Low-Affinity Na~+ Uptake In Wheat And The Physiological Role Of Na~+ In Adapting To Osmotic Stress In Zygophyllum Xanthoxylum

Soil salinization and water shortage are serious environmental factors limiting the growth and productivity of agricultural crops worldwide. Drought and salinity are in a sense belong to the same osmotic stress, but salinity also have ion-excess effect in addition to the role of osmotic stress. High concentration of Na+ in the soil may inhibit plant growth for two main reasons:first, the presence of salt in the soil solution reduces the ability of the plant to take up water, and this leads to reductions in the growth rate, which is referred to as the osmotic or water-deficit effect of salinity; second, if excessive amounts of Na+ enter the plant in the transpiration stream there will be injury to cells in the transpiring leaves and this may cause further reductions in growth. This is called the salt-specific effect of salinity. Studying Na+uptake pathway is an important means to cope with such stressful environments. Appropriate content of Na+could improve the growth and drought tolerance of succulent xenophile Zygophyllum xanthoxylum.In this paper, based on succulent xerophytes and wheat, through experiments on sand culture and water culture, we measured unidirectional 22 Na+influx to examine Na+ influx pathway. The main results are as follows:1. Unidirectional 22Na+influx in wheat presents a two-phase response at external salt concentrations of 25 and 125 mmol/L which further developed the two-phase growth response model of Munns. Phase 1 of Na+absorption includes the first 12 hours after exposured to external salt.In this phase, there is high 22Na+influx of wheat. After 12h,22Na+influx of wheat started to decline, and ultimately to achieve stability.2. The function of wheat HKT1 may vary in different external NaCl concentration. Under low salt concentration (25 mmol/LNaCl), HKT1 exercise the function of recoverying Na+from xylem; under high salt concentration (125 mmol/L NaCl), HKT1 is mainly responsible for absorbing Na+in roots.3. Under the different osmotic stress(-0.5,-1.0 and-1.OMPa),50 mmol/L NaCl alleviated growth inhibition of Zygophyllum xanthoxylum caused by drought stress, compared with control (0 mmol/L NaCl), net photosynthetic rate increased by 9%,40%,155%(P<0.05), respectively. At the same time, proline content decreased by 66%,55%,31%, respectively, and contents of total sugar content decreased by 41%,33%,30%(P<0.05), respectively. The activities of Superoxide Dismutase(SOD),Peroxidase(POD) and Catalase(CAT) increased significantly with 50 mmol/L NaCl treatment.