Estimating photosynthetic electron transport via chlorophyll fluorometry without Photosystem II light saturation |
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Authors: | Hugh J Earl Said Ennahli |
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Affiliation: | (1) Department of Plant Agriculture, University of Guelph, Canada;(2) Department of Crop and Soil Sciences, University of Georgia, GA, USA |
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Abstract: | Estimates of thylakoid electron transport rates (Je) from chlorophyll fluorometry are often used in combination with leaf gas exchange measurements to provide detailed information about photosynthetic activity of leaves in situ. Estimating Je requires accurate determination of the quantum efficiency of Photosystem II (ΦP), which in turn requires momentary light saturation of the Photosystem II light harvesting complex to induce the maximum fluorescence signal (FM′). In practice, full saturation is often difficult to achieve, especially when incident photosynthetic photon flux density (Q) is high and energy is effectively dissipated by non-photochemical quenching. In the present work, a method for estimating the true FM′ under high Q was developed, using multiple light pulses of varying intensity (Q′). The form of the expected relationship between the apparent FM′ and Q′ was derived from theoretical considerations. This allowed the true FM′ at infinite Q′ to be estimated from linear regression. Using a commercially available leaf gas exchange/ chlorophyll fluorescence measurement system, Je was compared to gross photosynthetic CO2 assimilation (AG) under conditions where the relationship between Je and AG was expected to be linear. Both in C4 leaves (Zea mays) in ambient air and also in C3 leaves (Gossypium hirsutum) under non-photorespiratory conditions the apparent ratio between Je and AG declined at high Q when ΦP was calculated from FM′ measured simply using the highest available saturating pulse intensity. When FM′ was determined using the multiple pulse / linear regression technique, the expected relationship between Je and AG at high Q was restored, indicating that the ΦP estimate was improved. This method of determining FM′ should prove useful for verifying when saturating pulse intensities are sufficient, and for accurately determining ΦP when they are not. This revised version was published online in June 2006 with corrections to the Cover Date. |
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Keywords: | chlorophyll fluorescence photorespiration photosynthesis saturating pulse thylakoid electron transport |
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