Genotypes of Brassica rapa respond differently to plant-induced variation in air CO₂ concentration in growth chambers with standard and enhanced venting

Growth chambers allow measurement of phenotypic differences among genotypes under controlled environment conditions. However, unintended variation in growth chamber air CO₂ concentration ([CO₂]) may affect the expression of diverse phenotypic traits, and genotypes may differ in their response to variation in [CO₂]. We monitored [CO₂] and quantified phenotypic responses of 22 Brassica rapa genotypes in growth chambers with either standard or enhanced venting. [CO₂] in chambers with standard venting dropped to 280 μmol mol^-1 during the period of maximum canopy development, ∼80 μmol mol^-1 lower than in chambers with enhanced venting. The stable carbon isotope ratio of CO ₂ in chamber air (δ¹³C_air) was negatively correlated with [CO₂], suggesting that photosynthesis caused observed [CO₂] decreases. Significant genotype × chamber-venting interactions were detected for 12 of 20 traits, likely due to differences in the extent to which [CO₂] changed in relation to genotypes' phenology or differential sensitivity of genotypes to low [CO ₂]. One trait, ¹³C discrimination (δ¹³C), was particularly influenced by unaccounted-for fluctuations in δ¹³C_air and [CO₂]. Observed responses to [CO₂] suggest that genetic variance components estimated in poorly vented growth chambers may be influenced by the expression of genes involved in CO₂ stress responses; genotypic values estimated in these chambers may likewise be misleading such that some mapped quantitative trait loci may regulate responses to CO₂ stress rather than a response to the environmental factor of interest. These results underscore the importance of monitoring, and where possible, controlling [CO₂].