Does Free-Air Carbon Dioxide Enrichment Affect Photochemical Energy Use by Evergreen Trees in Different Seasons? A Chlorophyll Fluorescence Study of Mature Loblolly Pine1

Previous studies of the effects of growth at elevated CO(2) on energy partitioning in the photosynthetic apparatus have produced conflicting results. The hypothesis was developed and tested that elevated CO(2) 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 CO(2) or ambient plus 20 Pa CO(2) using free-air CO(2) enrichment. During April and August, periods of shoot growth, light-saturated photosynthesis and linear electron transport were increased by elevated CO(2). In November, when growth had ceased but temperatures were still moderate, CO(2) treatment had no significant effect on linear electron transport. In February, when low temperatures were likely to inhibit translocation, CO(2) 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 CO(2) induced a more persistent photoinhibition. Both the summer increase and the winter decrease in linear electron transport in elevated CO(2) 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 CO(2) on photochemical energy sinks other than carbon metabolism. Our results suggest that elevated CO(2) may increase the effects of winter stress on evergreen foliage.