REVIEW PAPER Photosynthetic acclimation in the context of structural constraints to carbon export from leaves William W. Adams III Æ Amy M. Watson Æ Kristine E. Mueh Æ Ve ´ ronique Amiard Æ Robert Turgeon Æ Volker Ebbert Æ Barry A. Logan Æ Andrew F. Combs Æ Barbara Demmig-Adams Received: 31 August 2006 / Accepted: 7 December 2006 / Published online: 9 January 2007 Ó Springer Science+Business Media B.V. 2007 Abstract The potential role of foliar carbon export features in the acclimation of photosynthetic capacity to differences and changes in light environment was evaluated. These features included apoplastic vs. symplastic phloem loading, density of loading veins, plasmodesmatal frequency in intermediary cells, and the ratio of loading cells to sieve elements. In initial studies, three apoplastic loaders (spinach, pea, Ara- bidopsis thaliana) exhibited a completely flexible photosynthetic response to changing light conditions, while two symplastic loaders (pumpkin, Verbascum phoeniceum), although able to adjust to different long-term growth conditions, were more limited in their response when transferred from low (LL) to high (HL) light. This suggested that constraints im- posed by the completely physical pathway of sugar export might act as a bottleneck in the export of carbon from LL-acclimated leaves of symplastic loaders. While both symplastic loaders exhibited var- iable loading vein densities (low in LL and high in HL), none of the three apoplastic loaders initially characterized exhibited such differences. However, an additional apoplastic species (tomato) exhibited sim- ilar differences in vein density during continuous growth in different light environments. Furthermore, in contrast to the other apoplastic loaders, photosyn- thetic acclimation in tomato was not complete fol- lowing a transfer from LL to HL. This suggests that loading vein density and loading cells per sieve ele- ment, and thus apparent loading surface capacity, play a major role in the potential for photosynthetic acclimation to changes in light environment. Photo- synthetic acclimation and vein density acclimation were also characterized in the slow-growing, sclero- phytic evergreen Monstera deliciosa. This evergreen possessed a lower vein density during growth in LL compared to HL and exhibited a more severely lim- ited potential for photosynthetic acclimation to in- creases in light environment than the rapidly-growing, mesophytic annuals. Keywords Apoplastic loading Á Companion cells Á Intermediary cells Á Leaf vein density Á Phloem Á Phloem parenchyma cells Á Photosynthetic acclimation Á Plasmodesmata Á Symplastic loading Á Transfer cells Abbreviations A Antheraxanthin CC Companion cell HL High light LL Low light PC Phloem parenchyma cell PFD Photon flux density SE Sieve element W. W. Adams III (&) Á A. M. Watson Á K. E. Mueh Á V. Amiard Á V. Ebbert Á B. Demmig-Adams Department of Ecology & Evolutionary Biology, University of Colorado, Boulder, CO 80309-0334, USA e-mail: william.adams@colorado.edu R. Turgeon Department of Plant Biology, Cornell University, 256 Plant Science Building, Ithaca, NY 14853, USA B. A. Logan Á A. F. Combs Department of Biology, Bowdoin College, 6500 College Station, Brunswick, ME 04011, USA Present Address: V. Amiard Instituto de Investigaciones Agropecuarias, Unidad de Biotecnologı´a, INIA Carillanca, Casilla 58-D Temuco, Chile 123 Photosynth Res (2007) 94:455–466 DOI 10.1007/s11120-006-9123-3