Physiological Responses Of Six Ground Cover Plants To Sulfur Dioxide Stress

Ground cover plants are one of the essential plant species for landscape construction in current urban greening, so understanding the physiological mechanism of these plants'resistance to sulfur dioxide (SO2) can help us make decision on the suitable species which can not only display the effect of landscape but also exert ecological functions. In this study, the selected six ground cover plant species are divided into two categories:three kinds of heliophilous ground plants(Petasites japonicas, Trigonotis omeiensis,and Stachys lanata) and three kinds of sciophilous ground cover plants (Ophiopogon japonious, Hosta ventricosa and Saxifraga stolonifera). They were screened for tolerance for experimental materials. Three SO2 treatments including 5.71,11.43 and 17.43 mg·m-3 were used in the three kinds of heliophilous ground cover plants while four treatments, namely,5.71,11.43,17.43,22.86 mg·m-3 were used in the three kinds of sciophilous ground cover plants. The release of SO2 was achieved by using the artificial simulated fumigation method. The phenotypic injuries and the various physiological indexes among these six species in response to SO2 stress were assessed. Also the SO2 resistant ability of three plants in each category was compared. It is significant in providing scientific evaluation of SO2 resistance and sulfur purification and giving references for selecting proper species in landscape construction. Our results are as following:The visible injury of three kinds of heliophious ground cover plants varied with different SO2 concentration levels. Petasites japonicus had the slightest injury with only small necrotic spots when treated with the highest concentration of SO2 and the damage rate was 5%. Trigonotis omeiensis ranked the second with a damage rate of 25%. Stachys lanata suffered the severest injury. In the highest concentration SO2 treatment, large necrotic areas appeared on the leaf and the damage rate reached 45%. When treated, the chlorophy(a+b) content of the three heliophilous ground cover plants generally displayed a decreasing trend but the degree varied from one to another. Compared with controlle group, the chlorophy(a+b) content of Stachys lanata, Petasites japonicus and Trigonotis omeiensis decreased by 40.5%,29.6% and 20.8%, respectively treated by the highest SO2 concentration. Meanwhile, the leaf pH value of the three species decreased to different extent under SO2 stress. In comparison with controll group, Stachys lanata decreased by 0.86, Trigonotis omeiensis by 0.38 and Petasites japonicus by 0.17. They also underwent membrane lipid peroxidation and the relative conductivity rate of them increased with the surge in SO2 concentration. The ranking of their degree was Stachys lanata>Trigonotis omeiensis>Petasites japonicus. Furthermore, the content of MDA in leaf rose with the addition of SO2 concentration with Stachys lanata having the highest level, followed by Trigonotis omeiensis and Petasites japonicus being the lowest. Results indicated that Stachys lanata suffered the severest membrane lipid peroxidation under SO2 stress while Petasites japonicus the lightest. Different levels of SO2 treatment activated the vitality of resistance to oxidase SOD, POD and CAT in the three ground cover plants, all surging with the SO2 concentration. Increasing levels of Petasites japonicus and Trigonotis omeiensis superseded that of Stachys lanata, implying that Petasites japonicus and Trigonotis omeiensis can increase the vitality of resistance to oxidase under SO2 stress so that they can better cope with malign environment. All level of SO? concentration can induce the rising in osmotic adjustment substances in leaf to different extent and the denser the concentration, the more apparent the results. Trigonotis omeiensis had the highest level while Stachys lanata the lowest.The three sciophilous ground cover plants received injury as well and they all had the phenomenon of losing green. Among them, Saxifraga stolonifera experienced curling, lodging and appearing of small necrotic spots when the concentration of SO2 was 22.86 mg·m-3. Under the highest concentration, small necrotic spots emerged on Hosta ventricosa but Ophiopogon japonious suffered no obvious effect except several yellowing leaves. Observed from the appearance we can judge that the resistant ability of Ophiopogon japonious is stronger than that of Hosta ventricosa and Saxifraga stoloniferat. With the increasing level of SO2, the chlorophyll content, leaf pH value and relative moisture content all displayed decreasing tendency. Treated with highest SO2 stress, the chlorophy(a+b) content of Saxifraga stolonifera, Ophiopogon japonious and Hosta ventricosa reduced respectively by 36.9%,11.2% and 13.9% compared with controlled group, the leaf pH value of them by 0.163,0.094 and 0.132, relative moisture content by 20.2%,9.4% and 4.6%. Through SO2 treatment, the three sciophilous ground cover plants had membrane lipid peroxidation and their MDA content and relative conductivity rate rose with SO2 level. The ranking for increasing degree in MDA content was Hosta ventricosa>Saxifraga stolonifera>Ophiopogon japonious, while the ranking for relative conductivity rate rise was Saxifraga stolonifera>Hosta venlricosa>Ophiopogon japonious. Results indicated that the level of membrane lipid peroxidation for Ophiopogon japonious was lower than Saxifraga stolonifera and Hosta ventricosa. SO2 treatment also influenced the system of osmotic adjustment substances for the three plants. They increased level of soluble sugar and free proline to reduce the harm when faced with SO2 stress. Moreover, the vitality of resistance to oxidase in the three plants was activated and the level surged with SO2 level. In densest SO2 concentration, the resistant ability to oxidase of Ophiopogon japonious and Hosta ventricosa were stronger than that of Saxifraga stolonifera, implying that they were more resistant to SO2 than Saxifraga stoloniferat.Employing subjection function in fuzzy mathematics and principal component analysis, various physiological indexes of the 6 chosen ground cover plants were quantified and analyzed. The results showed that the resistant ability for heliophious ground cover plants ranked as Petasites japonicus>Trigonotis omeiensis>Stachys lanata; purification ability for SO2 ranked as Petasites japonicus>Stachys lanata>Trigonotis omeiensis. For the three sciophilous ground cover plants, the rank in resistant ability was Ophiopogon japonious>Hosta ventricosa>Saxifraga stolonifera and the rank in SO2 purification ability was Saxifraga stolonifera>Ophiopogon japonious>Hosta ventricosa.