Ecophysiological constraints of two invasive plant species under a saline gradient: Halophytes versus glycophytes B. Duarte a, * , D. Santos a , J.C. Marques b , I. Caçador a a MARE e Marine and Environmental Sciences Centre, Faculty of Sciences of the University of Lisbon, Campo Grande,1749-016 Lisbon, Portugal b MARE e Marine and Environmental Sciences Centre, Faculty of Sciences and Technology, University of Coimbra, 3000 Coimbra, Portugal article info Article history: Accepted 23 April 2015 Available online 2 May 2015 Keywords: chlorophyll fluorescence pigment profiles salt stress osmosolutes invasive species abstract Salt marsh environments are harsh environments where salinity comprises one of the most important species distribution shaping factor, presenting sediment salinities from 0 to 855 mM (0e50 ppt). Invasive species have often a high colonizing potential, due to its high plasticity and adaptation ability. Spartina patens is an invasive species already spread along several Mediterranean countries, like France and Spain. Cyperus longus is typically a freshwater species that has been spreading across the Mediterranean. In order to evaluate the ecophysiological fitness of these species, mesocosmos trials were performed subjecting both species to increasing realistic salinity levels and their photochemical and biochemical feedback was evaluated. Both species presented very different behaviours. S. patens appears to be insensitive to salt stress, mostly due to elevated proline concentrations in its leaves allowing it to maintain its osmotic balance, and thus preventing the damaging of its photochemical mechanisms. C. longus, on the other hand, was highly affected by elevated salt levels mostly due to the lack of osmotic balance driven by an incapacity to counteract the elevated ionic strength of the external medium by osmocompatible solutes. S. patens is physiologically highly adapted to saline environments and thus is capable to colonize all the marsh saline environments, while C. longus appears to be an opportunistic invader colonizing the marsh during periods of lower salinities typical from rainy seasons. © 2015 Elsevier Ltd. All rights reserved. 1. Introduction About 70% of the Earth's surface is covered by salt water, the oceans, with concentrations of Na þ around 500 mM, contrasting with the low K þ concentrations of 9 mM (Flowers, 2004). Also, the remaining 30% of the Earth's surface is severely affected by an increased salinization phenomenon, mostly due to the increased soil use on agriculture and its irrigation procedures (Zhang and Shi, 2014). The ongoing climatic changes are increasing drought, air temperature and salt-water intrusion in coastal soils (Duarte et al., 2013). Soil and water salinity substantially constrain crop and biomass production. Above 400 million hectares of land are affected by salinity and this area is increasing day by day due to the excessive irrigation practices, and to the tremendous increase of world population and consequent increasing demand for food supply. Crops cannot be grown on a salt-affected soils, but nature has provided us with a unique group of plants that can, the halo- phytes (Asla et al., 2011). Salinity-induced damage in plants in- cludes reduction of leaf expansion, stomata closure, reduce primary production, biomass losses due to water deficit and deficiency in essential nutrients like K þ (Mahajan and Tuteja, 2005; James et al., 2011; Rahnama et al., 2011). Although this is true for most of the Earth's flora, halophytes are the exception, being highly productive under saline conditions. Halophytes are defined as plant species that can survive and reproduce under growth conditions with more than 200 mM NaCl, comprising only 1% of world flora (Flowers and Colmer, 2008). By opposition, glycophytes are defined as species that cannot survive in a saline environment. The recent on-going environmental changes tend to modify the climatic habitats avail- able for colonization in the ecosystems. Salt marshes are no exception to these changes. In these environments species distri- bution is mostly modulated by marsh elevation, being this last a key factor affecting the sediment flooding, redox potential, pH and salinity (Caçador et al., 2007). In fact, this last abiotic driver is one of the most important shapers of the salt marshes morphology con- ditioning species distribution according to their salinity tolerance. Nevertheless, within the halophytes group, several salt tolerance * Corresponding author. Marine and Environmental Sciences Centre, Faculty of Sciences of the University of Lisbon, Campo Grande,1749-016 Lisbon, Portugal. E-mail address: baduarte@fc.ul.pt (B. Duarte). Contents lists available at ScienceDirect Estuarine, Coastal and Shelf Science journal homepage: www.elsevier.com/locate/ecss http://dx.doi.org/10.1016/j.ecss.2015.04.007 0272-7714/© 2015 Elsevier Ltd. All rights reserved. Estuarine, Coastal and Shelf Science 167 (2015) 154e165