Determination of the site of CO₂ sensing in poplar: is the area-based N content and anatomy of new leaves determined by their immediate CO₂ environment or by the CO₂ environment of mature leaves?
2011
Miyazawa, Shin-Ichi | Warren, Charles R. | Turpin, David H. | Livingston, Nigel J.
Exposure to an elevated CO₂ concentration ([CO₂]) generally decreases leaf N content per unit area (Narea) and stomatal density, and increases leaf thickness. Mature leaves can 'sense' elevated [CO₂] and this regulates stomatal development of expanding leaves (systemic regulation). It is unclear if systemic regulation is involved in determination of leaf thickness and Narea--traits that are significantly correlated with photosynthetic capacity. A cuvette system was used whereby [CO₂] around mature leaves was controlled separately from that around expanding leaves. Expanding leaves of poplar (Populus trichocarpaxP. deltoides) seedlings were exposed to elevated [CO₂] (720 μmol mol⁻¹) while the remaining mature leaves inside the cuvette were under ambient [CO₂] of 360 μmol mol⁻¹. Reverse treatments were performed. Exposure of newly developing leaves to elevated [CO₂] increased their thickness, but when mature leaves were exposed to elevated [CO₂] the increase in thickness of new leaves was less pronounced. The largest response to [CO₂] was reflected in the palisade tissue thickness (as opposed to the spongy tissue) of new leaves. The Narea of new leaves was unaffected by the local [CO₂] where the new leaves developed, but decreased following the exposure of mature leaves to elevated [CO₂]. The volume fraction of mesophyll cells compared with total leaf and the mesophyll cell density changed in a manner similar to the response of Narea. These results suggest that Narea is controlled independently of the leaf thickness, and suggest that Narea is under systemic regulation by [CO₂] signals from mature leaves that control mesophyll cell division.
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