Transpiration

Transpiration is the loss of water from the plant through evaporation at the leaf surface. The atmosphere to which the leaf is exposed drives transpiration, but also causes massive water loss from the plant. Up to 90% of the water taken up by roots may be lost through transpiration. Transpiration is the main driver of water movement in the xylem.

Leaves are covered by a waxy cuticle on the outer surface that prevents the loss of water. Regulation of transpiration, therefore, is achieved primarily through the opening and closing of stomata on the leaf surface (Figure 19.1). Stomata are surrounded by two specialized cells called guard cells, which open and close in response to environmental cues such as light intensity and quality, leaf water status, and carbon dioxide concentrations (Figure 19.1). Guard cells swell or shrink in response to osmotic changes. Stomata must open to allow air containing carbon dioxide and oxygen to diffuse into the leaf for photosynthesis and respiration. When stomata are open, water vapor is lost to the external environment, increasing the rate of transpiration. Therefore, plants must maintain a balance between efficient photosynthesis and water loss. The stomata are typically located on the underside of the leaf, which helps to minimize water loss due to high temperatures on the upper surface of the leaf.

Plants have evolved over time to adapt to their local environment and reduce transpiration. Desert plant and plants that grow on other plants have limited access to water. Such plants usually have a much thicker waxy cuticle than those growing in more moderate, well-watered environments. Aquatic plants also have their own set of anatomical and morphological leaf adaptations.

Main Structures and Summary of Photosynthesis

Photosynthesis is a multi-step process that requires specific wavelengths of visible sunlight, carbon dioxide (which is low in energy), and water as substrates (Figure 19.2). After the process is complete, it releases oxygen and produces glyceraldehyde-3-phosphate (G3P), as well as simple carbohydrate molecules (high in energy) that can then be converted into glucose, sucrose, or any of dozens of other sugar molecules. These sugar molecules contain energy and the energized carbon that all living things need to survive.

In plants, photosynthesis generally takes place in leaves, which consist of several layers of cells. The process of photosynthesis occurs in a middle layer called the mesophyll. The gas exchange of carbon dioxide and oxygen occurs through stomata, which also play roles in the regulation of gas exchange and water balance.

Photorespiration

Gas exchange poses a significant challenge to plants because oxygen is toxic to the photosynthetic process. A critical part of photosynthesis is carbon fixation – when carbon is converted from a gas (CO₂) to be incorporated as part of an organic molecule. The carbon fixation step is catalyzed by a very important enzyme called RuBisCO (which is an acronym for its full name, ribulose-1,5-bisphosphate carboxylase/oxygenase). Rubisco is so important to plants that it makes up 30% or more of the soluble protein in a typical plant leaf.

But rubisco also has a major flaw: instead of always using CO₂ as a substrate, it sometimes picks up O₂ instead. This side reaction initiates a pathway called photorespiration, which, rather than fixing carbon, actually leads to the loss of already-fixed carbon as CO₂. Photorespiration wastes energy and decreases sugar synthesis. Thus, there are strong selective pressures on plants to minimize photorespiration by keeping oxygen away from rubisco. 

Photosynthesis Variations

In response to selective pressures to minimize photorespiration, variations on the photosynthetic process have evolved. The variations on the photosynthetic process are called C3, C4, and CAM. These variations are directly relevant to global climate change because these processes affect how plants respond to changes in atmospheric carbon dioxide concentration and changes in temperature and water availability. Humans are currently dependent on plant species that do not thrive in hotter, dryer, and more erratic conditions. As the planet continues to warm up, researchers have begun exploring ways in which plants can be adapted to the changing environment. Modifying the photosynthesis processes may be one way to do that.

C3 Plants

The vast majority of land plants we rely on for human food and energy use the C3 pathway, which is the oldest of the pathways for carbon fixation, and it is found in plants of all taxonomies.

• Species: Grain cereals such as rice, wheat, soybeans, rye, and barley; vegetables such as cassava, potatoes, spinach, tomatoes, and yams; trees such as apple, peach, and eucalyptus
• Enzyme: Ribulose bisphosphate carboxylase oxygenase (Rubisco)
• Process: Convert CO₂ into a 3-carbon compound 3-phosphoglyceric acid (or PGA)
• Where Carbon Is Fixed: All leaf mesophyll cells
• Biomass Rates: -22% to -35%, with a mean of -26.5%

While the C3 pathway is the most common, it is also inefficient. Rubisco reacts not only with CO₂ but also O₂, leading to photorespiration, a process that wastes assimilated carbon. Under current atmospheric conditions, potential photosynthesis in C3 plants is suppressed by oxygen as much as 40%. The extent of that suppression increases under stress conditions such as drought, high light, and high temperatures. As global temperatures rise, C3 plants will struggle to survive—and since we’re reliant on them, so will we.

C4 Plants

Only about 3% of all land plant species use the C4 pathway, but they dominate nearly all grasslands in the tropics, subtropics, and warm temperate zones. C4 plants also include highly productive crops such as maize, sorghum, and sugar cane. While these crops lead the field for bioenergy, they aren’t entirely suitable for human consumption. Maize is the exception, however, it’s not truly digestible unless ground into a powder. Maize and other crop plants are also used as animal feed, converting the energy to meat—another inefficient use of plants.

• Species: Common in forage grasses of lower latitudes, maize, sorghum, sugarcane, fonio, tef, and papyrus
• Enzyme: Phosphoenolpyruvate (PEP) carboxylase
• Process: Convert CO₂ into 4-carbon intermediate
• Where Carbon Is Fixed: The mesophyll cells and the bundle sheath cells. C4 plants have a ring of bundle sheath cells surrounding each vein and an outer ring of Mesophyll cells surrounding the bundle sheath.
• Biomass Rates: -9 to -16%, with a mean of -12.5%.

C4 photosynthesis is a biochemical modification of the C3 photosynthesis process in which the C3 style cycle only occurs in the interior cells within the leaf. Surrounding the leaves are mesophyll cells that contain a much more active enzyme called phosphoenolpyruvate (PEP) carboxylase. As a result, C4 plants thrive on long growing seasons with lots of access to sunlight. Some are even saline-tolerant, allowing researchers to consider whether areas that have experienced salinization resulting from past irrigation efforts can be restored by planting salt-tolerant C4 species.

CAM Plants

CAM photosynthesis was named in honor of the plant family in which Crassulacean, the stonecrop family or the orpine family, was first documented. This type of photosynthesis is an adaptation to low water availability and occurs in orchids and succulent plant species from arid regions.

In plants employing full CAM photosynthesis, the stomata in the leaves are closed during daylight hours to lessen evapotranspiration and open at night in order to take in carbon dioxide. Some C4 plants also function at least partially in C3 or C4 mode. In fact, there’s even a plant called Agave Angustifolia that switches back and forth between modes as the local system dictates.

• Species: Cactuses and other succulents, Clusia, tequila agave, pineapple.
• Enzyme: Phosphoenolpyruvate (PEP) carboxylase
• Process: Four phases that are tied to available sunlight, CAM plants collect CO₂ during the day and then fix CO₂ at night as a 4 carbon intermediate.
• Where Carbon Is Fixed: Vacuoles
• Biomass Rates: Rates can fall into either C3 or C4 ranges.

CAM plants exhibit the highest water-use efficiencies in plants which enable them to do well in water-limited environments, such as semi-arid deserts. With the exceptions of pineapple and a few agave species, such as the tequila agave, CAM plants are relatively unexploited in terms of human use for food and energy resources.

C3 to C4 Adaptation

The evolutionary process that changed C3 plants into C4 species has occurred not once but at least 66 times in the past 35 million years. This evolutionary step led to enhanced photosynthetic performance and increased water- and nitrogen-use efficiency.

As a result, C4 plants have twice as the photosynthetic capacity as C3 plants and can cope with higher temperatures, less water, and available nitrogen. It’s for these reasons, biochemists are currently trying to find ways to move C4 and CAM traits (process efficiency, tolerance of high temperatures, higher yields, and resistance to drought and salinity) into C3 plants as a way to offset environmental changes faced by global warming.

At least some C3 modifications are believed possible because comparative studies have shown these plants already possess some rudimentary genes similar in function to those of C4 plants. While hybrids of C3 and C4 have been pursued more than five decades, due to chromosome mismatching and hybrid sterility success has remained out of reach.

References

Adapted from:

Hirst, K. Kris. “Adaptations to Climate Change in C3, C4, and CAM Plants.” ThoughtCo, Apr. 1, 2025, thoughtco.com/c3-c4-cam-plants-processes-172693.

Clark, M.A., Douglas, M., and Choi, J. (2018). Biology 2e. OpenStax. Retrieved from https://openstax.org/books/biology-2e/pages/1-introduction

“Photorespiration” Khan Academy, Retrieved from https://www.khanacademy.org/science/biology/photosynthesis-in-plants/photorespiration–c3-c4-cam-plants/a/c3-c4-cam-plants