Combined effect of NaCl-salinity and hypoxia on growth, photosynthesis, water relations and solute accumulation in Phragmites australis plants

Aim of the present study was to investigate the effects of two key environmental factors of estuarine ecosystems, salinity and hypoxia, on the physiological attributes in reed plants (Phragmites australis (Cav.) Trin. ex Steudel). Growth, leaf gas exchange, water (and ion) relations, and osmotic adjustment were determined in hydroponically grown plants exposed to hypoxia at varying NaCl-salinity concentrations (0, 50, 100, and 200mM). Plants grew well under hypoxia treatment with standard nutrient solution without added salt and at NaCl concentrations up to 100mM. Reed plants were able to produce and allocate phytomass to all their organs even at the highest salt level (200mM NaCl). In plants subjected to hypoxia at various water potentials no clear relationships were found between growth and photosynthetic parameters except for g s, whereas growth displayed a highly significant correlation with plant-water relations. A and g s of reed plants treated with hypoxia at varying water potential of nutrient solutions were positively correlated and the former variable also had a strong positive relationship with E. Leaf Ψ w and Ψ π followed a similar trend and declined significantly as water potential of watering solutions was lowered. Highly significant positive correlations were identified between leaf Ψ w and photosynthetic parameters. At all NaCl concentrations, the increase in total inorganic ions resulted from increased Na⁺ and Cl⁻ while K⁺, Ca²⁺, and Mg²⁺ concentrations decreased with increasing osmolality of nutrient solutions. Common reed has an efficient mechanism of Na⁺ exclusion from the leaves and exhibited a high leaf K⁺/Na⁺ selectivity ratio over a wide range of salinities under hypoxia treatment. In Phragmites australis grown in 200mM NaCl, K⁺ contributed 17% toΨ π, whereas Na⁺ and Cl⁻ accounted for only 11% and 6%, respectively. At the same NaCl concentration, the estimated contribution of proline to Ψ π was less than 0.2%. Changes in leaf turgor occurred with a combined effect of salinity and hypoxia, suggesting that reed plants could adjust their water status sufficiently.