Metrics of plant response to CO2 Enrichment

This LETTER discusses metrics that can be used to quantify plant response to fluctuating elevated CO₂ such as in Free-Air CO₂ Enrichment (FACE) compared to that at constant elevated CO₂, both with the same average elevated CO₂ concentration. The concept of Reduction in CO₂-stimulated uptake rate in oscillating elevated CO₂ (Holtum and Winter, 2003), abbreviated as RCS, value = 0.33, is the FIRST METRIC of plant response in oscillating elevated CO₂ compared to constant elevated CO₂. RCS, which includes ambient CO₂ in its calculation, inadequately describes the actual response of plants grown in fluctuating elevated CO₂. Furthermore, the SECOND METRIC, Relative Response Ratio of Allen et al. (2020b), abbreviated as RRR, where RRR = 1.0 – RCS with a value of 0.67, also inadequately describes the response of plants grown in fluctuating elevated CO₂. A THIRD METRIC of plant response to CO₂ enrichment, “Plant response in fluctuating elevated CO₂ / Plant response in constant elevated CO₂”, F_el/C_el, average value of 0.85, represents the response to fluctuating CO₂ in FACE. For completeness, a FOURTH METRIC (F_el/Amb) and FIFTH METRIC (C_el/Amb) are defined. A variation of the FOURTH METRIC, [(F_el –Amb)/Amb] X 100], has been widely used to report yield responses to FACE.