| Although the impact of saline stress on plants and mechanisms of salt resistance in plants are issues that has attracted extensive concern, no satisfactory theory is formulated in terms of the mechanisms of how plants fight against salt stress to absorb water under salination up to now, and still less is kown for salt-resistant mechanisms of arrowleaf saltbush which is a highly salt-resistant hylophyte and is cultivated as a novel vegetable. In this study, the characteristics of water and salt uptake, as well as the mechanisms involved in the accumulation, distribution and the osmoregulation in arrowleaf saltbush plants under salt stress were investigated with the employment of root pressure probe in combination with the Auger electron spectroscopy and atomic absorption spectroscopy. The effect of salinity on the transpiration and photosynthesis, as well as the mechanisms of adaptation of the plant to salt stress, were also studied. The results indicate that along with the increase in the intensity of saline stress in the exo-root solutions, the radial reflection coefficient of root declined stepwise accompanied by increase in the Na+ content in xylem exudates, but the Na+ content in xylem exudates based on the corresponding NaCl concentration in exo-root solutions showed a tendency of reduction following the decline of radial reflection coefficient and the elevation of NaCl level in the exo-root solutions. Although both the negative pressures and the Na+ content in xylem exudates increased with the intensity of saline stresses, the sum of the increase was far below the extent of the decrease of the osmotic potentials in exo-root solutions. The total water potential of xylem sap in arrowleaf saltbush roots was close to the osmotic potential of exo-root solutions when the saline stress was low; however, with the elevation in saline stress levels, the total water potential of xylem sap in arrowleaf saltbush roots overran the osmotic potential of the exo-root solutions gradually up to 1 MPa without the dehydration of the plants. Anyhow, the fact that the reduction in the radial reflection coefficient of roots in arrowleaf saltbush did not result in huge influx of NaCl in root xylem sap and the fact that the total water potential could be much higher than the osmotic potential of exo-root solution implied that the plants could employ small xylem pressure to counterbalance the saline stresses without the uptake of salt in large quantities.The experiments also showed that arrowleaf saltbush plants could localize the salt absorbed actively or passively to the vacuoles in order to regulate the osmotic potential of cells and thus avoided the injury of excessive salt to cells. Contrary to the conventional view, it was shown that the capacity of salt bladder in salt accumulation and salt secretion was very limited and no substantial effect of the salt bladders on the salt resistance of arrowleaf saltbush could be attributed.Additionally, it was observed that arrowleaf saltbush plants could reduce the transpiration and keep a relatively high level of photosynthesis via self regulation to reduce water loss, thus the water use efficiency was elevated and the growth could be maintained under intensive salt stress.It is therefore concluded that arrowleaf saltbush plants could employ a relatively small xylem tension to absorb water against the low osmotic potentials of the root bathing solution induced by salt; and the plants could get rid of salt injury via the relatively high ultrafiltration of root cells so as not to take excessive salt from the environment, and via the localization of salt to vacuoles to regulate the cell osmotic potential; and additionally, the plants could reduce the transpiration and keep a relatively high level of photosynthesis via self regulation to reduce water loss and enhance water use efficiency. All the factors mentioned above could be the physiological foundation of the high saline resistance in arrowleaf saltbush plants.
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