Cotton bracts are adapted to a microenvironment of concentrated CO ₂ produced by rapid fruit respiration

Background and Aims Elucidation of the mechanisms by which plants adapt to elevated CO₂ is needed; however, most studies of the mechanisms investigated the response of plants adapted to current atmospheric CO ₂. The rapid respiration rate of cotton (Gossypium hirsutum) fruits (bolls) produces a concentrated CO₂ microenvironment around the bolls and bracts. It has been observed that the intercellular CO₂ concentration of a whole fruit (bract and boll) ranges from 500 to 1300 μmol mol^-1 depending on the irradiance, even in ambient air. Arguably, this CO₂ microenvironment has existed for at least 1.1 million years since the appearance of tetraploid cotton. Therefore, itwas hypothesized that the mechanisms by which cotton bracts have adapted to elevatedCO₂ will indicate howplantswill adapt to future increased atmospheric CO₂ concentration. Specifically, it is hypothesized thatwith elevated CO ₂ the capacity to regenerate ribulose-1,5-bisphosphate (RuBP) will increase relative to RuBP carboxylation. Methods To test this hypothesis, the morphological and physiological traits of bracts and leaves of cottonwere measured, including stomatal density, gas exchange and protein contents. Key results Compared with leaves, bracts showed significantly lower stomatal conductance which resulted in a significantly higher water use efficiency. Both gas exchange and protein content showed a significantly greater RuBP regeneration/RuBP carboxylation capacity ratio (Jmax/Vcmax) in bracts than in leaves. Conclusions These results agree with the theoretical prediction that adaptation of photosynthesis to elevated CO₂ requires increased RuBP regeneration. Cotton bracts are readily available material for studying adaption to elevated CO₂.