Drought adaptation confers short-term but not long-term salt tolerance in cocksfoot, Dactylis glomerata

The effects of different levels of NaCl (0, 50, 100 and 150 mM) on growth, water relations, ions accumulation and nitrogen assimilation in two varieties of Dactylis glomerata, i.e., Hispanica from a semi-arid area and Frontier from a temperate climate, were studied in a greenhouse in hydroponic culture. Salinity induced a significant decrease in the growth of both varieties; however, their responses to NaCl stress were different. Based on weight and photosynthesis, Hispanica was found initially to be more resistant to salinity than Frontier. However, over a longer period of salinity treatments, Frontier displayed higher salt tolerance, especially at 50 mM NaCl concentration. The initial resistance to salinity of Hispanica, compared to Frontier, appeared to be related with a greater decrease in transpiration and water potential due to salinity, while there was no change in relative water content, suggesting better osmotic adjustment. In contrast, Frontier demonstrated more exclusion of Na⁺ and Cl^- from the xylem stream and a higher K⁺/Na ⁺ ratio, leading to a less harmful effect of salinity in the shoot, manifesting itself in better growth of this variety over the longer term. The results support the hypothesis that growth response to salinity has two phases. The first phase of growth reduction is mostly due to water (osmotic) stress, with the drought adapted variety, Hispanica, exhibiting higher tolerance. In the second phase, when toxic effects of accumulated Na⁺ and Cl ^- in tissues caused growth retardation, Frontier proved to have the higher resistance to salinity.