Abstract
Populus euphratica Olivier is known to exist in saline and arid environments. In this study we investigated the physiological mechanisms enabling this species to cope with stress caused by salinity. Acclimation to increasing Na+ concentrations required adjustments of the osmotic pressure of leaves, which were achieved by accumulation of Na+ and compensatory decreases in calcium and soluble carbohydrates. The counterbalance of Na+/Ca2+ was also observed in mature leaves from field-grown P. euphratica trees exposed to an environmental gradient of increasing salinity. X-ray microanalysis showed that a primary strategy to protect the cytosol against sodium toxicity was apoplastic but not vacuolar salt accumulation. The ability to cope with salinity also included maintenance of cytosolic potassium concentrations and development of leaf succulence due to an increase in cell number and cell volume leading to sodium dilution. Decreases in apoplastic and vacuolar Ca2+ combined with suppression of calcineurin B-like protein transcripts suggest that Na+ adaptation required suppression of calcium-related signaling pathways. Significant increases in galactinol synthase and alternative oxidase after salt shock and salt adaptation point to shifts in carbohydrate metabolism and suppression of reactive oxygen species in mitochondria under salt stress.
Highlights
Populus euphratica Olivier is known to exist in saline and arid environments
Osmometric measurements showed that P. euphratica was able to adjust the osmotic pressure of leaves to levels just exceeding those of the nutrient solution, which is important to maintain water uptake and to prevent dehydration (Fig. 1)
To identify major components contributing to the physiological adjustment of P. euphratica to salt stress, nutrient elements, carbohydrates, and amino acid concentrations were determined in leaves (Figs. 2 and 3; Supplemental Fig. 1)
Summary
Populus euphratica Olivier is known to exist in saline and arid environments. In this study we investigated the physiological mechanisms enabling this species to cope with stress caused by salinity. Soil salinization is still increasing mainly because of unsuitable irrigation practices To cope with this enormous problem, efforts are undertaken to enhance the salt tolerance of economically important plants by traditional plant breeding as well as biotechnological approaches. Appropriate strategies may include enhancing stress resistance of salt-sensitive plant species or using plants that naturally display high salt resistance (Flowers, 2003) For the latter option, the stress-tolerant tree Populus euphratica (Olivier) seems a promising candidate. At the wholeplant level many nonhalophytes try to exclude salt (Greenway and Munns, 1980) This strategy is not important in P. euphratica, which showed no restriction of Na1 uptake into roots compared with salt-sensitive poplar species (Chen et al, 2001). At the cellular level common metabolic answers to salt stress are the synthesis of stress-related enzymes like antioxidant systems, chaperons
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