Comparing two measures of leaf photorespiration rate across a wide range of light intensities.

Suppression of photorespiration by low O ₂ concentrations (Method 1) and simultaneous measurements of gas exchange and chlorophyll fluorescence (Method 2) are often used to estimate leaf photorespiration rate (R _p ) of C ₃ plants. However, it is largely unknown whether Method 1 and Method 2 can be used equivalently in estimating R _p . Using a field experiment on two wheat cultivars (T. aestivum JM22 and T. aestivum Z39-118) whose leaf gas exchange and chlorophyll fluorescence at low and normal O ₂ concentrations (2% versus 21% O ₂ ) were simultaneously measured across a wide range of light intensities (I), this study assessed the impacts of the two measures on R _p and its response under changing irradiance conditions. All the above quantities increased with the increasing I until reaching the cultivar-specific maximum values and the corresponding saturation light intensities. However, there were significant differences between R _p estimated by Method 1 and Method 2 at the I range from 150 to 2000 μmol m ^-2  s ^-1 for T. aestivum JM22 and from 150 to 1000 μmol m ^-2  s ^-1 for T. aestivum Z39-118. These findings demonstrated that the two methods cannot be used equivalently under changing irradiance conditions.
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