PARENT SESSION

Symposium S4C Controlling CO₂: Stomates and carbon concentrating mechanisms
Tuesday August 31st, 2004 2:40 PM-4:40 PM Room 510A
Chair: George Espie
Co-Chair: Murray Badger

Identifying internal gas diffusion limitations in leaves using chlorophyll fluorescence imaging. Emily Gallouët^{*,1, 2}, Tracy Lawson¹, James Morison¹, Gabriel Cornic², Neil Baker¹, ¹ Department of Biological Sciences, Colchester, Essex, UK² Laboratoire Ecologie Systématique et Evolution, Orsay, France

ABSTRACT- The exchange of gases between leaves and their surrounding environment takes place predominately through the stomata. In heterobaric leaves, where extension of the bundle sheath to the upper and lower epidermis prevent lateral gaseous CO₂ diffusion, patchy stomatal behaviour can result in heterogeneous assimilation rates across the leaf surface at the scale of 0.1 to 1 mm. It has been suggested that in homobaric leaves, where there is no compartmentalisation of the leaf to prevent rapid lateral CO₂ diffusion, such heterogeneity does not occur. However, mathematical modelling by Parkhurst (New Phytol, 1994, 126: 449-479) has predicted that densely packed palisade cells could significantly restrict CO₂ diffusion on an even smaller scale. The relationships between the internal CO₂ concentration and PSII operating efficiency for leaves was determined from simultaneous measurements of gas exchange and chlorophyll fluorescence imaging. Patterns of assimilation rates and internal CO₂ concentration were calculated from fluorescence images over large areas of leaf laminae. Patchy stomatal behaviour was simulated by applying circles (4 mm diameter) of grease to the upper, lower or both leaf surface(s) of Phaseolus vulgaris (heterobaric) and Commelina communis (homobaric), and the resulting patterns of internal CO₂ concentration, under different oxygen and carbon dioxide conditions, were calculated. These patterns revealed large decreases in CO₂ concentration from the patch boundary towards the centre of the patch. These data indicate that resistance to lateral gaseous resistance is high in packed mesophyll cells of both homobaric and heterobaric leaves, and that lateral diffusion is not effective over distances of more than 1 mm. The thinness of most leaves therefore means that mesophyll cells are primarily supplied with CO₂ by vertical, not lateral, diffusion. The presence or absence of vascular extensions in leaves plays no significant direct role in determining CO₂ diffusion in leaves, although it may influence stomata through water relations.

KEY WORDS: fluorescence imaging, heterogeneity, CO2 diffusion limitation, gas exchange