| A series of global environmental problems,such as global warming and increased nitrogen deposition,are affecting the survival and development of human society,and these problems have attracted more and more people’s attention.With the increasing temperature and the increasing shortage of water resources,the drought is becoming more and more serious,which seriously affects the survival,growth and yield of plants.Seedling is a fragile stage in plant life,and environmental factors such as water and nutrients play an important role in plant regeneration and growth.Therefore,exploring the growth characteristics,physiological characteristics and adaptive mechanism of forest seedlings under the combined effect of drought and nitrogen is of great significance for understanding the changes of forest ecosystem function and pattern,and also provides theoretical reference for reasonable regulation of water and fertilizer management in forest plantation breeding process,improvement of vegetation growth conditions under drought stress in nitrogen application system,and optimization of cultivation measures.In this study,the 2.5-year-old seedlings of Quercus aquifolioides in Xizang were selected as the research object.Potted plants were used to carry out water and nitrogen control experiments.With NH4NO3as the nitrogen source,four water gradients were set:normal water supply（NS:80%FC）,mild drought stress（LS:60%FC）,moderate drought stress（MS:40%FC）,and severe drought stress（SS:20%FC）,four nitrogen application gradiants:no nitrogen(N0:0kg/hm-2·a-1),low nitrogen(LN:50kg/hm-2·a-1),medium nitrogen(MN:100kg/hm-2·a-1),and high nitrogen(HN:150kg/hm-2·a-1).The results are as follows:（1）Under different drought stress,the seedling height,base diameter,leaf area,biomass accumulation of each component and biomass allocation showed significant differences.Under normal water supply conditions,the indexes showed an increasing trend with the increase of nitrogen application concentration.However,under the condition of drought stress,the growth and biomass of seedlings were significantly lower than the normal level.（2）Under drought stress,total chlorophyll,chlorophyll b,chlorophyll a and chlorophyll a/b in the leaves of Quercus aquifolioides seedlings decreased.The photosynthetic pigment of Quercus aquifolioides seedlings could be increased obviously with appropriate nitrogen application level.Under moderate and severe drought conditions,high nitrogen level inhibited the accumulation of photosynthetic pigments.In the condition of moderate drought water,the photosynthetic pigment level was higher in the condition of medium nitrogen,while in the condition of severe drought water,the photosynthetic pigment level was higher in the condition of low nitrogen.In normal water supply,Pn,Tr and Gs all increased gradually with the increase of nitrogen concentration,and showed the maximum value when nitrogen was high.Under the same conditions,Pn and Ci showed opposite trends.（3）The leaf relative water content（LRWC）of Quercus aquifolioides seedlings increased with the increase of nitrogen application level under normal water supply,but decreased with the increase of drought stress.Proline（pro）content of each component increased significantly under drought stress compared with normal water supply.Under severe drought stress and high nitrogen,the content of proline increased to the highest,which was much higher than that under normal water supply and high nitrogen.The content of proline（pro）in nitrogen treatment was significantly higher than that in no nitrogen treatment except under normal water supply condition,and it increased with the increase of nitrogen application.Under the three water stress treatments,soluble sugar content was significantly higher with the increase of nitrogen application concentration than without nitrogen application.Under different drought and nitrogen application levels,the content of MDA in the leaves of Quercus aquifolioides seedlings also showed significant changes.The MDA under normal water treatment（NS）was significantly lower than that under drought stress.With the increase of water stress intensity,the MDA value showed a significant increase under the same nitrogen concentration.There is a significant coupling effect between water and nitrogen.Under certain drought stress,proper nitrogen application can significantly increase the drought resistance and promote the growth of oak seedlings,while nitrogen application can inhibit the growth of plants under severe water shortage.Therefore,regulating water and nitrogen conditions is an important way to improve drought resistance and growth of Quercus mongolica. |
