Effects Of PM_2.5 Formed By NH₄HO₃ On Physiological Metabolism Of Two Typical Landscaping Herbs

In urban ecosystems where nitrogen deposition and atmospheric particulate matter pollution are increasing,plant absorption of particulate matter has a significant effect on purifying the air,and at the same time this process will also have a toxicological effect on plants.Iris and Purslane as two typical landscaping herbs with strong adaptability,the research on the resistance of both at home and abroad mainly focuses on drought stress and salt stress,while there are few researches on the resistance of nitrogen deposition and atmospheric particulate matter pollution,and there are differences in the metabolic mechanism of these two plants in adverse conditions due to different photosynthesis pathways.Therefore,it is very important to study the physiological and ecological changes of iris and Purslane under the background of nitrogen deposition and particulate matter pollution,and to compare the differences between the two,in order to provide theoretical basis for guiding the urban landscaping plant allocation.In this study,inorganic nitrogen particulate matter(PM_2.5)fumigation experiment was carried out in the nursery of Jiangsu Forestry Academy of Sciences to explore the metabolic response mechanism of iris and purslane under inorganic nitrogen PM fumigation,and the related physiological indexes were measured and analyzed from 3 aspects of photosynthetic physiology,antioxidant system and nitrogen storage strategy respectively.The results showed that:（1）Under the influence of stomatal restriction at the initial stage of fumigation and non-stomatal restriction at the later stage of fumigation,the net photosynthetic rate（Pn）,stomatal conductance（Gs）and transpiration rate（Tr）of Iris tectorum and Portulaca oleracea leaves were significantly correlated with fumigation time（p<0.05）,which showed as the trendacy of increased first and then decreased with the lasting of fumigation.It also showed the threshold phenomena of photosynthesis and transpiration process as the rule from promotion to inhibition.Only photosynthetic rate of Iris tectorum responded significantly to nitrogen fumigation,while all photosynthetic rate,transpiration rate and stomatal conductance of Portulaca oleracea changed significantly.（2）The antioxidant systems of Iris tectorum and Portulaca oleracea had the same response pattern to fumigation time.In the early stage of fumigation in non-growing season,the two types of plants relied more on POD to clear reactive oxygen species（ROS）,while in the later stage,SOD was more used to remove ROS.Iris tectorum and Portulaca oleracea showed different response mechanisms to fumigation concentration under inorganic nitrogen formed PM fumigation.When the concentration of added nitrogen was low,Iris tectorum relied more on POD to remove ROS,and Portulaca oleracea relied on CAT.When the nitrogen concentration was further increased,Portulaca oleracea began to improve CAT activity,while Iris tectorum relied more on POD to remove ROS.（3）Influenced by inorganic nitrogen formed PM_2.5 fumigation,mesophyll cells of Portulaca oleracea showed stronger storage ability to nitrogen in non-growing season under middle and low nitrogen concentration（p<0.05）,while mesophyll cells of Portulaca oleracea showed stronger storage ability to nitrogen under all nitrogen concentration（p<0.05）.In the growing season,the storage ability to nitrogen of epidermal and mesophyll cells of Portulaca oleracea was stronger than that of Iris tectorum（p<0.05）.Inorganic nitrogen formed PM fumigation had negative effects on nitrogen metabolism of Iris tectorum at longer time scale.In summary,the fumigation of inorganic nitrogen formed PM for both types of plants could stimulate the antioxidant mechanism to resist the damage of ROS.The difference is that the"wreath type"structure formed by the sheath cells and leaf flesh cells of purslane makes them more sensitive to inorganic nitrogen PM_2.5 fumigation than iris in photosynthetic physiology,and the ability of leaves to store nitrogen is stronger.The negative effects of inorganic nitrogen formed PM fumigation on nitrogen metabolism in Iris tectorum was more persistent,which indicated gernerally Portulaca oleracea was more dominant in coping with long-term and high concentration inorganic nitrogen formed PM_2.5 pollution.This conclusion can provide a theoretical basis for guiding the allocation of urban landscaping plants under the background of increasing atmospheric pollution of nitrogen-containing PM_2.5.