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Evaluating the role of feedbacks between atmosphere and canopy

The ability of plants to regulate gas exchange through stomata allows them to control water relations and carbon assimilation and the aperture of the stomatal pore reflects a compromise between the photosynthetic requirement for carbon dioxide (CO2) and the availability of water. Consequently, the study of stomatal response to change is important. As intercellular [CO2] is a key variable sensed by guard cells and used to co-ordinate stomatal opening, many studies have been designed to quantify stomatal responses to rising atmospheric [CO2]. Since stomatal responses are involved in determining CO2-induced enhancements of photosynthesis and leaf-level water use efficiency, differential stomatal responses under elevated atmospheric [CO2] have been of interest in the study of terrestrial carbon dynamics and the focus of multi-scale investigations of transpiration and water use efficiency. Few studies, however, have been completed where longterm (years) responses of ecosystems are considered; to our knowledge, however, no-one has previously reported the progression of stomatal frequency or development in response to elevated [CO2] during canopy closure in a FACE exposure, the focus of this study.

Scientific achievements from EuroFACE experiment are as follows:
After five growing seasons, the stomatal conductance of P. · euramericana still responded to FACE whereas the frequency of stomata, measured as stomatal density and index, did not. Consequently, stomatal aperture and not stomatal number was more important in determining leaf water loss in elevated [CO2]. Reductions in stomatal index and density may be a consequence of changes in the number of stomatal complexes, changes in epidermal cell density, or a combination of both, each of which may represent a separate response (Lake et al. 2002). In addition, stomatal density may be influenced by differential epidermal cell expansion. We have previously shown that the sensitivity of epidermal cell density and expansion to elevated [CO2] was altered with both leaf and tree age in this experiment and that stomatal density and index varied significantly with leaf age (defined as position) in the Populus clones Beaupre´ and Robusta. The data here suggest a similar alteration in sensitivity with age and possibly the length of exposure to elevated [CO2]. In younger trees stomatal density and index were reduced, whereas in older trees, exposed for a longer time, there was no significant reduction or stimulation. The mechanism for stomatal response to [CO2] is not well understood but aperture, at least, responds differentially to [CO2] in epidermal peels indicating that sensitivity is not due to a mesophyll signal but is controlled at the guard cell or cuticle.

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Bassett Crescent East
United Kingdom
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