A Review of CO2 Enrichment Studies: Does Enhanced Photosynthesis Enhance Growth?

Plants typically only convert 2 to 4% of available energy into actual growth and this natural inefficiency provides a reason for scientists to attempt to increase the efficiency of the process by increasing photosynthesis. One of the most common methods, other than genetic modification, to increase photosynthesis is to increase ambient CO2. Elevated CO2 can lead to growth increases ranging from 10 to 50%, depending on the plant’s carbon sink capacity and nutrient availability. Previous studies show that elevated CO2inevitably leads to increased growth, but the magnitude of the growth varies with the photosynthetic capacity of the plant. Photosynthesis is an inefficient process with a maximum of 8 to 10% of the energy in sunlight being converted into chemical energy. Realistically, only 2 to 4% of energy in sunlight is converted. In this paper, Kirschbaum examines previous studies and conducts experiments of his own in order to summarize the current knowledge on CO2enrichment studies, focusing on the ability of increased photosynthesis to ultimately increase plant growth. Kirschbaum studies the factors that affect plant growth under elevated CO2 in an attempt to determine if photosynthesis is the main factor increasing growth or if other factors are relatively more important.—Taylor Jones
Kirschbaum, M.U.F., 2011. Does Enhanced Photosynthesis Enhance Growth? Lessons Learned from CO2 Enrichment Studies.  Plant Physiology 155, 117-124.

          Kirschbaum first examines the photosynthetic response to increasing CO2concentrations and distinguishes between Rubisco-limited photosynthetic rates and ribose 1,5-bisphosphate (RuBP) regeneration-limited rates. For both photosynthetic rates, the relative responsiveness of increases in CO2concentration decreases as atmospheric CO2 continues to increase. Photosynthesis is limited by Rubisco-limited rates at low CO2concentrations and RuBP regeneration-limited rates at high concentrations, and scientists argue that the amounts of Rubisco plants have today is in excess of what is needed, so most plants experience RuBP regeneration-limited photosynthesis. Changes in plant photosynthesis are supported by previous studies, as 30 to 40% enhancements in photosynthesis were recently found in free-air CO2 enrichment experiments and a 58% increase was found in a controlled plot experiment. Kirschbaum notes that there are important limitations to any photosynthesis study as plants that experience less light and increased self-shading may have less enhancement of photosynthesis, while plants grown in high temperature conditions may have more. On average, increases in ambient CO2 lead to a 30% enhancement of photosynthesis, but does this translate to a 30% enhancement of growth?
          Studies show that the relative growth rate for plants is often similar among species and enhanced photosynthesis often leads to only a 10% increase in the relative growth rate. Kirschbaum suggests that previous studies show an increases in photosynthesis leads to 20% enhanced leaf area, but also a 6.5% increase in leaf weight due to increase amounts of carbohydrates and this leads to an ineffective transformation of increased photosynthetic rates to new growth. Extra amounts of carbon produced from photosynthesis can only be of use if the plant can utilize it through root growth, new foliage, or other carbon sinks. Also, carbon cannot be used efficiently if other vital resources, such as nitrogen, are lacking. Studies have shown that many plants show strong photosynthetic enhancement during the growth stages, reduced enhancement during the flowering stages and then increased enhancement during the fruiting stages. During the flowering stages, plants lost much of their potential carbon sink that exists in the growth phase and is regained through seed production in the flowering stage. Most plants show some increased growth response to elevated CO2, but the degree of this growth is determined by other limiting factors, such as carbon sink and nitrogen availability.
          Kirschbaum also notes that a large number of papers use biomass enhancement ratios to determine the effects of elevated CO2 on plant growth. Biomass enhancement ratios are often much greater than relative growth rates and also greater for single-plant studies and fast-growing plants. Under elevated CO2, plants often experience exponential growth in early stages, followed by average growth rates in intermediate stages. Plants that experience an overall relative growth rate of 10% can experience a biomass enhancement ratio of 50% in intermediate stages which eventually decreases to about 10% in later stages. This concept explains why fast-growing plants can have higher biomass enhancement ratios compared to slower-growing plants, but the same relative growth rate. Therefore, the length of an experiment is very important and should be considered when examining the biomass enhancement ratio of a plant to determine if real growth increases exist. The biomass enhancement ratio can often be a misleading value as it can be manipulated by varying the length of an experiment.
          Kirschbaum identifies other issues that may affect photosynthetic enhancement rates that need to be considered, such as natural competition and growth response in mixed-species communities. Also, some studies have shown a decrease in protein concentrations under elevated CO2. Plant herbivore interactions might also change as elevated CO2 usually leads to lower nutrient concentrations which reduces the rate of herbivores feeding on the plant and as a result, herbivores may attempt to consume more of the plant. All of these factors are important complicating issues and should be addressed further.
          In conclusion, photosynthetic enhancement due to elevated CO2 increases the carbon available to plants and whether or not this translates to growth depends on other colimiting factors, such as nutrient availability and carbon sink. Increases in carbon will exacerbate any other limitations. Plants are also subjected to genetic constraints and will only respond to increases in photosynthesis to levels within their genetic capability. By examining several CO2 enrichment experiments, Kirschbaum found that growth enhancements are modest and a 10% increase in relative growth rate can translate to a much higher relative growth rate in the early exponential phases of plant growth. Kirschbaum suggests that genetic manipulation of photosynthesis should include appropriate crop management and close examination of plant attributes to maximize photosynthetic enhancement.

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