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Daytime-Only vs. 24-Hour Atmospheric CO2 Enrichment

Paper Reviewed
Bunce, J.A. 2014. Limitations to soybean photosynthesis at elevated carbon dioxide in free-air enrichment and open top chamber systems. Plant Science 226: 131-135.

A little over a decade ago in a study of two species of tropical trees, Holtum and Winter (2003) presented evidence suggesting the possibility that free-air CO2 enrichment or FACE systems "may underestimate the potential fertilizing effects of above-ambient CO2 concentrations on plants," due to concerns about possible physiological impacts of the rapidly fluctuating CO2 concentrations that occur in response to the continual over- and under-shooting of the targeted CO2 concentration as the FACE apparatus continually adjusts to counteract the concentration-perturbing consequences of variations in wind speed and direction.

Two years later, Bunce (2005), grew soybean plants from seed to maturity out-of-doors in open-top chambers exposed to normal precipitation while continuously fumigating them with either ambient air (AC) or with air enriched with an extra 350 ppm of CO2 either 24 hours per day (ECdn) or 14 hours per day centered on solar noon (ECd) for a total of four entire growing seasons. And this sustained effort, in Bunce's words, revealed that "ECdn increased seed yield by an average of 62% over the four years compared with the ambient CO2 treatment, while ECd increased seed yield by 34%," indicative of the fact that the CO2-induced yield enhancement in the 24-hour CO2 enrichment treatment was almost twice as great as that of the daylight-only CO2 enrichment treatment.

Subsequent work by Bunce (2012, 2013) on cotton, wheat and rice also substantiated the much lower growth enhancement response of those plants to atmospheric CO2 enrichment via FACE techniques than the enhancement produced by non-FACE means. And most recently, Bunce (2014), in a FACE study of two soybean cultivars with either daytime-only or 24-hour CO2 enrichment, reports that "the seed yield of both elevated CO2 treatments exceeded that of the ambient CO2 controls, averaging 30% higher than the controls with daytime CO2 elevation, and about 40% higher with continuous CO2 elevation."

Consequently, Bunce (2014) concludes that "if night-time CO2 elevation has similar effects on the growth of other species" - and there is much evidence to suggest that it does - "FACE systems which only elevate CO2 during the daytime could underestimate crop responses to future CO2 concentrations." And this fact suggests that many of the FACE-derived plant growth responses to atmospheric CO2 enrichment that are tabulated in our Plant Growth Database may well be considerably less than what they are in reality.

Bunce, J.A. 2005. Seed yield of soybeans with daytime or continuous elevation of carbon dioxide under field conditions. Photosynthetica 43: 435-438.

Bunce, J.A. 2012. Responses of cotton and wheat photosynthesis and growth to cyclic variation in carbon dioxide concentration. Photosynthetica 50: 395-400.

Bunce, J.A. 2013. Effects of pulses of elevated carbon dioxide concentration on stomatal conductance and photosynthesis in wheat and rice. Physiologia Plantarum 149: 214-221.

Holtum, J.A.M. and Winter, K. 2003. Photosynthetic CO2 uptake in seedlings of two tropical tree species exposed to oscillating elevated concentrations of CO2. Planta 218: 152-158.

Posted 17 December 2014