title: Cotton bracts are adapted to a microenvironment of concentrated CO2 produced by rapid fruit respiration.
abstract: OBJECTIVE
  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 CO2. 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, it was hypothesized that the mechanisms by which cotton bracts have adapted to elevated CO2 will indicate how plants will adapt to future increased atmospheric CO2 concentration. Specifically, it is hypothesized that with elevated CO2 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.
  
  
  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.
authors.affiliation: The Key Laboratory of Oasis Eco-agriculture, Xinjiang Production and Construction Group, Shihezi University, Shihezi 832003, China.
authors.forename: Yuan-Yuan
authors.initials: YY
authors.lastname: Hu
authors.forename: Riichi
authors.initials: R
authors.lastname: Oguchi
authors.forename: Wataru
authors.initials: W
authors.lastname: Yamori
authors.forename: Susanne
authors.initials: S
authors.lastname: von Caemmerer
authors.forename: Wah Soon
authors.initials: WS
authors.lastname: Chow
authors.forename: Wang-Feng
authors.initials: WF
authors.lastname: Zhang
mesh_terms: D000222:Adaptation, Physiological; D000388:Air; D002245:Carbon Dioxide / Q000378:metabolism*; D005638:Fruit / Q000378:metabolism*; D003368:Gossypium / Q000502:physiology*; D010788:Photosynthesis / Q000502:physiology; D018515:Plant Leaves / Q000033:anatomy & histology / Q000378:metabolism / Q000502:physiology*; D054046:Plant Stomata / Q000502:physiology; D012273:Ribulose-Bisphosphate Carboxylase / Q000378:metabolism
chemical_list: D002245:Carbon Dioxide; D012273:Ribulose-Bisphosphate Carboxylase
keywords: Bract; CO2 acclimation; CO2 adaptation; Cyt b6f; Gossypium hirsutum; Jmax/Vcmax; Rubisco; cotton; photosynthesis; respiration; stomatal conductance; water use efficiency