Effects Of Simulated Acid And Nitrogen Deposition On Seed Germination,Seedling Morphology And Physiological Characteristics Of Main Tree Species In South China

With the rapid economic development and the frequent human activities in recent decades,there are induced climate and environmental changes on a global scale,such as the elevated atmospheric acid and nitrogen?N?deposition,which are expanding problems that affect the tree growth and development in forest ecosystems.To date,numerous studies have investigated the effects of a single type of acid or N deposition on seedling growth and development in forest ecosystems.However,natural atmospheric acid and N deposition contains a set of complex compounds.Atmospheric N deposition includes not only NO₃^--N deposition,but also NH₄⁺-N,or mixed N deposition.Thus,a single type of N addition may not accurately reflect their ecological effects on the tree seedling growth and development in forest ecosystems.Studying and identifying the effects of different types of acid and N deposition on tree seedling growth and development is an important task.Therefore,this thesis selects species of a typical subtropical area,which includes Cunninghamia lanceolata,Fokienia hodginisi,Liquidambar formosana,Pinus massoniana,Phoebe zhennan,and Schima superba as the research object.In this study,seeds were treated with simulated acid rain with pH 2.5,3.5,4.5,and 5.5 in a petri dish experiment.Seeds were supplied with solutions of 1 NH₂SO₄ and 1 NHNO₃ in the ratio of 10 to 1 of these pH values and distilled water as control and tested for germination in a controlled growth chamber.A similar solution was given to seedlings in the greenhouse in a pot experiment.Similarly,seed and seedlings were exposed to different doses of N deposition.In the N deposition experiment,two-level were used one control?Ck,without N addition?and the other with N supplementation 60 kg N hm^-2 a^-2.Ammonium sulfate?NH₄?₂SO₄,ammonium nitrate?NH₄NO₃?and potassium nitrate?KNO₃?were used as N sources.The objective of this research was?1?to obtain a better understanding of the effects of simulated acid rain and simulated N deposition in seed germination and seedling growth,which is a critical stage in the successful establishment of plants.?2?to investigate the possibility that these effects might be related to the difference in seedlings physiology,antioxidative defense,and sensitivity and?3?to check if these treatments could enhance seedling performance that would be useful for the protection of these species in southern China and to examine the level of acid deposition and simulated Nitrogen deposition that is detrimental for the establishment of the species.The results showed that?1?Acid rain with pH 2.5 adversely affected the germination capacity F.hodginisii?11.66±2.80?and P.zhennan?45±3.25?;while all acid solutions significantly increased germination P.Massoniana;but did not affect germination of C.Lanceolata seeds.Furthermore,a strong acid solution?pH 2.5?adversely affected the elongation of radicle and hypocotyl as well as fresh and dry weights of radicle and hypocotyl of tender seedlings.Seedling emergence was more sensitive than seed germination to acid deposition,and germination of conifer species are less sensitive than broad-leaved species to acid deposition.As a whole,the level of pH at 3.5 is the threshold level,and acid deposition below this value will have a detrimental effect on the germination of seeds and the emergence of seedlings.C.lanceolata and P.massoniana are coniferous specie and could adapt under the high acidic condition as compare to P.zhennan,S.superba,and L.formosana which are broadleaf species.Seedling height and diameter,chlorophyll content,photosynthesis characteristics,nutrients concentrations,sugar contents,and enzymatic antioxidant compounds showed different responses to simulated acid deposition at different levels.?2?Significant foliar damage decline in chlorophyll contents and retardation of seedling growth of all species were observed at pH 2.5.As the pH level decreased,height,stem basal diameter,and leaf area decreased.Under low levels of pH below 4.5,seedlings tended to grow well in height but with slow diameter,resulting in thin and tall seedlings;however,height and diameter growth of seedlings slowed down after some months because of high acid deposition especially in treatment with pH 2.5 and 3.5.?3?The effect of high acid deposition on the seedling antioxidative system and osmotic adjustment varied with extent of acid deposition.As acid deposition increased,the activity of superoxide dismutase?SOD?,catalase?CAT?and peroxidase?POD?,malonaldehyde?MDA?and proline increased,Whereas the content of soluble sugar and soluble protein decreased as acid deposition increased.Under high acid,deposition seedlings had higher antioxidant enzyme activity.L.formosana treated with simulated acid deposition endure more injury in cellular membranes during photosynthetic function than did S.superba,which was related to variations in the activities of the antioxidant enzyme.The results indicated that pH conditions,especially below 5.5,would be harmful to the growth and development of C.lanceolata seedlings,because,the cell peroxidation in leaves would damages the structure and function of plants.?4?Net photosynthetic rate?Pn?,transpiration rate?Tr?,stomatal conductance?Cond?,as well as the content of chlorophyll a?Chl a?,chlorophyll b?Chl b?and carotenoid?Caro?decreased as acid deposition increased.While intercellular CO₂ concentration?Ci?increased as acid deposition increased.Seedling had high photosynthetic rate and chlorophyll content under control condition.The results indicated that acid deposition significantly reduce seedlings growth and development.?5?In the N deposition experiment,better results in seed germination parameters were observed under 0.11% followed by 0.1 % while 0.5 % was lowest.N deposition at a rate of 0.5 % didn't influenced much to the germination capacity C.Lanceolata,P.Massoniana and F.hodginsii as compare to P.zhennan.N deposition at application rate of 0.1% significantly increased germination among three N deposition levels.Elongation of radicle,hypocotyl as well as fresh and dry weights of radicle and hypocotyl of tender seedlings were observed.?6?In the seedling experiment,chlorophyll,carbon and nitrogen contents,antioxidant enzyme activity,MDA content,and amount of N varied among treatments.Simulated N deposition decreased the content of MDA and the activities of antioxidant enzymes height,diameter and leaf area increased among all treatment however it decreased under high N deposition and increased under low deposition.The results indicated that under low N deposition,leaves became bigger and thinner.?7?The effect of nitrogen deposition on seedling antioxidative system and osmotic adjustment varied with amount of N added.As nitrogen deposition increased,the activity of SOD,CAT and POD,soluble sugar contents,soluble protein and proline increased with N deposition.The content of MDA decreased with increased deposition.Elevated activities of CAT,POD and SOD might be significant way of stopping membrane injury,reducing lipid peroxidation and delaying senescence in P.zhennan.The results indicated that high N deposition would be harmful to the growth and development of seedlings because the cell in leaves would experience peroxidation that damages the structure and function of plants.Low N deposition would be useful for growth and development of seedlings as seedlings would have high antioxidant activity,soluble sugar and soluble protein accumulation.?8?The photosynthetic characteristics of seedlings also varied with amount of N deposition.Net photosynthetic rate?Pn?,transpiration rate?Tr?and stomatal conductance?Cond?increased with N deposition.The contents of Chl a and Chl b and carotenoids content increased moderately under N deposition.While intercellular CO₂ concentration decreased with N deposition.Seedlings had high photosynthetic rate and chlorophyll content,and efficiency under low N deposition.The results indicated that lower N deposition results in better seedlings development and growth.?9?Experiment showed that the germination of seed and seedling emergence of tree species in south China were more sensitive to acid deposition below level 5.5.The response to acid rain also varies between species;with broadleaved species,Liquidambar formosana and Phoebe zhennan,are more sensitive than coniferous species Cunninghamia lanceolata and Pinus massoniana.In South China,this acidity level in the atmosphere occurs quite often,so the influence of less acidic precipitation than this level will perhaps have a slight effect on the establishment of the species.The outcomes here will enhance our knowledge of the physiological,photosynthetic,and antioxidant response mechanisms used by species throughout simulated acid and nitrogen deposition stress.?10?Thus,in an environment with a high risk of acid rain?pH < 3.5?,it would be advisable to plant coniferous species if no acid correction measures are made.Otherwise,acid correction measures,such as liming,would be required to ensure the establishment of broadleaved species.In our experiment,all species that belong to diverse forest types behave noticeably in response to increased N deposition.In combination with the increased leaf area,the transpiration demand also increased among all species.Therefore,low N deposition increased the development of seedlings,but adverse effects were noticed at high deposition rate.