Question

How does elevated ${\mathrm{CO}}_2$ influence rates of photosynthesis and transpiration?

Short answer

Answer: Elevated CO2 levels can increase the rate of photosynthesis and decrease the rate of transpiration in plants. This results in enhanced water-use efficiency, as the plants take in more CO2 for photosynthesis while conserving water. However, the overall impact on plant growth and productivity depends on interactions with other factors such as light intensity, temperature, and nutrient availability.

Step-by-step solution

Define photosynthesis and transpiration

Photosynthesis is the process by which green plants and some other organisms use sunlight to synthesize foods with the help of chlorophyll pigments, during which carbon dioxide (CO2) is taken in and oxygen (O2) is released. In this process, plants convert CO2, water, and light energy into oxygen and glucose, a sugar that serves as a source of energy for the plant. Transpiration is the process by which water is taken in by plant roots from the soil, transported throughout the plant, and eventually evaporated from the surfaces of leaves. This helps to cool the plant, transport nutrients, and maintain the state of the plant's cells.

Explain the role of CO2 in photosynthesis

Carbon dioxide is an essential component of the photosynthetic process. In the presence of light, CO2 combines with water (H2O) to form glucose and oxygen, through a series of chemical reactions. The CO2 concentration in the atmosphere affects the rate of photosynthesis. An increase in CO2 concentration generally causes an increase in the rate of photosynthesis, as more CO2 is available for the plants to use and convert into glucose and oxygen.

Discuss the effect of elevated CO2 levels on photosynthesis rates

When CO2 levels are elevated, the rates of photosynthesis tend to increase. This is because as CO2 concentration increases, the availability of this key photosynthetic reactant also increases, leading to a greater rate of CO2 being fixed into glucose and other organic molecules. However, there's a saturation point after which an increase in CO2 levels will not lead to a further increase in the rate of photosynthesis. This is because other factors, such as light intensity or the availability of other essential nutrients, may become limiting factors.

Explain the role of transpiration in plants

Transpiration plays an important role in the regulation of water balance, nutrient transport, and temperature management for plants. As water evaporates from the leaf surface, it helps maintain a constant flow of water and nutrients from the roots to the leaves. This water loss also helps to cool the plant, as the evaporation absorbs heat from the surrounding environment.

Discuss the effect of elevated CO2 levels on transpiration rates

Elevated CO2 levels can lead to a decrease in transpiration rates. This is because higher CO2 concentrations cause the stomata (small pores on the leaf surface) to partially close, reducing water loss through transpiration. By doing so, the plant conserves water while still taking in enough CO2 for photosynthesis. However, this reduced transpiration rate may also lead to an increase in leaf temperature, which could negatively affect plant growth and development, especially under conditions of high solar radiation. To summarize, elevated CO2 levels can result in an increase in photosynthesis rates and a decrease in transpiration rates, which can lead to enhanced water-use efficiency in plants. However, the overall impact on plant growth and productivity will depend on interactions with other factors such as light intensity, temperature, and nutrient availability.

Key concepts

Elevated CO2 Effects

Understanding the effects of elevated atmospheric carbon dioxide (CO2) levels on plant processes is crucial for predicting how plant life might change in the era of increasing greenhouse gases. Primary among these processes is photosynthesis, the fundamental mechanism by which plants convert light energy into chemical energy.

As CO2 concentrations rise, plants have more of the raw material they need for photosynthesis—carbon. This often results in a higher rate of photosynthesis, known as the CO2 fertilization effect. However, this isn't a straightforward boon for all plants; depending on other environmental factors like light availability, soil nutrients, and water, the actual benefit can vary significantly from species to species.

Photoinhibition and Stomatal Conductance

Additionally, at extremely high CO2 levels, a phenomenon called photoinhibition may occur, where the plant's photosynthetic machinery gets overwhelmed, actually reducing photosynthetic efficiency. Moreover, studies have shown that elevated CO2 can cause partial stomatal closure, which reduces water loss from the plant but can also limit the intake of CO2, effectively curbing the potential acceleration of photosynthesis.

Plant Growth and Development

Plant growth and development are tightly coupled with photosynthetic performance and water usage. Optimal conditions for these processes can bolster plant health and increase biomass production.

While elevated CO2 generally enhances photosynthetic rates, supporting better growth and robustness, the story doesn't end there. Growth also depends on plants' ability to transport water and nutrients via the transpiration stream. When elevated CO2 leads to reduced stomatal opening, transpiration decreases, which can slow nutrient uptake and potentially impact growth negatively, depending on the plant's water and nutrient needs.

Nutrient Concentration and Allocation

Furthermore, alterations in nutrient concentration and allocation within the plant can occur. This is because when CO2 levels are high, plants might allocate more resources towards above-ground structures (leaves and stems) rather than roots, sometimes leading to nutrient deficiencies. Another aspect to consider is the quality of the plant material produced; higher CO2 can lead to increased carbohydrate production but can dilute the nitrogen content, affecting the nutritional quality of the crops.

Water-Use Efficiency in Plants

Water-use efficiency (WUE) is a measure of how much biomass a plant produces per unit of water consumed. Elevated CO2 is known to increase WUE in many plant species. This results from the twin effects of increased photosynthesis (enhanced CO2 fixation) and decreased transpiration (reduced water loss), as already discussed.

The reduction in transpiration is primarily due to the partial closure of stomata in response to elevated CO2 conditions, which conserves water. Consequently, plants may grow better under drought-like conditions or in areas with limited water resources. However, it is also important to bear in mind that the reduction in transpiration, while conserving water, can lead to higher leaf temperatures and possibly stress the plant in hot conditions.

Drought Resistance and Plant Adaptation

Plants with improved WUE may exhibit better drought resistance, an increasingly valuable trait as climate change intensifies. The increased efficiency could mean that plants can sustain growth with fewer water resources, which has significant implications for agriculture and ecosystem sustainability. Nonetheless, the increase in WUE can be species-specific and influenced by other environmental factors such as soil type, humidity, and ambient temperature.