Fluctuations of CO2 in Free-Air CO2 Enrichment (FACE) depress plant photosynthesis, growth, and yield

Authors: ALLEN, LEON - RETIRED ARS EMPLOYEE; KIMBALL, BRUCE - RETIRED ARS EMPLOYEE; BUNCE, JAMES - RETIRED ARS EMPLOYEE; TOSHIMOTO, MAYUMI - NATIONAL INSTITUTE FOR AGRO-ENVIRONMENTAL SCIENCES; HARAZONO, YOSHINOBU - UNIVERSITY OF ALASKA; BAKER, JEFFREY; BOOTE, KENNETH - UNIVERSITY OF FLORIDA; WHITE, JEFFREY - RETIRED ARS EMPLOYEE

Submitted to: Agricultural and Forest Meteorology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/2/2020
Publication Date: 1/20/2020
Citation: Allen, L.H., Kimball, B.A., Bunce, J.A., Toshimoto, M., Harazono, Y., Baker, J.T., Boote, K.J., White, J.W. 2020. Fluctuations of CO2 in Free-Air CO2 Enrichment (FACE) depress plant photosynthesis, growth, and yield. Agricultural and Forest Meteorology. 284. https://doi.org/10.1016/j.agrformet.2020.107899.
DOI: https://doi.org/10.1016/j.agrformet.2020.107899

Interpretive Summary: In 2004-2008 publications, plant response to elevated carbon dioxide (CO2) in fluctuating Free-Air-CO2-Enrichment (FACE) exposure systems was purported to be only about 0.45 (45%) of that in constant elevated CO2 of the same mean constant concentration in chambered plant growth systems. Other ARS research reported in 2012 and later indicated that plant response to elevated CO2 in oscillating elevated CO2 was also low compared to plant response to constant elevated CO2 of the same mean concentration. However, in this review and analysis, ARS researchers and other scientists found that plant responses to elevated CO2 in fluctuating FACE exposure systems was about 0.65 (not 0.45) of that in constant CO2 after eliminating some problematic experimental data from the chambered systems that were compared with responses in FACE systems. Although this correction is vital, it does not explain why plant responses in fluctuating CO2 FACE exposure systems should be less than plant responses in constant CO2 exposure systems having the same CO2 concentration as the mean CO2 concentration of the FACE exposure systems. Our contribution is to point out the previously unidentified problem for fundamental Plant Photosynthetic Physiologists to solve.

Technical Abstract: Free-Air CO2 Enrichment (FACE) was conceived as an experimental method to measure plant responses to elevated CO2 in natural environments rather than in chambered or controlled environments. However, due to the difficulty of controlling elevated CO2 concentrations in turbulent air, the range of fluctuations of CO2 in FACE experiments are more than 10-fold greater than plants experience in natural conditions. One early study reported that photosynthetic increases of leaves in 40- and 80-s periods of oscillating elevated CO2 were only about 68% of those in leaves exposed to constant elevated CO2 with the same mean CO2 concentration. Later whole-plant studies reported smaller increases of responses in 60-s periods of oscillating elevated CO2 compared to constant elevated CO2 with the same mean concentration. After eliminating problematic data from studies that predicted plant responses in FACE to be only 45% of responses in open top chambers, we calculated that yields increased 65% as much in fluctuating elevated CO2 of FACE as in constant elevated CO2. The smaller plant responses in fluctuating elevated CO2 can be attributed partially to the non-linear, convex-upward curved response of photosynthesis to CO2 concentration, but other unknown mechanisms must exist. Future leaf chamber studies and FACE studies should focus on uncovering the mechanisms responsible for lower photosynthetic, biomass, and yield response in both regular waveform oscillating and irregular fluctuating elevated CO2.