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WHOLE PLANT AND ECOPHYSIOLOGY

The Penalty of a Long, Hot Summer. Photosynthetic Acclimation to High CO₂ and Continuous Light in "Living Fossil" Conifers¹

Department of Animal and Plant Sciences, University of Sheffield, Sheffield S10 2TN, United Kingdom

Deciduous forests covered the ice-free polar regions 280 to 40 million years ago under warm "greenhouse" climates and high atmospheric pCO₂. Their deciduous habit is frequently interpreted as an adaptation for minimizing carbon losses during winter, but experiments with "living fossils" in a simulated warm polar environment refute this explanation. Measured carbon losses through leaf abscission of deciduous trees are significantly greater than losses through winter respiration in evergreens, yet annual rates of primary productivity are similar in all species. Here, we investigate mechanisms underlying this apparent paradox by measuring the seasonal patterns of leaf photosynthesis (A) under pCO₂ enrichment in the same trees. During spring, A increased significantly in coastal redwood (Sequoia sempervirens), dawn redwood (Metasequoia glyptostroboides), and swamp cypress (Taxodium distichum) at an elevated pCO₂ of 80 Pa compared with controls at 40 Pa. However, strong acclimation in Rubisco carboxylation capacity (V_c,max) completely offset the CO₂ response of A in all species by the end of 6 weeks of continuous illumination in the simulated polar summer. Further measurements demonstrated the temporary nature of acclimation, with increases in V_c,max during autumn restoring the CO₂ sensitivity of A. Contrary to expectations, the acclimation of V_c,max was not always accompanied by accumulation of leaf carbohydrates, but was associated with a decline in leaf nitrogen in summer, suggesting an alteration of the balance in plant sources and sinks for carbon and nitrogen. Preliminary calculations using A indicated that winter carbon losses through deciduous leaf abscission and respiration were recovered by 10 to 25 d of canopy carbon fixation during summer, thereby explaining the productivity paradox.

¹ This work was supported by the Natural Environment Research Council (grant no. GR3/11900 to D.J.B.), by The Leverhulme Trust (to D.J.B.), and by Royal Society University Research Fellowships held by C.P.O. and D.J.B.

^* Corresponding author; e-mail c.p.osborne{at}sheffield.ac.uk; fax 44–114–222–0002.

Received May 7, 2003; returned for revision June 4, 2003; accepted June 23, 2003.

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