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Maintaining CO2 concentrations in the plant canopy can be critical for normal growth and development. Plants photosynthesize and consume CO2 during growth. In contained environments such as plant growth chambers, the consumed CO2 must be replenished in order to prevent appreciable drops in CO2 concentrations. Appreciable drops in CO2 concentrations can lead to reductions in biomass, delay or prevent flowering, and alter gene expression. Introducing ambient air into plant growth chambers is one method to replenish the CO2.

How do plant area and fresh air flow rates affect CO2 concentrations?

The more plant area inside a growth chamber, the greater the depletion of CO2 (Fig. 1). The rate of CO2 depletion is related to the total net CO2 assimilation rate (rate of photosynthesis) of the entire plant area. With well-watered plants, light intensity, temperature, and the CO2 concentration inside the chamber itself primarily affect the rate of CO2 assimilation1.

If growth chambers are filled to capacity, the fresh air flow rates through the chambers have a considerable effect on the CO2 concentrations inside. With their standard fresh air flow rates, CO2 concentrations dropped to 280ppm inside both a reach-in chamber filled with field mustard (Brassica rapa)2 and a walk-in growth room filled with poplar trees (Populus sp.)3. In chambers built before 1984, fresh air flow rates are generally even less and can result in CO2 concentrations of 150ppm and 50ppm inside a chamber filled with cotton (Gossypium sp.) and maize (Zea mays) respectively3-5. Increased fresh air flow rates increase the CO2 concentration around plants by replacing the CO2 consumed during photosynthesis at a greater rate (Fig. 2).

A BioChambers short plant chamber (model SPC-37) filled with young maize and soybean plants provides a fresh air flow rate that exceeds 50 ft3 min-1, mitigating CO2 depletion to less than 40ppm (Fig. 3). Assuming an ambient CO2 concentration close to atmospheric (~400ppm), the 51 ft3 min-1 fresh air flow rate of the SPC-37 is within this 10% CO2 drawdown, the most stringent recommendation of Morse (1963)6 in the Plant Growth Chamber Handbook7.

In addition, we estimate net CO2 assimilation rates per unit growth area for plants that use C3 and C4 photosynthesis at various growth temperatures using leaf level gas exchange data and comparable whole chamber experiments for a tall plant chamber (model TPC-19). These estimates were used to calculate the required fresh air flow rates to achieve a 10% CO2 drawdown and are based on an assumed leaf area index (leaf area/growth area, m2 m-2) of 1.5 for tall plants (Table 1). Theoretical estimates for the SPC-37 using this same approach are ±1.3 ft3 min-1 of actual measurements from the experiment with maize and soybean.

Photosynthetic Type
Leaf Temp (°C) Required Fresh Air
Flow Rate
(ft3 min-1)
C3 (eg. wheat) 21 31
C3 (eg. wheat) 30 33
C3 (eg. wheat) 35 29
C4 (eg. maize) 21 28
C4 (eg. maize) 30 44
C4 (eg. maize) 35 46

Table 1: Calculated fresh air flow rates (ft3 min-1) required to maintain a 10% reduction in ambient CO2 concentration inside a tall plant chamber (model TPC-19) filled to capacity for plants that use C3 or C4 photosynthesis at different growth temperatures. Calculations assume a 400ppm CO2 concentration entering the chamber and a leaf area index (leaf area/growth area, m2 m-2) of 1.5. Estimates are based on published leaf level net CO2 assimilation rates and comparable whole chamber CO2 assimilation studies8-15. These estimates incorporate the maximum photosynthetic photon flux density and growth area of the TPC-19 which are 1500 µmol m-2 s-1 and 1.87m2 respectively. The TPC-19 has a fresh air flow rate of 60 ft3 min-1, exceeding these requirements under the given assumptions.

How do low CO2 concentrations and inadequate fresh air intake affect plant growth and development?

When CO2 concentrations inside growth chambers drop below ambient (~400ppm), growth and development for plants that use C3 photosynthesis are reduced and impaired (eg. wheat, soybean, rice)16-26. The majority of plant species use the C3 photosynthetic pathway, where at low CO2 concentrations, CO2 itself becomes the primary limitation on carbon gain. Ribulose-1-5 carboxylase/oxygenase (Rubisco) is the photosynthetic enzyme that fixes CO2 into sugars for growth and development. When CO2 concentrations are low, Rubisco and carbon gain are limited by CO227,1. Rubisco also reacts with O2 in the energetically wasteful process of photorespiration28. Here O2 competes with and inhibits CO2 binding, exacerbating the CO2 limitation on carbon gain as CO2 concentrations decline, especially at warm temperatures29-32. Across a number of plants that use C3 photosynthesis, reductions in biomass are roughly proportional to reductions in CO2 concentrations below current atmospheric levels (~400ppm). Here, a 50% reduction in CO2 concentration results in a 50% reduction in biomass (see ref. 24, Fig. 3). Other effects of low CO2 concentrations on C3 plants include increased water use, a reduction in the ratio of root:shoot biomass, and delayed or failure to flower (reviewed in ref. 19). The stress of low CO2 concentration affects gene expression and quantitative traits, potentially confounding the expression profiles from treatment factor(s) of interest2. Plants that can concentrate CO2 around Rubisco experience reduced effects of low CO2 concentrations on growth and development, such as plants that use C4 photosynthesis (eg. maize, sugarcane)18,23.

In addition, gases emitted by plants such as isoprene and ethylene can potentially build up and have feedback effects if fresh air flow is inadequate. These feedbacks could have undesirable effects and further confound experiments. Increased isoprene exposure can hasten the onset of flowering and may artificially increase tolerance of high temperatures33-35. Low CO2 environments can increase isoprene emissions, further compounding the problem of inadequate fresh air supply36. Superambient ethylene exposure accelerates fruit ripening and can slow or accelerate plant growth depending on the species37,38.

Chart 2

Figure 1: The effect of greater plant mass/area on CO2 concentration inside a growth chamber. During photosynthesis plants consume CO2 from the surrounding air. The greater the leaf area inside a growth chamber, the greater the rate of CO2 drawdown from photosynthesis. Six plants (B) will lower the CO2 concentration more than only three plants (A) of equivalent size and development.

Chart 3

Figure 2: The effect of greater fresh air flow on the CO2 concentration inside a growth chamber. Greater fresh air flow replaces the CO2 consumed during photosynthesis at a faster rate, increasing the CO2 concentration or mitigating the drawdown from photosynthesis. With an equivalent number of plants of the same size and development, increasing the fresh air flow from 20 ft3 min-1 (A), to 50 ft3 min-1 (B), can prevent a nearly four-fold internal drawdown of CO2 concentrations (see Figure 3).

Chart 4

Figure 3: Drawdown of CO2 concentration as a function of fresh air flow inside a BioChambers SPC-37 filled with well-watered and fertilized maize and soybean (mean ±SE). Leaf temperatures ranged from 25.5 – 26.5°C and photosynthetic photon flux densities averaged 430 µmol m-2 s-1 across the upper leaves. The leaf area index (leaf area/growth area, m2 m-2) of all plants was 0.48. Flow rates were decreased by manually closing the fresh air intake valve from fully open (arrow). After each flow rate change, at least 45 minutes was given before measurements were recorded to allow for steady state conditions. Dotted line is the 10% recommended drawdown limit of Morse (1963)6 assuming the ambient CO2 concentration entering the chamber is current atmospheric (~400ppm).

For more information, and the full list of references, please view the downloadable PDF above.

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