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Does Free-Air Carbon Dioxide Enrichment Affect Photochemical Energy Use by Evergreen Trees in Different Seasons? A Chlorophyll Fluorescence Study of Mature Loblolly Pine1
Department of Biological Sciences, John Tabor Laboratories, University of Essex, Wivenhoe Park, Colchester CO4 3SQ, United Kingdom (G.J.H., N.R.B., S.P.L.); Environmental Biology and Instrumentation Division, Building 318, Brookhaven National Laboratory, Upton, New York 11973 (D.S.E.); and Departments of Crop Sciences and Plant Biology, University of Illinois, 190 Edward R. Madigan Laboratory, West Gregory Drive, Urbana, Illinois 61801 (S.P.L.) Previous studies of the effects of growth at elevated CO2 on energy partitioning in the photosynthetic apparatus have produced conflicting results. The hypothesis was developed and tested that elevated CO2 increases photochemical energy use when there is a high demand for assimilates and decreases usage when demand is low. Modulated chlorophyll a fluorescence and leaf gas exchange were measured on needles at the top of a mature, 12-m loblolly pine (Pinus taeda L.) forest. Trees were exposed to ambient CO2 or ambient plus 20 Pa CO2 using free-air CO2 enrichment. During April and August, periods of shoot growth, light-saturated photosynthesis and linear electron transport were increased by elevated CO2. In November, when growth had ceased but temperatures were still moderate, CO2 treatment had no significant effect on linear electron transport. In February, when low temperatures were likely to inhibit translocation, CO2 treatment caused a significant decrease in linear electron transport. This coincided with a slower recovery of the maximum photosystem II efficiency on transfer of needles to the shade, indicating that growth in elevated CO2 induced a more persistent photoinhibition. Both the summer increase and the winter decrease in linear electron transport in elevated CO2 resulted from a change in photochemical quenching, not in the efficiency of energy transfer within the photosystem II antenna. There was no evidence of any effect of CO2 on photochemical energy sinks other than carbon metabolism. Our results suggest that elevated CO2 may increase the effects of winter stress on evergreen foliage. 1 G.J.H. was supported by a studentship from the Natural Environment Research Council (United Kingdom). This research is part of the Forest-Atmosphere Carbon Transfer and Storage (FACTS-1) project at Duke Forest. The FACTS-1 project is supported by the U.S. Department of Energy (DOE), Office of Health and Environmental Research, under DOE contract nos. DE-ACO2-76CH00016 at Brookhaven National Laboratory and DE-FG05-95ER62083 at Duke University. * Corresponding author; e-mail stevel@life.uiuc.edu; fax 217-244-7563.
Plant Physiol. (1999) 120: 1183-1192
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