| The physiological response of saltgrass(halophyte) and tall fescue (glycophyte) to salt stress, as well as the distribution and transport of Na+, were studied experimentally at both cellar and whole plant levels to better understand the mechanism of salt tolerance and Na+ transport in higher plants. The research work and main results are summarized below.1. The physiological response and the activity of antioxidative enzymes of saltgrass and tall fescue under salt stress were studied. Plant growth, net photosynthesis, relative electrolyte leakage percentage (RELP), proline content and activity of antioxidative enzymes were measured after 0, 5,10,15,20 days of 250mM NaCl treatment. The results showed that the shoot weight and net photosyntheis of both saltgrass and tall fescue were decreased significantly with prolonged salt stress, with the reduction being more evident in tall fescue than in saltgrass. Salt stress led to an increase in RELP in leaf tissues in both species but it is higher in tall fescue, while no significant change in proline cotent was detected in both species. Salt stress also increased the SOD activity in leaves and roots of both species, with that of saltgrass being higher than in tall fescue, suggesting that the saltgrass had better ability of O2.- radical scavenging. With increased salt stress, leaf APX activity in saltgrass was significantly higher than in tall fescue, but there was no significant difference in root APX between the the two species. These suggested that APX and SOD enzymes jointly contributed to scavenging of H2O2 resulted from SOD reaction and the actitivy took place mainly in leaf tissues. The activity of CAT in leaf tissues of saltgrass increased significantly with increased salt stress, however that of tall fescue was not affected significantly by salt stress.2. The effects of exogenous NaCl on ion distribution and transport were investigated in both species. Na+, K+,Ca2+,Mg2+ content in root, shoot and leaf were measured after 0, 5,10,15,20 days of 250mM NaCl treatment. The results showed that Na+ content in root, shoot and leaf tissues increased as a response to increased salt stress. After 15 days of NaCl treatment, Na+ content in leaf of saltgrass was significantly higher than that of tall fescue (P<0.05). K+ content decreased in leaves of both species, while that of saltgrass was higher than that Tall fescue after 25 days of NaCl treatment. K+ content in leaf tissue, Ca2+ content in shoot and leaf of tall fescue was higher than in saltgrass, indicating a better condition for tall fescue to maintain the normal metabolic function . K+/Na+ ration in root of saltgrass was lower than that of tall fescue, indicating more Na+ leaked into root tissue.Higher K+-Na+ selectivity was detected in the root-to-shoot and shoot-to-root transport in saltgrass as compared with that in Festuca arundinaces. The results indicated that saltgrass had more control on K-Na selectivity.3. Microdistribution of mineral ions in roots, stems and leaves of saltgrass and tall fescue was investigated under salt stress using an energy dispersive X-ray microanalyzer (EDX) to explore its relations to salt-tolerance. Plants of the two species were grown hydroponically with 250 mM NaCl . Root tips, stem and leaves were harvested for microanalysis 20 days after transplanting. X-ray percentage of content of atom and the peak of X-ray energy spectra intensity were measured in the sections. It was shown that high Cl and Na X-ray peaks were recorded in root tissues of the both specieswith the X-ray percentages of content of Na and Cl in stelar cells of roots of tall fescue being higher than that of Distichlis spicata. In contrast with tall fescue, saltgrass had relatively high percentage of content Na+ in xylem, and Cl- in phloem. The percentage of Na+ in leaves of tall fescue was 2 times of that of saltgrass.The results suggested that saltgrass regulated the absorption of toxic irons by the mechanism of selective absorption in root, recirculated Na back to root through phloem or retrieval of Na+ from xylem so that less toxic ions were transported to leaf tissues. It was clear that a micro-distribution of mineral ions in plants was involved in the salt tolerance machanism.4. The micro-structure, distribution of salt glands, salt excretion and the ecophysiological environments in saltgrass were observed using scanning electron microscopyin relation to diurnal changes in net photosynthesis, transpiration rate and other ecophysiological factors. Diurnal dynamics of net photosynthesis, transpiration rate, ionic secretion and ion content were measured after 30 days of NaCl treatment. The results showed that there was generally a positive correlation between the atmosphere humidity and ion secretion during the day (R>0.843), with the excretion rate being the highest at 9:00 AM (61.3μmol Na+ g-1f.wt h-1). The ions could be ranked, according to their excretion rate, as Na>>K>Ca>Mg.5. A segment of vacuolar Na+/H+ antiporter gene was cloned from saltgrass using PCR and a sequence of 600 bp in length was obtained. The sequence was highly homologous with the Na+/H + antiporter genes from other plants. X-ray microanalysis were used to investigate the Na+ cellar distribution in saltgrass after 0,5,10,15,20,25 days of NaCl treatment. The results showed that Na+ in cytoplasm was compartmented into vacuole and apoplast. Meanwhile the activity of vacuolar H+-ATPase increased after salt treatment, while the activity of plasma H+-ATPase did not increase. And the expression of vacuolar Na+/H+ antiporter increased using real-time PCR. In sum, we found that a part of Na+ which increased in leaf of saltgrass was accumulated in vacuolar and was compartmented. In the proceed on compartment, more energy was supplied for Na+ transport by the increase of vacuolar H+-ATPase, in coordination expression of vacuolar Na+/H+ antiporter as carrier was also increasing. However the activity of H+-ATPase in plasma had not significant change although the Na+ in apoplast increased. It suggested that maybe some ion channels were cotrolled to inhibtated Na+ into cytolplasm.
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