Moderate salinity stimulates growth and photosynthesis of Phragmites karka by water relations and tissue specific ion regulation

•Salt resistance was evaluated in Phragmites karka: a candidate for biofuel crop.•Growth of P. karka increased at 100 mM NaCl by ionic homeostasis.•Higher root biomass supports the water and mineral uptake at moderate salinity.•Decrease in water relations caused growth inhibition in 300 mM NaCl.•Photosynthesis was positively related to plant growth.

The domestication of non-food salt resistant plants could be helpful for biofuel as well as phyto-remediation of salt-affected lands. Limited eco-physiological studies on halophytes are the major bottleneck in developing such plants as energy crops. The aim of the present work was to determine the effect of salt stress on growth, ion homeostasis, water status and photosynthesis of Phragmites karka – a potential biofuel crop. Plants were grown in 0, 100 and 300 mM NaCl (equivalent to non-saline, 20 and 60% seawater). Our results indicate that the addition of 100 mM NaCl to the growth medium increased plant biomass. Leaf osmotic adjustment was primarily achieved by inorganic solutes (K+ and Na+) while soluble sugars and proline remained unchanged. Higher K+ selectivity and Na+ exclusion from shoots improved photosynthesis and growth under moderately saline conditions. Decreased growth in 300 mM NaCl could be attributed to leaf tissue dehydration and turgor loss leading to stomatal closure and decreased CO2 absorption. Our results indicate that P. karka could produce high ligno-cellulosic biomass for ethanol production using brackish water irrigation on saline soils avoiding competition with conventional agriculture.