisms are affected differently by elevated CO2 concentrations.

C3 Photosynthesis

• C3 plants are the most common type of plant, employing the simplest photosynthetic pathway.
• The C3 pathway involves the enzyme RuBisCO which catalyzes the carboxylation of ribulose-1,5-bisphosphate (RuBP).
• This forms two molecules of 3-phosphoglycerate (3-PGA), a 3-carbon compound, hence the name C3.
• C3 plants thrive in environments with moderate temperatures and sufficient water.
• Under high CO2 concentrations, C3 plants generally exhibit increased photosynthetic rates.
• This is because RuBisCO's affinity for CO2 increases relative to its affinity for O2, reducing photorespiration.
• Photorespiration is a process where RuBisCO oxygenates RuBP, leading to a loss of energy and carbon.
• Elevated CO2 can lead to increased biomass production and water use efficiency in C3 plants.
• However, the magnitude of the response varies depending on species, environmental conditions, and nutrient availability.
• Some C3 plants may experience photosynthetic acclimation, where the initial increase in photosynthesis diminishes over time due to various factors such as nutrient limitations or changes in Rubisco activase activity.

C4 Photosynthesis

• C4 plants have evolved a mechanism to concentrate CO2 in specialized bundle sheath cells, improving photosynthetic efficiency in warm and dry environments.
• Initial carbon fixation occurs in mesophyll cells, where phosphoenolpyruvate carboxylase (PEP carboxylase) catalyzes the carboxylation of phosphoenolpyruvate (PEP) to form oxaloacetate (OAA), a 4-carbon compound.
• OAA is then converted to malate or aspartate, which are transported to bundle sheath cells.
• In bundle sheath cells, these C4 acids are decarboxylated, releasing CO2.
• The released CO2 is then fixed by RuBisCO in the Calvin cycle, similar to C3 plants.
• The higher CO2 concentration in bundle sheath cells minimizes photorespiration, increasing photosynthetic efficiency.
• C4 plants generally show a less pronounced response to elevated CO2 compared to C3 plants.
• This is because their CO2 concentrating mechanism already ensures high CO2 levels around RuBisCO.
• In some C4 species, elevated CO2 may still lead to a small increase in photosynthesis, particularly under conditions where the CO2 concentrating mechanism is not fully saturated.
• Water use efficiency in C4 plants is already high due to their ability to fix carbon efficiently at lower stomatal conductance.
• Elevated CO2 may further improve water use efficiency by allowing for even lower stomatal conductance without compromising carbon gain.

CAM Photosynthesis

• CAM plants are adapted to extremely arid environments.
• They temporally separate carbon fixation and the Calvin cycle to minimize water loss.
• At night, CAM plants open their stomata and fix CO2 using PEP carboxylase, similar to C4 plants.
• The resulting OAA is converted to malic acid and stored in vacuoles.
• During the day, stomata close to conserve water.
• Malic acid is decarboxylated, releasing CO2.
• The released CO2 is then fixed by RuBisCO in the Calvin cycle.
• CAM plants typically exhibit a small or negligible response to elevated CO2.
• Their internal CO2 concentrations are already high during the day due to the decarboxylation of malic acid.
• The primary limitation to photosynthesis in CAM plants is often water availability rather than CO2 concentration.
• Elevated CO2 may indirectly affect CAM plants by influencing stomatal behavior and water use efficiency.
• Under elevated CO2, CAM plants may be able to reduce stomatal opening, further conserving water.

Effects of Elevated CO2

• The effects of elevated CO2 on C3, C4, and CAM plants depend on their photosynthetic mechanisms and environmental conditions.
• C3 plants generally benefit the most from elevated CO2 due to reduced photorespiration and increased carboxylation efficiency.
• C4 plants exhibit a smaller response due to their CO2 concentrating mechanism.
• CAM plants often show little direct response due to water availability being the primary limiting factor.
• However, elevated CO2 can still influence water use efficiency and stomatal behavior in all three types of plants.
• The overall impact of elevated CO2 on plant communities will depend on the relative abundance and responses of C3, C4, and CAM species, as well as changes in other environmental factors such as temperature, water availability, and nutrient availability.
• Understanding these differences is crucial for predicting the effects of climate change on plant productivity and ecosystem function.