Effects Of Simulated Nitrogen Depositionon Seed Germination And Seedling Growth Characteristics Of Toona Sinensis

With the development of global industrial and agricultural modernization,the characteristics and processes of various ecosystems have been deeply affected under the current situation of increasing atmospheric nitrogen deposition.Toona sinensis,which is widely distributed in China,is not only a precious fast-growing timber species in China,but also a troditional medicine and vetegable in China.The seeds and annual seedlings of Toona sinensis were studied as experiment material in this paper.The nitrogen deposition control experiment was carried out,according to the characteristics of nitrogen deposition in China.The experiment used urea as the nitrogen source,and levels of nitrogen addition were setted up as:0,2.5,5,7.5,10,12.5,15,17.5,20 kg N?hm^-2?a^-1for gemination experiment and 0,20,40,80,120,180 180 kg N?hm^-2?a^-1.for seedling experiment.In order to explore the threshold of nitrogen deposition that can be tolerated in the seedling stage,and provide a theoretical basis for the cultivation of camphor,the germination rate,germination index,vigor index and other indicators of seedling diameter and seedling height,seedling height and seedling growth,biomass and photosynthetic characteristics,gas exchange characteristics,antioxidant enzyme activities such as SOD and POD,malondialdehyde content,soluble sugar and soluble protein content of Toona sinensis were mersured and analysised.The results are as follows:1.The germination rate,germination index,radicle length and vigor index of Toona sinensis seeds decreased significantly with the increase of nitrogen addition,and were lowest at20 kg N?hm^-2?a^-1nitrogen addition level.2.Nitrogen addition can promote the growth of Toona sinensis seedlings,and the ground diameter,seedling height and biomass index are highest at the level of 180 kg N?hm^-2?a^-1nitrogen addition.Affected by the addition of nitrogen,the biomass of the seedlings of Toona sinensis was more distributed to roots and leaves.3.Nitrogen addition enhances photosynthetic capacity of leaves of Toona sinensis seedlings.The relative chlorophyll content（SPAD）of the leaves of Toona sinensis increased with the increase of nitrogen addition level,and the highest was under the addition of 80 kg N?hm^-2?a^-1nitrogen addition level,which increased by 73.9%compared with the control.The apparent quantum efficiency（AQY）,maximum photosynthetic rate(Pn_max),light saturation point（LSP）,light compensation point（LCP）and dark respiration rate（Rd）of the seedlings increased at first and then decreased with the increase of nitrogen addition.Except for the highest LCP of 80 kg N?hm^-2?a^-1,AQY,Pn_max,LSP,and Rd were the highest at 120 kg N?hm^-2?a^-1.The net photosynthetic rate（Pn）,stomatal conductance（Gs）,transpiration rate（Tr）and water use efficiency（WUE）of the seedlings of Toona sinensis increased first and then decreased with the increase of nitrogen addition level,and the highest was at 120 kg N?hm^-2?a^-1level of nitrogen addition;the intercellular CO₂concentration first decreased and then increased,and the lowest was at 120 kg N?hm^-2?a^-1.nitrogen addition level.4.Nitrogen addition affected SOD and POD activities of Toona sinensis seedlings to some extent.The activities of SOD and POD increased first and then decreased with the increase of nitrogen addition level,reaching the maximum at the nitrogen addition level of 120 kg N?hm^-2?a^-1.At the same time,nitrogen addition also caused certain damage to the cell membrane structure of the seedling leaves,and the MDA content increased with the increase of nitrogen addition level,reaching the maximum at the nitrogen addition level of 180 kg N?hm^-2?a^-1.5.The content of soluble sugar and soluble protein in the seedlings of Toona sinensis seedlings changed under the action of nitrogen addition,and both of them increased first and then decreased.Soluble sugar reached the highest level of nitrogen addition at 120 kg N?hm^-2?a^-1;soluble The protein was highest at the nitrogen addition level of 180 kg N?hm^-2?a^-1.Studies have shown that the seeds of Toona sinensis are very sensitive to nitrogen addition,and nitrogen deposition can inhibit the germination of seed and the growth of seed radicle,resulting in a decrease in seed vigor.But the addition of nitrogen at a certain level can promote the growth of seedlings,and the photosynthesis and stress tolerance of the seedlings will also increase.However,the tolerance of the seedlings to the increase of nitrogen deposition was limited.When the nitrogen level increased to a certain extent,the photosynthetic capacity began to decrease,and the cell membrane structure showed signs of damage.Therefore,the threshold for nitrogen tolerance of Toona sinensis seeds is less than 2.5 kg N?hm^-2?a^-1,and the threshold for annual nitrogen tolerance of Toona sinensis seedlings is 120 kg N?hm^-2?a^-1.