CO2 enrichment system and environmental conditions

For the CO₂ treatment, we used four sunlit, closed chambers. Fresh atmospheric (outdoor) air was supplied to the chambers from outside of the greenhouse using flexible dryer aluminum ducting (15.24×609.6 cm) with variable speed inline fans (Model FR, Fantech, Sarasota, FL, USA). Together with the fresh air, CO₂ gas balanced with N₂ was supplied through clear Tygon tubing (Saint-Gobain Performance Plastics, Akron, OH, USA) from a 22.7-kg tank (Praxair, Seattle, WA, USA). The CO₂ gas was provided to two of the four chambers to provide the elevated [CO₂] treatment (ELE, ~800 ppm). The other two chambers remained at ambient [CO₂] (AMB, ~400 ppm). [CO₂] in the chambers was monitored every 30 min by diffusion-type CO₂ probes (GMP343, Vaisala Oyj., Vantaa, Finland). The mean [CO₂] of the AMB and ELE chambers was 407 and 843 ppm, respectively, for Expt 1, and 437 and 886 ppm, respectively, for Expt 2.

A temperature and relative humidity probe (CS215, Campbell Scientific) was installed 20 cm from the top of each chamber and a quantum sensor (SQ-100, Apogee Instruments, Logan, UT, USA) was set up around each probe. Data from these instruments were collected every 15 min by a datalogger (CR1000, Campbell Scientific). Supplementary artificial lights (high-pressure sodium 400 W single-phase bulbs, Phillips Electronics) were used throughout both experiments to maintain consistent light intensity in the greenhouse during the daytime from 06.00 to 20.00 (14/10 h photoperiod). Mean day/night temperatures and relative humidity in Expt 1 were 23/19 °C and 57/64%, and in Expt 2 were 23/19 °C and 60/66%. Mean light intensity and daily light integral in Expt 1 were 216.7 μmol m^–2 s^–1 and 11.6 mol m^–2 d^–1 photosynthetic photon flux density (PPFD), and in Expt 2 were 176.3 μmol m^–2 s^–1 and 9.1 mol m^–2 d^–1 PPFD.

Specifications for the CO₂ chamber system used in the current study can be also found in Kinmonth-Schultz and Kim (2011) and Nackley et al. (2016).