Effect of climate on the salt tolerance of two Phragmites australis populations. I. Growth, inorganic solutes, nitrogen relations and osmoregulation

This study estimated the effects of Danish (temperate nemoral) and Spanish (mediterranean) climatic conditions on the salt tolerance of two Phragmites australis (Cav.) Trin. ex Steud. populations. Plants propagated from seeds collected in Denmark (=Danish population) and Spain (=Spanish population) were exposed to salinity levels ranging from 0 to 30 parts per thousand at outdoor experimental plots situated in Denmark (56 degrees N) and Spain (41 degrees N). Plants were measured for growth rate, mortality, below-ground/ above-ground ratio, water content, ash content, nitrogen content, C:N and i ons (K+, Na+, Mg2+, Ca2+, Cl-). Growth was significantly enhanced at the Sp anish growth site, especially for salinity levels ranging from 10 to 20 par ts per thousand. In this interval, growth rates of both the Danish and the Spanish population were 20-100% higher than at the Danish growth site. Thes e growth data point to higher salt tolerance at the Spanish site when using relative salt tolerance and absolute salt tolerance as criteria. Salt tole rance is ascribed to two important mechanisms, cation adjustment and water loss. Despite higher transpiration at the Spanish site, plants were able to maintain or improve K+/Na+ ratios of leaf laminas, indicating enhanced cat ion adjustment under the Spanish climatic conditions. Primarily, this was d ue to higher K+ accumulation at the Spanish site (305-378 mmol kg(-1)) as c ompared to the Danish site (220-268 mmol kg(-1)), while a climatic effect o n Na+ level only was evident for the Danish population. Climatically induce d differences in water content may also have contributed significantly to t he higher salt tolerance observed for plants at the Spanish site. Shoot wat er content was lower at the Spanish site (1.9-2.4 g water g(-1) DW) than at the Danish site (2.6-4.0 g water g(-1) DW), suggesting that water loss is an important mechanism contributing to osmotic adjustment. Water loss may a ct by concentrating solutes in the cell sap, thereby reducing the need for synthesis of metabolically expensive organic osmotica as well as reducing t he need for Na+ uptake for osmotic adjustment. This study also showed that II australis populations differ in salt tolerance. A relatively higher salt tolerance observed for the Danish population was related to lower mortalit y, lower leaf Na+ and Cl- content, higher leaf K+/Na+ ratio, higher leaf ni trogen content, higher below-ground/above-ground ratio and lower ash conten t of below-ground parts and is discussed in relation to partitioning. (C) 1 999 Elsevier Science B.V. All rights reserved.