Effects Of Drought Stress On The Growth Of Phoebe Zhennan Seedlings And Its Alleviation By Nitrogen Application

This research regarded Phoebe zhennan seedlings as the research object,the pot experiment which combined with artificial water control was carried to explore the responses of photosynthetic physiology,water physiology and growth of two-year-old Phoebe zhennan seedlings in different drought conditions to nitrogen application,whether nitrogen application can enhance the drought resistance of Phoebe zhennan seedlings,and recovery of Phoebe zhennan seedlings'growth in different drought treatments when drought was relieved.Aimed at giving full play to potential drought tolerance of Phoebe zhennan seedlings,so that promoting the improvement of afforestation efficiency and the improvement of regional ecological environment.At the same time,providing theoretical basis and technical reference to scientific water management of artificial young forest of Phoebe zhennan and the cultivation of forest trees in seasonal arid water shortage area.The soil water content was firstly adjusted to four treatment levels,i.e.80%of field water holding capacity?80%FC?,50%FC,30%FC and 15%FC,respectively.The physiological variables?water physiology and photosynthetic physiology?and growth of plants were measured after one week,and then three nitrogen application rates,control?N0?,medium nitrogen?MN?and high nitrogen?HN?were performed at an interval of 7 days for four times?7d,14d,21d and 28d,respectively?.The above-mentioned index and leaf trait were determined again one month after the accomplishment of nitrogen application.In September,soil samples were collected,and soil nutrient content was measured.After the collections were completed,normal watering was restored.The gas exchange parameters of leaves were measured in October 2016.In the following spring,the time of apical growth was recorded.After growth of new shoots were finished?May?,the photosynthetic physiological variables and growth of plants were measured.The main results are as follows,1.Effects of drought stress on the osmotic adjustment and active oxygen metabolism of Phoebe zhennan seedlings and its alleviation by nitrogen application1)The free proline?Pro?and soluble sugar?SS?contents in the leaves increased significantly with the aggravation of drought stress after 7 days of drought,but the content of soluble protein?SP?was firstly increased and then declined..After nitrogen application,the content of Pro raise further,but the values varied in drought treatment.The SS contents under sufficient water supply?80%FC?and mild drought?50%FC?were decreased by MN,but it did not change significantly when supplied with HN despite the soil water content.After nitrogen application,the SP contents under 80%FC and 50%FC were lower than those of no exogenous N,while they were opposite response under 30%FC and 15%FC;2)Before nitrogen application,with the aggravation of drought stress,the hydrogen peroxide?H₂O₂?content,superoxide dismutase?SOD?activity,catalase?CAT?activity increased significantly,and the peroxidase?POD?activity showed an up-down trend.After nitrogen application,the content of H₂O₂ was generally deceased at each water condition,with the maximum decrease at MN,while the HN treatment was not conducive to reduce the content of H₂O₂;3)Before nitrogen application,the content of malondialdehyde?MDA?in leaves increased significantly when the soil water content declined to and below 50%FC.The relative electrical conductivity?REC?was decreased at first,and followed by significant increase.Except severe drought?15%FC?stress,the MDA content showed a decreasing trend at MN,but a rebound at HN.As regards severe drought stress,however,the content of MDA increased at both MN and HN.2.Effects of drought stress on Photosynthetic physiology of Phoebe zhennan seedlings and its alleviation by nitrogen application1)With the aggravation of drought stress,photosynthetic pigment content was firstly increased and then declined in leaves of Phoebe zhennan seedlings,while leaf net photosynthetic rate?P_n?,stomatal conductance?G_s?,apparent quantum efficiency?AQY?,maximum net photosynthetic rate?Pmax?,light saturation point?LSP?and dark respiration rate?Rd?decreased,and intercellular CO₂?C_i?concentration was firstly increased and then declined,but the optical compensation point?LCP?rised.Under 80%FC,50%FC and30%FC,nitrogen application?MN?significantly increased photosynthetic pigment content,P_n,G_s,AQY,Pmax,LSP and Rd,reduced C_i and LCP at the same time.However,nitrogen application played the opposite role under severe drought stress;2)Moderate and severe drought stress can cause photoinhibition,while this phenomenon did not appear under mild drought stress and sufficient water;3)Before and after nitrogen application,the diurnal variation of G_s showed bimodal pattern under 80%FC and 50%FC,but it was a single peak pattern under 30%FC and 15%FC;4)The diurnal variation of P_n under four drought levels showed bimodal pattern before nitrogen application,but it showed a single peak pattern after nitrogen application.3.Effects of drought stress on water physiology of Phoebe zhennan seedlings and alleviation of nitrogen application1)With the aggravation of drought stress,the water condition of the leaves in Phoebe zhennan seedlings gradually deteriorated,transpiration was weaken at the same time,but the water use efficiency?WUE?of the leaves increased.Under 80%FC?50%FC?30%FC,application of MN improved the water condition of the leaves,while the effect of nitrogen application under severe drought was the opposite.2)Before nitrogen application,the diurnal variation of leaf transpiration rate showed bimodal pattern under 80%FC and 50%FC,but the diurnal variation showed a single peak pattern under 30%FC and 15%FC.After nitrogen application,the diurnal variation of Leaf transpiration rate showed bimodal pattern under80%FC,but the others were a single peak pattern.4.Effects of drought stress on leaf trait and stem growth of Phoebe zhennan seedlings and its alleviation by nitrogen applicationWith the aggravation of drought stress,leaf area?LA?,leaf biomass?LB?,leaf thickness?LT?,the increase of ground diameter,length and diameter of treetops of Phoebe zhennan seedlings decreased,but leaf length/width ratio?L/W?and leaf nitrogen?LN?content increased.Phoebe zhennan seedlings almost stopped high growth under 15%FC 30 days after the last nitrogen application.Application of nitrogen promoted LA,LB,LT,LN content and the stem growth of Phoebe zhennan seedlings excepting severe stress.5.Soil fertility after drought stress,and the response of photosynthetic physiology,water physiology and Stem growth in Phoebe zhennan seedlings to rewateringThe contents of soil nutrients in the pots increased with the aggravation of water deficit.Under the same drought level,the content of ammonia nitrogen and nitrate nitrogen increased with the increase of nitrogen level.However,there was no significant difference in available phosphorus between different nitrogen treatments.The recovery rate of P_n,and T_r slowed down with the aggravation of drought stress in the first year.Nitrogen application could promote the recovery of P_n and T_r in Phoebe zhennan seedlings,while HN was not conducive to recovery under severe stress.The growth rate of the stem was faster than that of the control under mild and moderate drought stress in the following spring,but it was slower than that of control when drought stress was serious.