Contrasted effects of water limitation on leaf functions and growth of two emergent co-occurring plant species, Cladium mariscus and Phragmites australis Anna Saltmarsh a,1 , Andre ´ Mauchamp b, * , Serge Rambal a,1 a CEFE CNRS Montpellier BP 5051, Montpellier Cedex, France b Station Biologique de la Tour du Valat, Le Sambuc, 13200 Arles, France Received 10 June 2004; received in revised form 31 August 2005; accepted 20 September 2005 Abstract Phragmites australis tolerates a wider range of ecological conditions than Cladium mariscus and thus may be better adapted to disturbance resulting from human manipulations. We hypothesised that the difference in ecological conditions was related to differences in photosynthetic ability under water stress. We compared the reactions of the two species to water deficit related to the summer drawdown in the Mediterranean regions that may be enhanced by drainage for human activities. Stress tolerance was evaluated in a pot experiment measuring pigment contents, chlorophyll fluorescence and gas exchanges, growth rates, living and dead biomass, for plants grown in waterlogged conditions or submitted to permanent or temporary drainage. Reed was favoured by the first days of drainage with an increase in CO 2 assimilation by approximately 5 mmol m 2 s 1 and an increase of the elongation rates from 1 to 1.5 cm d 1 , indicating waterlogged conditions were not optimal for this species. While pigment content showed no significant effects, growth rate, percentage live above-ground biomass, chlorophyll fluorescence and net rate of CO 2 assimilation declined significantly for both species from days 5 to 7 after drying started. Assimilation rates reached 2.3 and 0.3 mmol m 2 s 1 on day 9 for sawsedge and reed, respectively. Sawsedge had a 50% greater assimilation rate in the waterlogged conditions, but its responses to water loss in terms of declining growth rate and CO 2 assimilation started on day 5 versus 7 for reed. By contrast, the condition of PSII as indicated by chlorophyll fluorescence was affected more in reed. Both growth rates and rates of CO 2 assimilation had completely recovered for both species within 3 days of re-wetting. Our results reflect the evergreen character of sawsedge leaves that are hardly damaged by stress but decrease their activity. Reed leaves die upon excessive stress and plants may shed leaf as an adjustment to stress. Sawsedge photosynthesis will be optimal in waterlogged conditions whereas that of reed will be constant within a broad range from slightly drained to mild stress. Those responses to drainage at the leaf scale may partly explain the difference in habitat where both species are usually found. # 2005 Elsevier B.V. All rights reserved. Keywords: Sawsedge; Sawgrass; Common reed; Water stress; Fluorescence; Photosynthesis; Wetland 1. Introduction With some exceptions in continental wetlands in semi-arid countries (Humphries and Baldwin, 2003; McMahon and Finlayson, 2003), effects of water deficit on wetland plants have seldom been studied. The occurrence of such events may however increase after human-made manipulations of the water cycle, particularly drainage (Alvarez-Cobelas et al., 2001), or as a consequence of climate change. Sawsedge, or sawgrass, Cladium mariscus (L.) R.Br., a Cyperaceae, and common reed Phragmites australis (Cav.) Trin. ex Steud., a Poaceae, are commonly found in mixtures or forming mosaics of neighbour- ing patches. Sawsedge vegetation is the only habitat dominated by a tall emergent species classified under the European Habitat Directive, thus recognising its value for conservation of biodiversity. It may be long-lasting provided water levels and quality remain favourable and prevent competitive displacement by other species. Christanis (1994) described in Greece a wetland with a 15 m thick layer of sawsedge peat aged approximately 19 000 years, thus underlying its value as a C sink. Both species are perennial but while sawsedge is evergreen, aboveground biomass of reed dies back in winter. Interactions www.elsevier.com/locate/aquabot Aquatic Botany 84 (2006) 191–198 * Corresponding author. Tel.: +33 4 90 97 20 13; fax: +33 4 90 97 20 19. E-mail addresses: mauchamp@tourduvalat.org (A. Mauchamp), serge.rambal@cefe.cnrs.fr (S. Rambal). 1 Tel.: +33 4 67 61 32 89; fax: +33 4 67 41 21 38. 0304-3770/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.aquabot.2005.09.010