| Plant senescence refers to the phenomenon in which the survival or reproductive capacity of plants declines with increasing age.Nitrogen deposition has an important impact on the survival,growth and reproduction of plants.Perennial caespitose grasses have important economic value as common species in grasslands and artificial grasslands.The process of plant senescence under the influence of different ecological factors has always been controversial.In view of this,this study took the caespitose excellent forage Elymus dahuricus as the research object,and established the experimental plots of different age cohorts established by sowing in 4 years,combined with 3 levels of nitrogen application(0,10,20 g N m-2yr-1),carried out a systematic study on the effects of nitrogen on the growth,reproduction,production,allocation and physiological characteristics of E.dahuricus,and then enrich the theory of plant senescence of clones,and provide theoretical basis for the management practice of grassland establishment,rejuvenation and production.The main results are as follows:（1）For reproductive characteristics,nitrogen application promoted the growth of vegetative,reproductive and hibernating members of E.dahuricus.The vegetative fertility of tiller tuft was increased,the sexual reproduction allocation of tiller tuft was decreased.With the extension of the growth years,the vegetative fecundity and sexual reproduction number characteristics of E.dahuricus tiller tuft showed a downward trend under different nitrogen application levels,while the growth of vegetative plants and hibernating components was inconsistent between two years.At the same time,nitrogen fertilization delayed the heading date of E.dahuricus tiller tuft,N1 and N2delayed by 2.1 and 3.2 days on average,respectively,and shortened the reproductive growth time of tiller tuft.The longer the growth years,the longer the heading date and the shorter the reproductive growth time.The heading date of the 4-year-old E.dahuricus was delayed by an average of 6.2 days compared with the 1-year-old.The shortening of reproductive growth time is not conducive to the sexual reproduction of the population.Inflorescence length,inflorescence biomass,seed biomass,seed number,reproductive plant height and biomass generally decreased with the shortening of reproductive growth time.（2）Regarding the characteristics of material production,nitrogen application promoted the biomass accumulation of E.dahuricus module to a certain extent,Under N2 treatment,the biomass of roots,stems,leaves,inflorescences and total biomass of E.dahuricus tiller tuft increased by 109.7%,84.8%,176.7%,86.7%and 134.7%,respectively,compared with N0,and the biomass of each component under N1treatment was less than N2 treatment.Among them,nitrogen application had a significant effect on the biomass accumulation of 1-year-old and 2-year-old tiller tuft.With the extension of the growth years,the root biomass of the tiller tuft at each nitrogen application level gradually increased,and the changes of the biomass of other components varied between years.In 2020,when the climate is extremely dry,the aboveground biomass of the tiller tuft showed a decreasing trend with the increase of plant age after nitrogen application,and the root biomass showed an increasing trend;The biomass of each component and the total biomass are the highest.Nitrogen fertilization significantly increased leaf biomass allocation.As the growing age increases,E.dahuricus will allocate more energy to the root system to better sustain its survival.（3）For physiological characteristics,in two years,compared with N0 without nitrogen application,the catalase（CAT）and peroxidase（POD）activities of the leaves treated with N2 nitrogen application increased by an average of 56.1%and 16.8%,and the content of malondialdehyde（MDA）decreased by 75.3%.Nitrogen fertilization promotes the photosynthesis of E.dahuricus to a certain extent,improves the ability of E.dahuricus to scavenge oxygen free radicals,and effectively relieves oxidative damage.With the extension of growth years,net photosynthetic rate（Pn）,stomatal conductance（GS）,transpiration rate（Tr）,water use efficiency（WUE）decreased,intercellular CO2 concentration（Ci）increased,and antioxidant enzyme activities decreased,the malondialdehyde content increased.The photosynthetic capacity and related activity indexes of the leaves of E.dahuricus decreased gradually with the prolongation of the growth years.（4）A comprehensive analysis of the effects of the aging process found that sexual reproduction traits of E.dahuricus（such as reproductive allocation RAI and inflorescence biomass.）and related photosynthetic traits（such as stomatal conductance and water use efficiency.）had a greater impact on the aging process in two years.Nitrogen application can reduce the of E.dahuricusiller tuft of distribution of sexual reproduction and improve the efficiency of photosynthesis and delay its the senescence.In this study,two years of nitrogen application experiments had different effects on the phenotypic and physiological characteristics of E.dahuricus.The sexual reproduction of E.dahuricus reached the maximum in the first year of establishment,and the material production reached its peak in the second year of establishment.After3 years of establishment,the reproductive characteristics and material production characteristics of E.dahuricus will decline gradually.With the extension of growth years,the sexual reproduction ability,photosynthesis efficiency and antioxidant enzyme activity of E.dahuricus will decline gradually.Nitrogen fertilization reduced the distribution of sexual reproduction of E.dahuricus,improved photosynthesis efficiency and antioxidant enzyme activity,delayed the senescence of E.dahuricus to a certain extent,and improved its material production. |
