During Earth’s history, CO2 levels in the atmosphere varied highly, as did the concentrations of other greenhouse gases and the average global temperature. These fluctuations affected life greatly, but life also affected them. During the Cambrian era, some 530 million years ago, plants first emerged on our planet. The Cambrian Earth was a lot different than the one we know and love today, with staggering levels of atmospheric CO2 that reached 7,000 ppm. Since then, we can see a general trend of CO2 level decline, with a few pronounced ups and downs during the Mesozoic. Fast forward to today, the CO2 levels are almost 17 times lower, and the Cambrian plants are long gone. Other species slowly took their place, the ones better adapted to lower CO2 and higher O2 levels.
Although today’s CO2 levels are at a “mere” 419 ppm, those are the highest CO2 levels Earth has seen in the last 14 million years. How are these increases affecting the plants exactly? The logic might lead you to believe that increasing CO2 levels are increasing photosynthesis and plant growth. While that is true to a degree, if we look at the whole picture, we’ll discover a bit more complex situation. CO2 assimilation and related processes within the plants are quite intricate, and 1+1 doesn’t always equal 2. Different plant species react differently and there are thresholds of the stimulating effect. To understand these complexities, we should delve a bit into the mechanisms of CO2 assimilation and find out exactly how plants absorb and use this gas.
The next section is going to be a bit technical, so if the mechanisms of CO2 are not particularly interesting to you, I recommend going straight to ‘How does elevated atmospheric CO2 affect plants?’.
How do plants assimilate CO2?
Plants use microscopic, lip-like pores that we call stomata to absorb and release gases – O2, CO2, and water vapor. On average, one square millimeter of leaf’s surface has about 300 stomata, and most of them are found on the lower, shaded side. By opening and closing the stomata, plants control the rate and direction of gas exchange in their tissues.
When talking about CO2 assimilation, we first need to differentiate between the two phases of photosynthesis – the light and the dark phase. The light phase begins with photons hitting the chloroplasts – the cell organelles in which photosynthesis takes place. The photons excite the photosynthetic pigment chlorophyll, which uses the acquired energy to produce molecules of NADPH and ATP, essential for the dark phase. The dark phase of photosynthesis is a complex chain of biochemical reactions that convert CO2 into sugars. These sugars are the primary product of photosynthesis and the building blocks of a variety of complex organic compounds necessary for the normal functioning of the plant. We call these complex compounds secondary products of photosynthesis. As its name implies, the dark phase doesn’t require light, but nevertheless happens in light and dark conditions.
Depending on the specific type of primary photosynthetic products, i.e., the exact sugars plants initially create, we can classify plants into three groups: C3, C4, and CAM plants.
The C3 pathway, widely known as the Calvin cycle, is the most widespread type of CO2 assimilation. About 85% of plant species use this photosynthetic pathway. The name C3 comes from the fact that the first stable compound these plants produce in the process has three carbon atoms. The name of this compound is glyceraldehyde-3-phosphate (GAP). Through a series of biochemical reactions, GAP condenses into more complex sugars – glucose and fructose. These six-carbon sugars are then further used as building blocks for even more complex compounds.
The key enzyme that makes this process possible, and photosynthesis in general, is RuBisCo – the most abundant enzyme on Earth. The activity of RuBisCo is quite interesting and complex, and we’ll talk a bit about it in a jiffy.
The C3 pathway comprises three phases:
- CO2 fixation – with the help of RuBisCo, CO2 attaches to the acceptor molecule (a sugar, Ru-1,5-BP), which then reacts with water to create two 3-phosphoglyceric acid (3-PGA) molecules
- C reduction – aided by NADPH and ATP (products of the light phase), GAP is synthesized from the simple 3-PGA molecules
- Regeneration of acceptor molecules – in order to maintain the continuity of the cycle, it is necessary to regenerate the acceptor molecules, which includes the transformation of different sugars into Ru-1,5-BP
C3 plants are most resource-efficient in wet and temperate climates, where water availability is rarely a problem, and where hot spells don’t last long. C3 plants include fruits, vegetables, various herbs, all trees, wheat, oats, barley, cannabis, sunflower, rice, cotton, tobacco, and many others.
Photorespiration is a process that sometimes follows photosynthesis, during which gas exchange occurs. Although it is similar to respiration, these two processes have different phases and take place at different sites. Also, photorespiration doesn’t have the same energy output as respiration since it doesn’t produce ATP. For this reason, and a few others, photorespiration is a wasteful process that can significantly decrease yield quality and quantity. The complex, dual activity of RuBisCo is the primary cause of this process.
RuBisCo has an affinity toward both CO2 and O2, with the affinity toward O2 being significantly weaker. In normal circumstances, O2 and CO2 are both present in the cell in specific concentrations, and CO2 is almost always the one to interact with RuBisCo and be processed further down the photosynthetic chain. However, if the concentration of O2 in the cells is higher than it should be, O2 molecules can outcompete CO2 for the spot on RuBisCo molecules. This oxygen excess often happens when the plant closes its stomata due to dry and/or hot conditions. RuBisCo then attaches the O2 to the acceptor molecule, as it would with CO2, and the result is a compound that cannot be included in the next steps of the Calvin cycle. Photorespiration is also referred to as the C2 pathway because the end product has two C atoms.
In a nutshell, photorespiration uses energy and resources to create a useless product. C3 plants have no efficient adaptations to help them deal with the dual nature of RuBisCo, making them susceptible to damage from this mechanism. However, some plant species discovered other pathways to assimilate CO2 and circumvent photorespiration – C4 and CAM plants.
C4 plants are different from C3 in a couple of ways. Firstly, light and dark phases of photosynthesis happen at distinct locations in the leaf tissues of C4 plants. The dark phase occurs in the specialized cells surrounding leaf nerves, while the light phase occurs in the spongy tissues. Secondly, C4 plants use phosphoenolpyruvate carboxylase (PEPc) instead of RuBisCo in the initial assimilation phases. PEPc has no affinity toward O2, so an increased oxygen concentration does not affect its activity.
The C4 pathway comprises four phases:
- CO2 fixation – the PEPc enzymes take the CO2 molecules and attach them to acceptor molecules; the first product of this reaction is unstable and quickly converted into malate
- Transport of C4 acids – malate travels from the spongy tissues to the specialized cells by the leaf nerves
- Decarboxylation of C4 acids – CO2 is separated from the malate and then its included in the Calvin cycle with the help of RuBisCo; after this step, the Calvin cycle continues as it would in C3 plants
- Regeneration of acceptor molecules – similar to the C3 pathway, C4 assimilation also necessitates the need for regeneration of acceptor molecules in order to maintain the process
Although giving C4 plants an advantage over C3 plants in arid and hot conditions, this adaptation comes with a price. It requires investing more energy compared to the C3 pathway. As such, C4 plants can be less energy and resource-efficient than C3 plants in temperate and cooler climates. However, in cases of limited water availability and hot temperatures, C4 plants far surpass C3 plants in resource efficiency, photosynthesis intensity, and productivity.
C4 plants are less abundant than C3 plants, but we can find their species across many different families. They include many members of the grass family (Poaceae) but also other families, like amaranths (Amaranthaceae), daisies (Asteraceae), sedges (Cyperaceae), spurges (Euphorbiaceae), borages (Boraginaceae), etc. Some widely cultivated species include corn, sorghum, switchgrass, and millet. Both terrestrial and aquatic plant species use the C4 assimilation pathway, and such species usually originate from tropical and subtropical regions.
The natural habitats of CAM plants are some of the most scorching, dry areas of planet Earth. No wonder these plants developed adequate structural adaptations – thick, succulent leaves and/or stems, leaves reduced to spines, etc. However, their physiological adaptations are of greater interest to us at the moment – particularly how they manage their CO2 assimilation. We talked about how the light and dark phases in C4 plants happen at different locations; in CAM plants, these two phases happen at different times.
During the night, when the temperature cools down and the Sun isn’t burning the entire landscape, CAM plants open their stomata to take in atmospheric CO2 and release O2. They convert the absorbed CO2 into malate or some other organic acid (C4 pathway) and store the products in their vacuoles. During the day, CAM plants perform photosynthesis (C3) but keep their stomata closed to minimize water loss. They achieve this by using the CO2 stored throughout the night and involving it directly in the Calvin cycle. Such gradual, controlled release of CO2 allows CAM plants to keep the RuBisCo molecules satiated with enough CO2 to prevent photorespiration while using minimal water.
The name CAM comes from the Crassulaceae Acid Metabolism acronym, as this type of CO2 assimilation was first discovered in the stonecrops family (Crassulaceae). However, besides stonecrops, CAM plants include members of other families, like cacti (Cactaceae), lilies (Liliaceae), daisies (Asteraceae), and spurges (Euphorbiaceae). Among the most commonly cultivated CAM plants are pineapple, opuntia, vanilla, aloe, and agave. Numerous ornamentals also belong to this group (kalanchoe, wax plant, jade plant, snake plant, etc.).
There are plant species that defy the simple C3, C4, and CAM classification. These plants use the C3 or C4 pathway in normal conditions, but when exposed to stress, most commonly drought, they switch to the CAM pathway. We call this mechanism facultative CAM. Examples of such plants include the common iceplant (Mesembryanthemum crystallinum), common purslane (Portulaca oleracea), and waterleaf (Talinum fruticosum). In optimal conditions, the common iceplant uses the regular ol’ C3 pathway. However, if drought, heat, or salt stress strikes, it switches to the CAM pathway.
How does elevated atmospheric CO2 affect plants?
Global mean CO2 levels almost doubled since the last Ice Age, and most of the increase happened over the last 250 years. Before the advent of the First Industrial Revolution in the mid-18th century, CO2 levels were sitting around 280 ppm. Three fossil-fuel-hungry industrial revolutions later, we are well on our way to double the pre-Industrial era levels. If we put the global mean CO2 levels from the 1750s to the 2020s in a graph, we can see an exponential increase.
According to the most recent data from May 2022, the current mean CO2 levels in the atmosphere are at a staggering 421 ppm. The trend of CO2 increase in the last 60 years has been 100 times faster than the trend of natural increase between the last ice age and the pre-Industrial era. And it keeps on going. By some estimations, CO2 levels will reach 900 ppm by the end of the 21st century.
How are the plants handling this state of affairs? Initially, most researchers were very optimistic, estimating that elevated CO2 levels will intensify the rate of photosynthesis. More intensive photosynthesis means that the plants are taking up more CO2, growing faster and bigger. And of course, they are sequestrating the carbon in the process, removing it from the atmosphere and helping us reverse the effects of climate change. Sounds pretty simple and straightforward, but is it like that actually? We’ve come to learn that the reality is a bit more complex.
The double-edged sword
Plants exposed to elevated CO2 levels display certain physiological and chemical changes within their cells, as well as structural modifications of the tissues. Some of these changes can be very advantageous and help plants adapt to climate change, while some can get very problematic. CO2 concentration impacts a very broad set of functions in plants, so it can get very challenging to see where the cause and effect of one function end and the other begin. We’ll start with the functions that are primarily affected by CO2 increase – respiration and photosynthesis, as the shifts in their activity are the ones to cause a cascade of changes in other processes.
Plants respond to elevated CO2 levels by reducing their stomatal conductance, which means that they close their stomata and reduce gas exchange. As the CO2 concentration increases in leaves, so does the acidity of the plant’s cells, causing the stomata to close. This mechanism helps the plants to lose less water and increase water use efficiency – a measure that shows how much biomass is produced per unit of water used in the process. Since the elevated CO2 levels are creating an increasingly hotter and drier climate, this mechanism will be vital in helping plants cope with environmental changes. Changes in respiration are most pronounced in C3 plants, which is very fortunate since they are the most sensitive to hot and dry conditions.
The increase in photosynthetic activity is the most evident impact of the elevated CO2 levels on plants. Plants exposed to CO2 concentrations of 475-600 ppm increase their photosynthetic activity by 40% on average. Such an increase leads to the intensive production of carbohydrates. The resulting abundance of carbohydrates allows the plants to build more biomass and grow faster. Indeed, in experimental conditions, plants grown under elevated CO2 levels were more robust than the ones grown under regular conditions. They had a greater root-to-shoot ratio and achieved higher yields.
However, this proliferation comes at a cost. The elevated carbohydrate content decreases nitrogen levels in plant tissues. This is most likely an outcome of a combination of factors – decreased uptake of minerals from the soil, dilution of nitrogen in the carbohydrate-rich solution, allocating nitrogen to the regeneration of RuBisCo, and decreased water uptake which affects the process of transforming nitrates into organic compounds.
Nitrogen is the key element for building proteins and many essential organic compounds, so what this information implies is that our crops, and most other plants, will get bigger but less nutritious as the CO2 levels increase. The high carbohydrate content increases the quantity, but not the quality of the plant biomass. Empirical studies have shown that protein levels of wheat, rice and barley grains, as well as potato tubers, decrease under elevated CO2 levels by 5–14%. Concentrations of 25 nutritionally-important minerals like potassium, magnesium, calcium, iron, and zinc also show a significant decline.
This is very concerning information considering the current population trend and increasingly limited fertile agricultural land. Not just us humans, but also our livestock and all wild herbivores will need to eat more plants to satisfy their daily nutritional needs.
Differences between C3, C4 and CAM plants
When talking about the general impact of elevated CO2 levels, many researchers present general estimations, which can be very misleading. Since C3, C4, and CAM plants manage their CO2 assimilation differently, they do not react the same to CO2 increase. The vast amount of studies that talk about the aforementioned increase in photosynthesis and decrease in respiration mostly refer to C3 plants, as most of the research efforts were focused on them as the most economically important group. C4 plants are another story. Due to their specific CO2 assimilation process, C4 plants are able to maintain the high CO2 levels in the area of the cell where RuBisCo is active, so the increase in the atmospheric CO2 has very little effect on them, and no effect after a certain point.
The situation gets very interesting when the C3 and C4 plants interact under elevated CO2 levels. Studies that have looked into the competition of C3 and C4 plants concluded that C3 plants are far more advantageous in these conditions. Although the root-to-shoot ratio increased in both types, C3 plants showed a significant increase in biomass and more powerful competitive ability. However, it is important to note that not all C3 plant species are equally efficient at adapting to elevated CO2 levels. For example, legumes create a symbiotic relationship with nitrogen-fixing bacteria that provide them a steady source of nitrates in return for sugars. As such, legumes can mitigate the suppressive effects of high carbohydrate concentration and maintain their nitrogen levels.
The response of CAM plants to elevated CO2 levels is more similar to C3 than to C4 plants. At 700 ppm, CAM plants increase their biomass production on average by 35%, conduct photosynthesis more efficiently and display enhanced water use efficiency.
These are all very interesting findings, but we have still a lot to learn before we fully understand how will the increasing CO2 levels affect plants and their relationships with other organisms and each other. What we know for certain is that elevated CO2 changes the chemical content and physiological functions of plants in different ways, depending on the CO2 assimilation pathway and specific attributes of different species. C3 and CAM plants show a significant increase in photosynthetic activity, biomass, and drought tolerance, while these effects are far more limited in C4 plants.
High carbohydrate content in C3 plants grown under elevated CO2 affects the nutritive qualities of their fruits and seeds, as it interrupts the absorption of nitrogen and essential minerals. Planning the food production of the future and mitigating climate change must take into account these effects, as they will only become more prominent in the decades to come.
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