RESEARCH COMMUNICATIONS CURRENT SCIENCE, VOL. 115, NO. 1, 10 JULY 2018 129 *For correspondence. (e-mail: raji.swaminathan@gmail.com) Responses of selected C3 and C4 halophytes to elevated CO 2 concentration under salinity Saranya Jothiramshekar 1 , Jenifer Joseph Benjamin 1 , Rani Krishnasamy 1 , Anand Kumar Pal 1 , Suja George 1 , Rajalakshmi Swaminathan 1, * and Ajay K. Parida 1,2 1 Department of Plant Molecular Biology, M.S. Swaminathan Research Foundation, III Cross Street, Taramani Institutional Area, Taramani, Chennai 600 113, India 2 Institute of Life Sciences, Department of Biotechnology, Government of India, Bhubaneswar 751 023, India Halophytes have superior capacity to withstand soil salinity and are appropriate resources to study the mechanism of salt tolerance which can be harnessed to develop crops to withstand salinity. In this communi- cation, we report the effect of salinity (200 mM NaCl) and elevated carbon dioxide (CO 2 ) treatments in tan- dem, on select halophytes that have different photo- synthetic pathways: C3 and C4. The plants were raised in ambient (380 ppm) and enriched (500 ppm) concentrations of CO 2 using a mini-FACE facility. Total chlorophyll content, total soluble sugar concen- tration, lipid peroxidation level and electrolyte leak- age were measured from fresh leaf samples collected at different time points. The results show a positive effect for elevated CO 2 concentration on salt tolerance in both C3 and C4 plants, and indicate that halophytes may benefit from rising atmospheric CO 2 concentra- tion. The results also suggest that C4 halophytes may benefit from the rising atmospheric CO 2 concentra- tion than C3 halophytes. Keywords: Elevated carbon dioxide, halophytes, salinity, photosynthetic pathway, Sesuvium portulacastrum, Suaeda nudiflora. ALTHOUGH the earth’s climate has never been static, the planet is currently experiencing faster changes in climate/weather patterns than it had in the past, due to increased anthropogenic activities. Since the beginning of the Industrial Revolution, carbon dioxide (CO 2 ) concen- tration in the atmosphere has increased from 280 to approximately 400 ppm (ref. 1) and is estimated to increase by 2.25 ppm each year according to the US National Oceanic and Atmospheric Administration (NOAA). Increasing concentrations of greenhouse gases (GHGs), including CO 2 are directly linked to changes in temperature, rainfall, near-surface radiation, higher solar radiation and desertification 2–4 . Soil salinization affects almost 7% of the total global land area and 20–50% of the global irrigated farmland, and is largely due to deser- tification 5 . Soil salinity is a major abiotic stress that impacts plant growth and productivity 6 . High salt concentrations decrease water uptake leading to water stress and inhibit key metabolic processes such as photosynthesis. Soil salinization also inhibits plant growth due to osmotic and ionic stress 7 . High soil salinity leads to enhanced produc- tion of reactive oxygen species (ROS) in plants, which is accompanied by increased membrane lipid peroxidation 8 . Most plant species are sensitive to salinity even at low concentrations (glycophytes), while some species can tolerate and complete their life cycle even at high saline concentrations (halophytes). Increase in atmospheric CO 2 coupled with salinity adds another component to the range of complex physiological and morphological responses in plants that directly impact photosynthetic processes. Plant response to increased CO 2 concentration and/or salinity differs, and is based on the photosynthetic pathways followed, which impacts growth rates and other factors. Most species that use the C3, photosynthetic pathway respond favourably to elevated atmospheric CO 2 (refs 9, 10). However, long- term exposure to elevated levels of CO 2 has been reported to substantially suppress photosynthesis 11 . Many plant species that use C4 and CAM pathways respond posi- tively to increase in atmospheric CO 2 too, but the res- ponses are generally less vigorous than that of C3 plants 12,13 . At increased atmospheric CO 2 concentration, plants perhaps counteract the water stress better, caused due to salinity, by reducing transpiration and increasing water potential 14 . As more carbon dioxide is assimilated due to greater diffusion of this gas into the leaf, it causes an increase in the supply of carbohydrates. Increased carbo- hydrate concentration will reduce the osmotic potential and hence turgor pressure is maintained 15 . Additionally, plants use water more efficiently due to decrease in stomatal conductance 16,17 . Some studies support the hypothesis that CO 2 enrichment stimulates plant growth via increased photosynthesis and not necessarily improved water relations 18 . All these reports point towards the need for more comprehensive studies on the interactions between elevated CO 2 and salinity conditions with different plant species. Hence in this study, we evaluated the combined effect of salinity and elevated CO 2 on photosynthetic pigment content, lipid peroxidation level, electrolytic leakage and osmolyte concentration (total soluble sugars) of two halophytes. Sesuvium portulacastrum (Aizoaceae) is an important mangrove-associated, facultative halophyte and follows the C3 pathway. This plant has the ability to grow under high salinity with lower nutrition availability; it is used as a soil cover and landscaping plant, and plays a vital role in alleviating saline soil, desalination, desert greenification and as an alternative to problem soils in