How Rising CO2 Levels Are Reshaping Plant Life - Part 1

by Aqui Griffin, 11/10/24

As atmospheric carbon dioxide (CO2) levels continue to rise, understanding the impacts on plant life becomes increasingly crucial. In this post we delve into the complex relationship between elevated CO2 and plant physiology, exploring how plants have evolved unique adaptations to fix carbon, and how they respond to the environmental changes brought on by Climate Change. In a later post I talk about the implications for our sustainable practices and the issues and risks modern agriculture poses. To read Part 2 now click here.

The Calvin Cycle & Plant Strategies

To begin let's familiarize ourselves with the science behind photosynthesis, specifically the carbon fixation process called the Calvin Cycle. This process happens within small structures called chloroplasts that can be found in any of the green tissues of a plant. They are especially abundant in the leaves where photosynthesis primarily occurs. These tiny structures perform the amazing task of turning CO2 into carbohydrates or sugars, which in turn fuels plant growth and contributes to the production of root exudates. The Calvin Cycle consists of 3 main stages: Carbon Fixation, Reduction, and Regeneration. We won't go into detail here, but feel free to learn more about this process here.

Instead we will focus on the photosynthetic pathways plants have developed to adapt to different environmental conditions. There are 3 commonly sited pathways: C3, C4, and CAM. Let's take a look at each one in detail.

C3 Plants

The C in the name stands for carbon and 3 is the number of atoms. So in this case, C3 refers to a set of 3-carbon molecules that are fixed during the Reduction step of the Calvin Cycle. C3 plants rely solely on this process and do not have any other adaptations for carbon fixation. They are the most common and evolutionarily oldest type of plants and perhaps the biggest contributors to the expansion of our own species on this planet. C3 plants include:

wheat
rice
soybean
oats
okra
tomato
potato
most trees & shrubs

C4 Plants

C4 plants have evolved a more complex carbon fixation pathway that concentrates CO2, thereby reducing photorespiration. C4 photosynthesis evolved much more recently, around 30-40 million years ago. It has evolved independently in at least 66 different plant lineages, and is a perfect example of convergent evolution. It's interesting to note that the emergence of C4 plants coincided with a period of declining atmospheric CO2 levels and increasing aridity, which may explain why C4 plants tend to do well in hot, dry environments with high light intensity. Some examples include:

maize (corn)
sugarcane
sorghum
purslane (both C4 and CAM)
lemongrass
malabar spinach
many tropical grasses

CAM Plants

CAM (Crassulacean Acid Metabolism) plants have adapted to extremely arid conditions. They are unique in that they can temporally separate CO2 fixation from the Calvin Cycle. They do this by fixing CO2 at night and store it as malic acid. Afterwards the CO2 is processed during the day when stomata are closed. Because of this, they are highly water-efficient but generally slower-growing. Like C4 photosynthesis, CAM has evolved independently multiple times in various plant lineages, but is especially concentrated in the cactus family, cactaceae. Examples include:

cacti
pineapples
agaves
aloe
purslane (both C4 and CAM)
many succulents

Photosynthetic Pathways Timeline vs. Atmospheric CO2 (Simplified)

Simplified representation of the approximate date (billions of years) of the emergence of different photosynthetic pathways against atmospheric CO2 concentration (ppm)

C2 Plants

Although not often mentioned, a few species are considered C2 plants. This type is considered an evolutionary intermediate between C3 and C4 pathways. C2 plants use both the C3 pathway and a limited CO2 concentrating mechanism. Scientists are studying this pathway with hopes it can be used to genetically modify more resilient C3 plants in the face of increasing atmospheric CO2. Some examples include:

Flaveria
Moricandia

A more comprehensive list can be found in this research paper that talks about C2 photosynthesis.

Effects of Increased CO2 on Different Plant Types

As atmospheric CO2 levels continue to rise, plants respond in various ways depending on their photosynthetic pathway. Understanding these responses is crucial and can help us design agricultural systems and practices that are appropriate for our specific climate. Now let's take a look at how each plant type responds.

C3 Plants

Based on current research C3 plants generally show the most pronounced response to elevated CO2 levels:

Enhanced growth: Many C3 plants exhibit increased biomass and yield under elevated CO2. In other words the extra CO2 acts like a lightweight fertilizer.
Improved water-use efficiency: Higher CO2 concentrations reduce photorespiration and stomatal conductance, reducing water loss and uptake.

However, the benefits may be severely limited by other factors:

Photosynthetic downregulation: boosted growth from CO2 may be temporary as downregulation occurs fairly quickly within 3-6 weeks.
Decreased Nutrition: The down-side of reduced water uptake, is reduced nutrient uptake leading to lower nutritional content
Decreased Sugars : In dry environments reduced photorespiration can lead to decreased carbohydrate formation.

C4 Plants

C4 plants, which already concentrate CO2, show a less dramatic response to elevated CO2:

Minimal direct CO2 effect: Photosynthesis in C4 plants is generally saturated at current CO2 levels.
Indirect benefits: C4 plants may still benefit from improved water-use efficiency under elevated CO2.

CAM Plants

CAM plants, adapted to arid conditions, show varied responses to increased CO2:

Moderate growth enhancement: Some CAM plants show increased biomass production under elevated CO2. Continue reading about it here.
Improved water-use efficiency: Like C3 and C4 plants, CAM plants may benefit from reduced water loss.
Potential shift in carbon uptake patterns: Elevated CO2 might alter the balance between daytime and nighttime carbon fixation in some CAM species.

C2 Plants

As an intermediate form, C2 plants' response to elevated CO2 falls between that of C3 and C4 plants:

Moderate photosynthetic enhancement: C2 plants may show some increase in photosynthetic efficiency, though less than C3 plants.

Interactions with Water Levels and Increased Temperatures

We should keep in mind, elevated CO2 will not happen in isolation. We must also be aware of changes in water availability and temperature. Both these factors along with CO2 may have positive and negative effects.

Precipitation and Climate Change - desert land represents drought

Water Availability

We are already seeing major changes to the hydrological cycle, but it is predicted that precipitation patterns will likely change such that places that already receive a lot of rain will get even more rain, and places that are drier will receive even less. This translates to more drought and flooding.

Improved drought resistance: Increased water-use efficiency under elevated CO2 can help plants better withstand drought conditions.
Altered hydrological cycles: Changes in plant water use can affect local and regional water cycles, potentially altering precipitation patterns. (Nature Climate Change)

Increased Temperatures

When it comes to heat, average seasonal temperatures will only increase for the next 30 years even with the immediate cessation of ghg emissions. You may even already see these impacts occurring in your region.

Extended growing seasons: Warmer temperatures, often associated with increased CO2, can lengthen growing seasons in temperate regions.
Heat stress: While CO2 can enhance growth, extreme temperatures can negate these benefits and cause heat stress in plants. Although C4 plants can tolerate higher heat stress than C3 plants, approximately 5 deg C higher tolerance, both plant types will be negatively effected if heat cannot be dissipated.

Complex Interactions

Finally, all these factors together can have a very unpredictable behavior.

CO2-temperature trade-offs: While elevated CO2 generally enhances growth, higher temperatures can increase respiration rates, potentially offsetting some CO2 benefits.
Altered plant-water relations: Changes in stomatal behavior due to elevated CO2 can interact with temperature effects on evapotranspiration, complicating predictions of plant water use.
Ecosystem-level effects: Changes in individual plant responses can cascade through ecosystems, affecting biodiversity, species interactions, and ecosystem services.

Conclusion

We just learned about how plants have evolved different strategies for fixing carbon from CO2 in the air; how each strategy responds to increased atmospheric CO2; and how combined effects from increased heat and water stress can further complicate these responses. As research progresses on plant responses to elevated CO2 and climate changes, it's crucial to apply this knowledge to promote ecological balance, support biodiversity, and ensure long-term agricultural viability. In the next post we will talk about how we can prioritize sustainable practices that consider the plant responses we just learned. Continue to Part 2.

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