Effects Of Nitrogen And Phosphorus Addition On Photosynthetic Performance Of Alfalfa And Adaptability Of Different Fall-dormancy Alfalfa Varieties

Alfalfa is one of the leguminous forages with the largest cultivated area in the world.It is characterized by high yield and quality,cold and drought resistance,salt and alkali resistance,soil improvement and economic benefits.There is still a big gap between the development of Chinese alfalfa planting industry and that of foreign countries,mainly manifested in the slow progress of Alfalfa breeding,low yield per unit,low crude protein content,etc.,to further improve scientific fertilization management technology and carry out field introduction experiments,which will play a scientific guiding role in the alfalfa planting industry.Objective:To determine the anatomical structure,photosynthetic physiological characteristics and agronomic characteristics of Alfalfa Leaves by drip irrigation with nitrogen and phosphorus fertilizer application.The purpose of this study is to clarify the change rules of growth and photosynthetic,physiological and biochemical characteristics of Alfalfa under nitrogen and phosphorus addition,to reveal the mechanism of photosynthetic capacity,anatomical characteristics and growth differences of Alfalfa leaves under different fertilization conditions,and to clarify the relationship between photosynthetic rate and structure and physiology.To provide theoretical basis for establishing high-quality and high-yield alfalfa cultivation.At the same time,taking different fall dormancy grade alfalfa varieties（fall dormancy grade 1-10）as research objects,the differences of their production performance,nutrient value and Overwintering Rate in Shihezi area of Xinjiang were analyzed and comprehensively evaluated,and how the C,N and P stoichiometric characteristics of different physiological functional organs responded to the difference of fall dormancy of alfalfa was explored,as well as the nutrient cycling characteristics and adaptation mechanism of alfalfa plants were understood,in order to screen out high hay yield and high hay yield.Alfalfa varieties with high quality and adaptability provide theoretical basis for introduction of regional high-quality alfalfa varieties and establishment of high-quality and high-yield alfalfa cultivated grassland.Methods:Fertilization test of alfalfa and adaptability study between varieties were carried out simultaneously.The fertilization test was designed by two-factor random block design with 4 gradients at N:0（N₀）,60（N₁）,120（N₂）and 180（N₃）kg·hm^-2 and 4 gradients at P application level,i.e.P₂O₅:0（P₀）,50（P₁）,100（P₂）and 150（P₃）kg·hm^-2.There were 16 treatments applied with N and P application.Fertilizers are dripped with water for 3 to 5 days after rejuvenation and the first three harvesting by using a high-efficiency water-saving and fertilizer-saving method of"fertilizer follows water".Completely random block design was used for the adaptability study among varieties.95 alfalfa varieties were planted in the test field.For the purpose of the study,we selected 3 common alfalfa varieties in each fall dormancy grade,a total of30 alfalfa varieties.Result:1)The results showed that all alfalfa photosynthetic"photosynthetic noon-break"phenomenon under different phosphorus application treatments,and stomatal limitation is the main factor for the decline of photosynthetic rate.Photosynthetically active radiation（PAR）has the greatest impact on the net photosynthetic rate（P_n）of alfalfa leaves,while atmospheric CO₂ concentration（C_a）,atmospheric temperature（T_a）,and atmospheric relative environmental factors such as relative humidity（RH）and vapor pressure deficit based on leaf temp(V_pdl)affect P_n in the second place.Principal component analysis（PCA）of daily average photosynthesis,dry matter yield,aboveground soluble sugar and starch content under different phosphorus application treatments showed that the photosynthetic efficiency of alfalfa in descending order is P₂>P₁>P₃>CK treatment.When the phosphorus level was 120 kg·ha^-1,the dry matter yield of alfalfa increased significantly;the soluble sugar content of leaves,stems,and roots increased by11.6%,5.0%,and 4.6%,respectively,and the starch content increased by 15.2%,9.6%and 5.3%,respectively.The more resource allocation of soluble sugar and starch is manifested by increased allocation to leaves,and relatively less allocation to roots and stems.The diurnal changes of P_n,T_r,and water use efficiency（WUE）of alfalfa leaves were improved compared with the control treatment,and the daily change of C_i was significantly lower than the control treatment.2)The results showed that the net photosynthetic rate,transpiration rate,and water use efficiency of alfalfa leaves under the nitrogen application treatment were higher than those under the non-nitrogen application treatment,and the intercellular CO₂ concentration of alfalfa leaves under the nitrogen application treatment was lower than that under the non-nitrogen application treatment.The environmental factor that had the greatest combined effect on net photosynthetic rate and transpiration rate was photosynthetic effective radiation.The leaf length,leaf width,leaf area,and specific leaf weight of alfalfa,as well as leaf dry weight,stalk dry weight,dry matter yield,leaf N content,starch,and soluble sugar content of alfalfa showed a trend of first increasing and then decreasing as N application increased.At different nitrogen concentrations,leaf area was the most significant factor affecting leaf morphological structure,followed by leaf specific weight,leaf length,and leaf width.The effects on dry matter yield of alfalfa were,in descending order,leaf N content>net photosynthetic rate>leaf area>transpiration rate>specific leaf weight.The decrease in photosynthetic rate in the no N and high N treatments was primarily due to the inhibition of photosynthetic activity,this is a non-stomatal factor.Based on the principal component analysis,dry matter yield,leaf morphology,and photosynthesis were the highest for the N₂ treatment,followed by N₃,N₁,and CK treatments.Therefore,nitrogen fertilization promoted the synergistic improvement of photosynthetic rate and area in alfalfa,it facilitates the production of photosynthetic products,thereby increasing alfalfa dry matter yield,especially at the nitrogen application rate of 120 kg·ha^-1.3)The results showed that the dry matter yield（DMY）and net photosynthetic rate（Pn）were increased by2.10%～11.82%and 4.95%～11.93%in the N application treatment compared with the non-N application treatment,while the DMY and Pn were increased by 3.19%～17.46%and 6.15%～24.95%in the P application treatment compared with the non-P application treatment.N and P increase the thickness of the spongy tissue（ST）of alfalfa,which will facilitate the entry and exit of gas and water,and will further affect the photosynthetic indexes,such as stomatal conductance（Gs）and transpiration rate（Tr）of alfalfa leaves.Increased palisade tissue（PT）thickness will also enhance the adaptability of plant leave to strong sunlight,thereby increasing the maximum net photosynthetic rate(P_max)and light saturation point（LSP）.Fertilization treatment showed the highest utilization efficiency for low light and better adaptation to strong light,but the R_d decreased.The comprehensive score of principal component analysis for anatomical structure,photosynthetic performance and agronomic traits were N₁P₂>N₀P₂>N₁P₃>N₁P₁>N₀P₃>N₀P₁>N₁P₀>N₀P₀.4)The results showed that the biomass per plant,plant height,stem diameter and crude protein content of alfalfa tended to increase and then decrease with the increase of fall dormancy rate,whereas the stem to leaf ratio,neutral detergent fiber and acid detergent fiber content tended to initially decrease and subsequently increase with the increase of fall dormancy rate,and the overwintering rate gradually decreased as fall dormancy rate increased.The comprehensive evaluation scores of the principal component analysis were from high to low are fall dormancy level 4>level 3>level 2>level 5>level 6>level 8>level 7>level 9>level10,and the cluster analysis showed that fall dormancy level 4 and level 3 alfalfa were the best class in terms of overall index.Therefore,alfalfa with fall dormancy level 4 is the closest to the ideal variety,followed by fall dormancy level 3.Both of the above two fall dormancy levels of alfalfa are suitable for planting in Shihezi,Xinjiang.5)The results showed that the N and P contents of both alfalfa leaves and stems tended to decrease and then increased with the increase of fall dormancy rate,while the C contents and C:N,C:P and N:P showed the opposite trend.The contents of each element in the leaves of alfalfa varieties with different fall dormancy rates were:the leaf C content of‘Xinjiang Daye’alfalfa with fall dormancy rate 4 and the leaf N content of‘WL656HQ’with fall dormancy rate 9 were significantly greater than those of other alfalfa varieties with fall dormancy rates（P<0.05）.The variation of C content in alfalfa stems was not significant（P>0.05）among alfalfa varieties of fall dormancy rate,and the variation of N and P content was the same as that of alfalfa leaves,and the coefficients of variation of C,N and P content in leaves and stems of the second cut were lower than that of the first cut,and both cuts showed the same pattern.Except for the leaves of the 2nd cut of alfalfa,the fall dormancy rate had the greatest effect on the P content of alfalfa leaves and stems,followed by the N content,and the least effect on the C content.Between these two cuts,the fall dormancy rate had the greatest effect on alfalfa leaf C:P and the least effect on N:P.Carbon content of alfalfa leaves was positively correlated with C:N and C:P of leaves and stems,while it was negatively correlated with N content of leaves and stems（P<0.05）;leaf N and P contents were positively correlated with N and P contents of stems（P<0.05）,respectively.Leaf C:N and C:P were positively correlated with stem C:N and C:P（P<0.05）,respectively,while negatively correlated with stem N content（P<0.05）;stem C content was significantly positively correlated with stem C:N（P<0.05）,and stem N content was negatively correlated with stem C:N（P<0.05）;and stem C:P was significantly positively correlated with stem N:P（P<0.05）.In conclusion,the growth of non-fall-dormant and fall-dormant alfalfa is limited by N elements,and the growth of medium fall-dormant alfalfa is affected by both N and P elements.6)With the increase of autumn dormancy level,the C of mature and senescent leaves of Alfalfa increased first and then decreased,while the N,P decreased first and then increased.The nutrient reabsorption of Alfalfa showed a downward trend with the decrease of stubble times.The medium autumn dormant alfalfa had more reabsorption of N,P,but the increase was not significant until the last stubble.Alfalfa persistence showed a downward trend with autumn dormancy grade alfalfa,and the downward trend in 2021 was more obvious than that in 2020,which may be caused by the growth years.Conclusion:The application of N and P fertilizer is conducive to the changes of anatomical structure adaptability and photosynthetic capacity of Alfalfa leaves,and to the improvement of dry matter yield,growth characteristics and nutrient quality of alfalfa.The improvement effect is most obvious when applying N 120kg·hm^-2 and P 100 kg·hm^-2.In Shihezi region of Xinjiang,alfalfa with fall dormancy grade 4 is closest to the ideal variety,followed by fall dormancy grade 3.At the same time,it was found that the growth of non-fall dormant and fall dormant alfalfa was limited by N element,while that of medium fall dormant alfalfa was affected by N and P element together.In the production process of different fall dormant alfalfa,the balance of supply and demand of N and P should be considered.