Elevated CO₂ Effects on Plant Growth: Key Insights from FACE Experiments

Introduction

The atmospheric concentration of CO₂ was stable at approximately 270 parts per million (ppm) for at least 1000 years before the Industrial Revolution. However, since then, CO₂ levels have been rising rapidly. In 2009, the concentration had reached 384 ppm, a 40% increase from historical levels[1]. Projections suggest that this could exceed 550 ppm by the middle of the 21st century and 700 ppm by the century's end[1].

Understanding how plants respond to these changes is crucial. Free-Air CO₂ Enrichment (FACE) experiments are essential as they expose plants to elevated CO₂ concentrations in natural settings over extended periods. Here are six important lessons learned from nearly two decades of FACE studies[1].

Enhanced Photosynthesis in C₃ Plants

title: 'Fig. 1. Average plots of A/Ci response curves for Trifolium repens grown at ambient [CO2] (;365 lmol mol�1) and elevated [CO2] (;600 lmol mol�1) during spring (May 2001) and autumn (October 2000) at the Swiss FACE array in Eschikon, Switzerland (adapted from Ainsworth et al., 2003b and reproduced by kind permission of Oxford University Press). The maximum RuBP-saturated rates of carboxylation in vivo (Vc,max) were estimated from the initial slopes of each curve, and the maximum in vivo rates of electron transport contributing to RuBP regeneration (Jmax) were estimated using points after the inflection. The black dashed lines indicate the supply functions and resultant photosynthetic rates for plants measured in October, while the grey dashed lines indicate the supply function and resultant photosynthetic rates for plants measured in May.'
title: 'Fig. 1. Average plots of A/Ci response curves for Trifolium repens grown at ambient [CO2] (;365 lmol mol�1) and elevated [CO2] (;600 lmol mol�1) during spring (May 2001) and autumn (October 2000) at the Swiss FACE array in Eschikon, Switzerl...Read More

One key finding is that elevated CO₂ stimulates photosynthetic carbon gain in C₃ plants despite a process known as acclimation. Acclimation is characterized by a decrease in the maximum carboxylation rate of Rubisco (Vc,max) and the electron transport rate for ribulose-1,5-bisphosphate (RubP) regeneration (Jmax)[1]. However, even with these reductions, the overall rate of photosynthetic carbon uptake (Asat) saw a marked increase. This is particularly true for species limited by Rubisco capacity, such as trees and grasses, which showed more significant photosynthesis increases compared to legumes and other functional groups[1].

Improved Nitrogen Use Efficiency

Table 1. Estimates of the maximum theoretical saving in leaf N due to Rubisco acclimation at the elevated [CO2] used in FACE experiments
Table 1. Estimates of the maximum theoretical saving in leaf N due to Rubisco acclimation at the elevated [CO2] used in FACE experiments

The second lesson from FACE studies is the improvement in Photosynthetic Nitrogen Use Efficiency (PNUE) in C₃ plants. Theory suggests that as photosynthesis increases, nitrogen use should become more efficient. Findings reveal that PNUE indeed increases by about 31%, driven by elevated photosynthesis rather than a significant reduction in leaf nitrogen content[1]. While the maximum potential nitrogen savings due to down-regulated Rubisco were initially overestimated, the actual figures are lower but still significant[1].

Reduced Water Use

Plants grown in elevated CO₂ have consistently shown reduced stomatal conductance (gs), which leads to lower water use[1]. Evidence from FACE experiments shows a consistent decrease in canopy evapotranspiration (ET) by 5% to 20%, depending on the species and environmental conditions[1]. For example, decreased gs in soybean leaves led to reduced whole-canopy water usage, ensuring higher soil moisture availability, particularly beneficial during drought periods[1].

Increased Dark Respiration in Soybeans

title: 'Fig. 3. Graphical representation of transcripts encoding enzymes of carbohydrate metabolism and respiration whose abundance is significantly altered by growth at elevated [CO2] during the day and night in the leaves of soybean grown at SoyFACE. Each blue or yellow box represents the statistically significant treatment response (P <0.05) of a unique transcript encoding an enzyme or protein structure. Insets show mean treatment values (6SE) of the (A) night-time rates of respiratory CO2 efflux and (B) respiratory O2 uptake of soybean leaves grown at ambient and elevated [CO2]. Means sharing a common letter are not statistically different. All data are average responses to elevated [CO2] (550 lmol mol�1) compared to ambient [CO2] (380 lmol mol�1), redrawn from data in Ainsworth et al. (2006) and Leakey et al. (2008).'
title: 'Fig. 3. Graphical representation of transcripts encoding enzymes of carbohydrate metabolism and respiration whose abundance is significantly altered by growth at elevated [CO2] during the day and night in the leaves of soybean grown at SoyFAC...Read More

Dark respiration, the process by which plants break down carbohydrates and produce CO₂ in the dark, was stimulated in soybean leaves grown at elevated CO₂[1]. This was attributed to greater gene expression relating to enzymes involved in carbohydrate metabolism and respiration. Along with increased photosynthesis, this stimulated dark respiration due to more abundant carbohydrate substrates[1]. While this result was particularly evident in soybeans, other species exhibited varied responses[1].

Limited Direct Effects on C₄ Photosynthesis

title: 'Fig. 4. Comparing the dependence of photosynthesis (A) on intercellular CO2 concentration (Ci) for C4 maize and C3 soybean. The maize data were fit using the equations for C4 photosynthesis (von Caemmerer, 2000). The soybean data fit according to the C3 leaf biochemical model of photosynthesis of Farquhar et al. (1980). The grey lines illustrate the supply function for CO2, starting at the atmospheric [CO2] and ending at the operating Ci of photosynthesis. This is illustrated for current atmospheric [CO2] (384 lmol mol�1, solid line), elevated [CO2] anticipated for 2050 (550 lmol mol�1, dashed line), and elevated [CO2] anticipated for 2080 (700 lmol mol�1, dashed and dotted line). This figure was redrawn using data from Leakey (2009).'
title: 'Fig. 4. Comparing the dependence of photosynthesis (A) on intercellular CO2 concentration (Ci) for C4 maize and C3 soybean. The maize data were fit using the equations for C4 photosynthesis (von Caemmerer, 2000). The soybean data fit according...Read More

Contrary to C₃ plants, the direct stimulation of photosynthesis in C₄ plants by elevated CO₂ is negligible. This is because C₄ photosynthetic pathways are already saturated at current atmospheric CO₂ levels[1]. Nonetheless, under drought conditions, elevated CO₂ indirectly enhances photosynthesis in C₄ plants by conserving water and delaying drought stress. For instance, Sorghum and maize benefited from improved water status under dry conditions, leading to better photosynthetic performance and crop yields[1].

Smaller Than Expected Crop Yield Increases

FACE studies have shown that crop yield stimulations in response to elevated CO₂ are smaller than previously expected from theories or controlled environment experiments[1]. For major crops like soybean, the increase in light-saturated photosynthesis and daily carbon uptake at elevated CO₂ fell short of predictions made by chamber studies[1]. While theoretical projections suggested a significant boost in productivity with rising CO₂ levels, FACE results showed more modest increases. This discrepancy between theory and real-world data has profound implications for future food supply projections, highlighting the need for better understanding and optimization of crops under future atmospheric conditions[1].

Conclusion

FACE experiments provide invaluable insights into plant responses to elevated CO₂ levels, highlighting complexities that controlled environment studies cannot replicate. While enhanced photosynthesis and improved nitrogen and water use efficiency were key benefits, the expected yield gains in crop plants were lower than anticipated. These findings underscore the need for continuous research and innovation in agriculture to adapt to increasing CO₂ levels and ensure food security in the future[1].

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