Metrics of plant response to CO2 Enrichment

• Holtum and Winter (2003) is citation #1 and Allen et al.(2000) is citation #2. • #1: Reduced Plant CO 2 uptake in oscillating elevated CO 2 = 0.33 is re-examined. • Relative Response Ratio = 0.67 found in citation #2 is superseded by the next metric. • Plant response in fluctuating elevated CO 2 /response in constant elevated CO 2 = 0.85. • Plant response in FACE should be ∼ 0.85 compared to response in constant elevated CO 2 . This LETTER discusses metrics that can be used to quantify plant response to fluctuating elevated CO 2 such as in Free-Air CO 2 Enrichment (FACE) compared to that at constant elevated CO 2 , both with the same average elevated CO 2 concentration. The concept of Reduction in CO 2 -stimulated uptake rate in oscillating elevated CO 2 (Holtum and Winter, 2003), abbreviated as RCS, value = 0.33, is the FIRST METRIC of plant response in oscillating elevated CO 2 compared to constant elevated CO 2 . RCS, which includes ambient CO 2 in its calculation, inadequately describes the actual response of plants grown in fluctuating elevated CO 2 . 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 CO 2 . A THIRD METRIC of plant response to CO 2 enrichment, “Plant response in fluctuating elevated CO 2 / Plant response in constant elevated CO 2 ”, F el /C el , average value of 0.85, represents the response to fluctuating CO 2 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 e l –Amb)/Amb ] X 100], has been widely used to report yield responses to FACE.