Effects Of Different Nitrogen Application Levels On Tree Growth And Sugar Metabolism In Leaf And Fruit Of Pyrus Pyrifolia

Nitrogen is an important component of the organic nitrogen in plants containing protein,nucleic acid,chlorophyll,hormones and a variety of enzymes.It is a key element in the essential mineral elements in fruit trees.Considering the important roles of nitrogen in plant life,it is also known as the elements of life.Pear is one of the most important commercial fruits in China,which ranks second only to apple and citrus.Soluble sugar is an important component of fruit,and it is also an important index to measure the quality of fruit.Effects of nitrogen application on fruit quality of pear have been reported,but the mechanism of effect of nitrogen fertilizer on sugar metabolism in pear leaves and fruit was rarely reported.The application of nitrogen fertilizer in China is still in the state of Blind fertilization’ and ’experience fertilization’ in the pear orchard.Pear(Pyruspyrifolia.Kotobuki shinsui)managed by trellis cultivation was used as materials of field experiment from 2014 to 2015.Four nitrogen(N)treatments of 0.25,0.5,1.0,1.5 kg N per plant(N-0.25,N-0.5,N-1.0,N-1.5)were applied to the pear trees during each growth season.No N application(N-0)was used for control and all the five treatments were conducted by random disposal teams.Study on the effects of different nitrogen levels on pear growth yield and quality.Study on the response of key genes of pear leaf and fruit sugar metabolism of nitrogen at the transcriptional level by using qRT-PCR.Our results will be helpful to explain the regulation mechanism of nitrogen application rate on the sugar metabolism of leaves and fruits and provid the theoretical basis for determining the optimum amount of nitrogen application.The main results are as follows:With the increase of N application,an increasing trend on the girth of trunk and leaf weight was found,while the single fruit weight and yield increased firstly but then decreased.There was no significant difference on yield between treatments when the amount of N application rate was more than 0.5kg N.With the increasing of N application rate,soluble solid content,soluble sugar and the ratio of sugar to acid were found increasing at first but then decreased during two years.The maximum values of the three indexes above were found in N-0.5 treatment.Leaf nitrate reductase activity was positively correlated with the amount of nitrogen application,the more nitrogen application rate,the more vigorous of the nitrogen metabolism.Chlorophyll a and chlorophyll b in leaves increased with the increase of N application rate,and reached the peak value in N-1.0 treatment,and then decreased.The content of sorbitol in leaves decreased from young fruit stage to expanding stageⅡ,while the content of sorbitol in fruits increased.Compared to N-0 treatment,the sorbitol content in leaves and fruit of N-0.5 treatment was significantly increased by 25.3%and 90.9%,respectively,at expanding stage Ⅱ.With the increasing amount of N application,the activity of NAD-SDH and S6PDH in leaves was increased at first but then decreased,and a minimum value was found in N-0 compared to the other N application treatments.The expression level of S6PDH of the N-0.5 treatment was significantly up-regulated than the N-0 treatment,which resulted in the promotion of the sorbitol synthesis.At mature stage,the expression of NAD-SDH3 in fruit was found significantly increasing under the N-0.5 treatment,which could enhance the decomposition of sorbitol;while compared to the N-0.5 treatment,the expression of that under the N-1.5 treatment was decreased by 42.5%.Leaf sorbitol transporter protein SOT2 relative expression of N-0.5 treatment was significantly higher than those of the N-1.5 108%,95.6%at expanding stage Ⅱ.Compared with other treatments,N-0.5 treatment significantly increased the relative expression of SPS1 at expanding stage Ⅰ,which resulted in the promotion of the sucrose synthesis.Leaf sucrose content of N-0.5 treatment was significantly higher than other treatments at expanding stage Ⅰ.The relative expression of AIV1 and AIV2 was higher in the mature stage.The relative expression levels of nitrogen treatments were higher than that of N-0 treatment.The relative expression of AIV1 and AIV2 in N-0.5 treatment was significantly higher than that in N-0 treatment 198%and 229%.N-1.5 treatment significantly inhibited the expression of AIV1 and AIV2 genes,and decreased by 30.4%and 61.9%compared with N-0.5 treatment.The relative expression of SUT1 at expanding stageⅡ was significantly higher than that in other stages.N-0.5 treatment was the highest,which promoted the translocation of sucrose from the source to the sink and increased sucrose content in the leaves.In summary,N-0.5 treatment can meet the tree nutrition growth and promoting the sugar metabolism of pear leaf and fruit at the same time.N-1.0 treatment has a certain yield advantage.Therefore the recommendation on the amount of nitrogen fertilizer for trellis cultivated pear was 0.5kg-1.0 kg N per plant.