| In saline-alkali area, applicable plants'varieties are few, bring about plant landscape monotony and single color. Therefore, breeding fine ornamental plant with salt-tolerant capability is the key of enriching plant landscapes in saline-alkali area. The Spiraea is garden foliage shrubs with high ornamental value. This research selected three-year-old cutting seedlings of 4 spiraeas (Spiraea thunbergii, Spiraea japonica, Spiraea×bumalda'Gold Mound', Spiraea×bumalda'Gold Flame') as test material, which were treated under 20 groups of artificially-simulated complex saline-alkali stresses with different level of salinity and alkalinity. By studying on the change lows of root vitality, osmoregulation substance accumulation and active oxygen metabolism level in roots of 4 spiraeas under saline-alkali stress, to revealing salt-tolerant ability among different varieties and discussing spiraeas'adaptability to saline-alkali stress. It will provide scientific theoretical basis for selecting excellent landscape green seedlings for saline-alkali area. This is the first time to study on the root's physiological response to saline-alkali stress, the results are showed as follows:1. The changes of root vitality of 4 spiraeas increased firstly and then decreased with enhanced saline-alkali stress. It indicated that spiraeas had a certain tolerant ability to saline-alkali stress, root vitality increased, but when salinity and alkalinity exceeded certain range, the roots of spiraeas were severely injured, root vitality decreased. During the stress, the root vitality of Spiraea thunbergii was strongest, and the corresponding salinity and pH value were highest while root vitality declines, the next strongest in turn were Spiraea×bumalda'Gold Flame', Spiraea×bumalda'Gold Mound', Spiraea japonica.2. Along with the increases of the salinity and pH value, the electrolyte leakage rates, the contents of MDA and oxygen free radicals all showed an increasing tendency, and there were significantly positive correlation among three indices, the increasing rates were greater with the increase of stress degree. The ranges of variation of electrolyte leakage rates, the contents of MDA and oxygen free radicals in roots of Spiraea thunbergii and Spiraea×bumalda'Gold Flame'were obviously smaller than them in roots of Spiraea japonica and Spiraea×bumalda'Gold Mound'.3. Under saline-alkali stress, the contents of proline, soluble sugar and soluble protein in roots of 4 spiraeas all had an increased trend, and the increasing ranges of these three osmoregulation substance content were larger with higher salnity and pH value. But while exceeding certain salinity and pH, the roots injured seriously, the contents of osmoregulation substance decreased. The corresponding salnity and pH value of Spiraea thunbergii were highest when the contents of three osmoregulation substance declines, that of Spiraea×bumalda'Gold Flame'was second, and that of Spiraea japonica and Spiraea×bumalda'Gold Mound'were lowest. 4. Under saline-alkali stress, the change regulations of SOD, POD and CAT in roots of 4 spiraeas all increased firstly and then decreased. It indicated that, three varieties of protective enzymes could timely eliminated oxygen free radicals under certain salt-alkali stress, but along with the stress enhanced, the structure and function of protective enzyme might be damaged, the enzyme activities declined. When the activities of three protective enzymes declined, the corresponding salnity and pH value were highest by Spiraea japonica, second by Spiraea×bumalda'Gold Mound'and Spiraea×bumalda'Gold Flame', and lowest by Spiraea japonica.5. At lower salinity with increased pH value (such as salinity 60mmol·L-1, pH 7.12~10.21), or at lower pH value with increased salinity (such as neutral salt treatment group A, salinity 60~240mmol·L-1), the increasing extents of each physiological index in roots of 4 spiraeas were low, but with increased salinity and pH value, the changes of each index were gradually obvious. Higher salinity and pH more seriously affected roots'normal physiological metabolism, root vitality, osmoregulation substances and protective enzyme activities all declined, it showed that, the interrelation of salinity and alkalinity will make a stronger damage to plants.6. The correlation analysises between seven kinds of stress factors and each physiological index in roots of 4 spiraeas indicate that, the effects of salinity, pH value, [Na+], [CO32-] to spiraeas are the greatest, and that of [Cl-] and [SO42-] are the smallest can be ignored. The stress effect of [Na+] can be completely represented by salinity (r=1).Synthesizing each physiological index, the order from strong to weak in saline-alkali tolerant ability of 4 spiraeas in turn is Spiraea thunbergii, Spiraea×bumalda'Gold Flame', Spiraea×bumalda'Gold Mound', Spiraea japonica.
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