Plant growth chamber design for subambient pCO₂ and δ¹³ C studies

Subambient pCO₂ has persisted across the major Phanerozoic ice ages, including the entire late Cenozoic (ca 30 Ma to present). Stable isotope analysis of plant-derived organic matter is used to infer changes in pCO₂ and climate in the geologic past, but a growth chamber that can precisely control environmental conditions, including pCO₂ and δ¹³ C value of CO₂ (δ¹³ C_CO2 ) at subambient pCO₂ , is lacking.

We designed and built five identical chambers specifically for plant growth under stable subambient pCO₂ (ca 100 to 400 ppm) and δ¹³ C_CO2 conditions. We tested the pCO₂ and δ¹³ C_CO2 stability of the chambers both with and without plants, across two 12-hour daytime experiments and two extended 9-day experiments. We also compared the temperature and relative humidity conditions among the chambers.

The average δ¹³ C_CO2 value within the five chambers ranged from -18.76 to -19.10‰; the standard deviation never exceeded 0.14‰ across any experiment. This represents better δ¹³ C_CO2 stability than that achieved by all previous chamber designs, including superambient pCO₂ chambers. Every pCO₂ measurement (n = 1225) was within 5% of mean chamber values. The temperature and relative humidity conditions differed by no more than 0.4°C and 1.6%, respectively, across all chambers within each growth experiment.

This growth chamber design extends the range of pCO₂ conditions for which plants can be grown for δ¹³ C analysis of their tissues at subambient levels. This new capability allows for careful isolation of environmental effects on plant ¹³ C discrimination across the entire range of pCO₂ experienced by terrestrial land plants.