8.6 Impact of Environmental Change on Plant Physiology

Environmental Responses and Adaptations

The form of photosynthesis that we discussed in Chapter 7 is better known as C3 photosynthesis based on the fixation of CO₂ into 3-phosphoglycerate (a 3-carbon molecule), the first product of the Calvin cycle (see 7.4 Photosynthesis: Calvin Cycle for more details). C3 plants struggle in hot, arid environments where they keep their stomata closed to limit water loss. This results in excess O₂ levels trapped in the leaves that compete with CO₂ for Rubisco and interfere with the efficiency of the Calvin Cycle. Certain plants have evolved adaptations to increase their photosynthetic efficiency in these conditions. Desert plants, such as CAM plants, have adapted to only open their stomata at night. This prevents water loss during the day, where extreme heat would quickly dry out the leaf’s reserves. CAM plants take in CO₂ at night and store it in the form of an organic acid. During the daytime, the plant can perform photosynthesis without having to open its stomata. Common examples of CAM plants include cacti and pineapples.

C4 plants have also adapted to perform photosynthesis while in hot and dry conditions. While most plants fix CO₂ from the atmosphere and convert it to a three carbon molecule, the C4 plant converts it into a four carbon molecule using an enzyme in the mesophyll called PEP carboxylase. This small molecular tweak allows the C4 plant to deliver CO₂ in high concentrations to the enzyme Rubisco, located in bundle sheath cells that are away from where O₂ levels accumulate when stomata are closed.

Both C4 and C3 are important crops, and changing CO₂ levels may impact their productivity differently. C3 plants, such as rice and wheat, tend to benefit more from rising atmospheric CO₂ concentrations. This increase in carbon can potentially increase photosynthetic efficiency. In contrast, C4 plants like maize and sugarcane may not show as significant a response. Concentrating CO₂ internally by its complex physiology would imply added CO₂ is of no benefit.

Video 8.6.1. Photosynthesis: Comparing C3, C4 and CAM by RocochetScience

FACE Experiments

For the last three decades across the globe, Free Air Carbon Enrichment (FACE) experiments have exposed plants to heightened levels of carbon dioxide to study how ecosystems will respond to future atmospheric levels. FACE experiments help scientists observe changes in plant growth, water use, behavior, and nutrient cycling under close-to-identical environmental conditions. Thus far, results have shown that while increased CO₂ can boost photosynthesis and growth, responses vary widely depending on species, soil nutrients, and water availability.

Video 8.6.2. Free-Air CO2 Enrichment Experiments by Methods in Soil Organic Matter and Biogeochemistry

Figure Descriptions

Figure 8.6.1. The image is a comparative diagram illustrating the processes of photosynthesis in C3, C4, and CAM plants. It is divided into three sections, each labeled at the top with the type of plant. On the left, the C3 Plants section has a CO2 circle at the top, with an arrow pointing downward to a rectangular cell labeled “Mesophyll,” which is shaded green. Inside the cell, the Calvin Cycle is depicted with circular arrows. At the bottom, there is an orange oval labeled “Sugar.” In the center, the C4 Plants section features a similar CO2 circle at the top, with an arrow leading to a rectangular cell labeled “Mesophyll.” This cell is divided into two sections; the top is light green and contains another rectangle labeled “CA,” while the bottom is yellowish and leads to another cell labeled “Bundle Sheath,” which houses the Calvin Cycle depicted with circular arrows. An arrow connects the Bundle Sheath to an orange oval labeled “Sugar.” On the right, the CAM Plants section shows a CO2 circle with an arrow pointing down to a blue-shaded “Mesophyll” cell. Inside, a “CA” rectangle is present with adjacent night and day symbols, indicating a temporal separation of processes. The cell also leads to the Calvin Cycle depicted with circular arrows, and an arrow continues to the “Sugar” oval. [Return to Figure 8.6.1]