Cotton bracts are adapted to a microenvironment of concentrated CO₂ produced by rapid fruit respiration
2013
Hu, Yuan-Yuan | Oguchi, Riichi | Yamori, Wataru | Von Caemmerer, S. (Susanna) | Chow, Wah Soon | Zhang, Wang-Feng
Background and Aims Elucidation of the mechanisms by which plants adapt to elevated CO ₂ 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 CO ₂ microenvironment around the bolls and bracts. It has been observed that the intercellular CO ₂ 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 CO ₂ microenvironment has existed for at least 1·1 million years since the appearance of tetraploid cotton. Therefore, it was hypothesized that the mechanisms by which cotton bracts have adapted to elevated CO ₂ will indicate how plants will adapt to future increased atmospheric CO ₂ concentration. Specifically, it is hypothesized that with 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 cotton were 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 (J ₘₐₓ/ V cₘₐₓ) in bracts than in leaves. Conclusions These results agree with the theoretical prediction that adaptation of photosynthesis to elevated CO ₂ requires increased RuBP regeneration. Cotton bracts are readily available material for studying adaption to elevated CO ₂.
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