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

Abstract

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‰ at outdoor experimental plots situated in Denmark (56°N) and Spain (41°N). Plants were measured for growth rate, mortality, below-ground/above-ground ratio, water content, ash content, nitrogen content, C:N and ions (K +, Na +, Mg 2+, Ca 2+, Cl −). Growth was significantly enhanced at the Spanish growth site, especially for salinity levels ranging from 10 to 20‰. In this interval, growth rates of both the Danish and the Spanish population were 20–100% higher than at the Danish growth site. These growth data point to higher salt tolerance at the Spanish site when using relative salt tolerance and absolute salt tolerance as criteria. Salt tolerance 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 cation adjustment under the Spanish climatic conditions. Primarily, this was due to higher K + accumulation at the Spanish site (305–378 mmol kg −1) as compared to the Danish site (220–268 mmol kg −1), while a climatic effect on Na + level only was evident for the Danish population. Climatically induced differences in water content may also have contributed significantly to the higher salt tolerance observed for plants at the Spanish site. Shoot water 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 act by concentrating solutes in the cell sap, thereby reducing the need for synthesis of metabolically expensive organic osmotica as well as reducing the need for Na + uptake for osmotic adjustment. This study also showed that P. australis populations differ in salt tolerance. A relatively higher salt tolerance observed for the Danish population was related to lower mortality, lower leaf Na + and Cl − content, higher leaf K +/Na + ratio, higher leaf nitrogen content, higher below-ground/above-ground ratio and lower ash content of below-ground parts and is discussed in relation to partitioning.