C4 plants possess special adaptations that enable them to minimize photorespiration. Examples of such crops include crabgrass, sugarcane and corn.
This is especially important in hot climates, where high temperatures can lead to excessive photorespiration. Under such circumstances, the C4 pathway becomes an efficient means of fixing carbon dioxide and producing sugars.
They do not carry out the Calvin cycle
C4 plants (such as maize, sorghum and sugarcane) boast approximately 50% greater photosynthesis efficiency than their C3 counterparts due to a different method for fixing carbon dioxide called the C4 pathway.
C4 plants begin by fixing atmospheric CO2 into their mesophyll cells in their leaves. They then use these carbon molecules as fuel for the Calvin cycle.
To accomplish this goal, researchers first must convert mesophyll CO2 to an organic acid known as oxaloacetate which is converted by PEP carboxylase – a non-rubisco enzyme found deep inside leaves. From there, malate is formed using bundle-sheath cells.
Once this occurs, oxaloacetate is broken down to two 3-carbon molecules and rubisco is then used in the Calvin cycle to create sugars from these two compounds. This process is sometimes referred to as the “standard” Calvin cycle due to its prevalence across all plants.
After this, the two 3-carbon molecules are split and each produces two more molecules of 3-phosphoglycerate, a type of sugar plants can use in their tissues. This process of breaking down oxaloacetate to make these two 3-carbon molecules is known as carboxylation and plays an essential role in the Calvin cycle.
Oxaloacetate is converted to glyceraldehyde 3-phosphate, a glycolytic molecule plants can use in their tissues for energy production. This glyceraldehyde then breaks down into pyruvate for further use in the Calvin cycle’s subsequent step: cell energy production.
Finally, pyruvate is oxidized to acetyl coenzyme A – a type of molecule which can be utilized by cells for creating proteins and other molecules. This process is similar to how carbohydrates are utilized in the Krebs cycle.
Combining these two processes is highly inefficient and that’s why many C3 plants have developed special adaptations that minimize photorespiration. Some of these organisms can separate the two steps in time so one takes place at night and the other during daytime; this is known as crassulacean acid metabolism (CAM for short).
They use a more efficient enzyme
C4 plants use an efficient enzyme to photosynthesize with no apparent photorespiration, giving them one major advantage over C3 plants. This enables C4 plants to grow faster in conditions which promote photorespiration in C3s such as drought or hot climates.
C4 plants achieve this by localizing Rubisco into a bundle sheath compartment and separating Calvin cycle from rubisco in mesophyll cells (Welkie and Caldwell, 1970). This separation allows for CO2-concentrating mechanisms which reduce oxygenase activity of rubisco while minimizing photorespiration’s inhibitory effects.
The initial step of carbon fixation is carried out by an enzyme distinct from rubisco, called phosphoenolpyruvate carboxylase (PEPc). PEPc lacks oxygenase activity and has a much higher affinity for CO2 than rubisco, increasing the number of ATPs fixed per mole of CO2, thus improving C4 photosynthesis efficiency.
The increased ATP consumption is offset by an increased rate of biomass production. Furthermore, PEPc increases malate concentration in mesophyll cells, which can then be transported into bundle sheath cells where Rubisco can fix it into sugars.
Once malate is produced, it is broken down into pyruvate by the Calvin cycle and returned to the mesophyll cell. Pyruvate then undergoes NADP-dependent malate dehydrogenase for conversion to glucose and other carbohydrates within the Calvin cycle.
However, this process can also lead to a decrease in photosynthesis efficiency if there is an excess of oxygen present in the leaf. In such instances, another enzyme called phosphate dikinase is employed to break down pyruvate back into malate.
In warm climates and arid habitats, atmospheric CO2 concentrations often lead to substantial inhibition of photosynthesis by photorespiration in C3 plants. To combat these challenges, many C4 plant species developed CO2-concentrating mechanisms which enhance photosynthesis efficiency while decreasing oxygenase activity of Rubisco, enabling them to outcompete C3 species and spread across the planet – leading to the rise of native C4 species like corn or sorghum.
They are adapted to cold climates
C4 plants are able to photosynthesize with no apparent photorespiration (loss of energy from the cell’s internal processes) because they have developed a CO2-concentrating mechanism. This is achieved through a unique leaf anatomy and biochemistry that allows C4 plants to bind carbon dioxide at the leaf surface and produce a 4-carbon compound that transfers and concentrates carbon dioxide in specific cells around the Rubisco enzyme, significantly improving their photosynthetic and water use efficiency.
The C4 cycle is a two-stage process that begins in the mesophyll of the plant leaf. It involves the fixation of CO2 to phosphoenolpyruvate by PEP-carboxylase and the formation of a 4-carbon intermediate, typically malate (malic acid). This 4-carbon intermediate is then actively pumped across the cell membrane into a thick-walled bundle sheath cell where it is decarboxylated.
This action creates a CO2-rich environment in the mesophyll that is adapted to be diffused through plasmodesmata into the thick-walled bundle sheath cells where Rubisco is localized. This enables Rubisco to act more efficiently in a CO2-rich environment, and thus suppresses photorespiration.
In addition, the mesophyll and bundle sheath are differentiated from each other by a diffusive barrier between the chloroplasts of the mesophyll and the cytosol of the bundle sheath. This separation creates a space for CO2 to be centered on RuBisCO where the oxygenase reaction is negligible, thus avoiding photorespiration by increasing the concentration of CO2 in this area.
In warm climates, the additional energy cost of concentrating CO2 in this location is compensated for by the lower water and nitrogen demands of the resulting C4 metabolism. This adaptation is thought to be one of the reasons why C4 plants can dominate open landscapes in areas with high temperatures, such as tropical grasslands or savannas. This is because the higher temperatures enable them to grow faster and more vigorously. These plants are also more efficient at utilizing sunlight than C3 plants. They are particularly suited to the hot and arid regions of the planet. This makes them ideal weeds, as they can spread rapidly and aggressively. They are also able to withstand extreme weather conditions, such as drought and salinity.
They exclude oxygen from their tissues
C4 plants do not utilize the Calvin cycle, instead using PEP carboxylase to fix CO2. This adaptation allows them to thrive in cold and wet climates by conserving water more efficiently.
Photorespiration competes with carboxylation for oxygen, and as the ratio of photochemistry to oxygenation decreases at low temperatures and CO2 partial pressures , plants’ photosynthetic efficiency declines. Furthermore, oxygenation generates toxic intermediates which must be metabolized through photorespiration before they can be consumed or recycled and carbon recovered.
C4 plants’ remarkable ability to avoid photorespiration allows them to operate at higher efficiency in hot and dry climates with low photosynthetic rates but limited water availability. This advantage stems from their spatial separation of initial assimilation and Calvin cycle stages.
This type of cell anatomy, commonly seen on C4 leaves, is known as ‘Kranz anatomy’. It features an enlarged BSC relative to its major cell, close vein spacing and a dimorphic form of chlorenchyma with one layer at the outermost edge.
These plants can metabolize glycolate and glycine in a light-independent, O2-sensitive manner (Voznesenskaya et al., 2001). The enzymes involved are NADP-malic and PEP carboxylase.
Hydrilla verticillata uses a c4-like CO2-concentrating mechanism in its aquatic plant to fix bicarbonate and produce malate. The produced CO2 is then fixed by Rubisco through the Benson-Calvin cycle.
However, the C4-like CO2-concentrating mechanism requires a long diffusion path from the inner end of the cell to its outermost point, and chloroplast lipid bilayer can only diffuse CO2 so far. This explains why some c4 plants can tolerate lower CO2 partial pressures than others.
No matter how the C4 system functions in air, studies have demonstrated that plants with C4 systems can produce CO2 more concentrated than their C3 counterparts do. This is likely because c4 plants use their leaf phospholipids to create membranes highly permeable to CO2.
These mechanisms have been demonstrated to be beneficial in a variety of habitats, such as open tropical rainforests. They increase photosynthesis and decrease water loss from plants while decreasing their tissue cost – making them more competitive in shaded conditions. These adaptations have played an integral role in the evolution of the C4 pathway.