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Plant and Cell Physiology Advance Access published online on March 12, 2009
Plant and Cell Physiology, doi:10.1093/pcp/pcp041
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© The Author 2009. Published by Oxford University Press on behalf of Japanese Society of Plant Physiologists. All rights reserved. For Permissions, please e-mail: journals.permissions@oxfordjournals.org

Potential errors in electron transport rates calculated from chlorophyll fluorescence as revealed by a multilayer leaf model

John R Evans

Research School of Biological Sciences, The Australian National University, Canberra, Australian Capital Territory 2601, Australia

Corresponding author: Dr. John R Evans. Fax 61 2 61254919, email John.Evans{at}anu.edu.au


  Abstract

Increasingly, photosynthetic electron transport rate is being calculated from chlorophyll fluorescence measurements. The fluorescence signal is a complex mixture of contributions from different depths within the mesophyll. One condition required for electron transport calculated from fluorescence to accurately represent the rate is that the ratio of photosynthetic capacity to light absorbed be constant throughout the leaf. In order to explore the fluorescence properties of leaves where this assumption is not true, a new approximation for {phi}PSII is used to generate Fm' and Fs values throughout the leaf. Fs is assumed to be proportional to the amount of light absorbed from the fluorescence measuring beam and constant i.e. independent of the actinic irradiance or CO2 concentration. This assumption is validated by measurements from Eucalyptus maculata, Flaveria bidentis and Triticum aestivum, with two different types of fluorometer, where irradiance or CO2 response curves were measured with normal or inverted leaf orientations. The new approach enables fluorescence values to be generated at each layer in a multi-layer model. Two applications using this approach are presented. Firstly, the model is used to show that when quantum yield varies through a leaf, then fluorescence will lead to an incorrect estimate of electron transport rate. Secondly, since chlorophyll fluorescence is also used to calculate the CO2 concentration at the sites of carboxylation within chloroplasts, Cc, the model is also used to show that Cc may vary with depth. Significant variation in Cc through the mesophyll could lead to an apparent dependence of internal conductance on irradiance or CO2.

Keywords: Chloroplast - Internal conductance - Leaf anatomy - Light profiles - Mesophyll conductance - Rubisco

(Received December 18, 2008; Accepted March 5, 2009)
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