Effect Of Nitrogen Application And Interstem Rootstock On Calcium Absorption And Fruit Quality Of Fuji Apples

Nitrogen and calcium are essential mineral elements of fruit trees,which are the basis of yield formation and play an important role in the formation and maintenance of fruit quality.At present,the problem of calcium deficiency in apples caused by unreasonable application of nitrogen fertilizer and improper selection of interstem rootstock is becoming more and more prominent,followed by the occurrence of physiological diseases such as bitter pit and reduction of fruit quality.Therefore,it is an urgent problem for the development of apple industry to select an appropriate nitrogen application rate to facilitate the absorption of calcium in apples and to explore the influence of interstem rootstock on the absorption and transport of calcium in apples.In this paper,the effect of nitrogen on calcium uptake was investigated by applying different levels of nitrogen fertilizer after flowering for two consecutive years using Malus baccata Borkh/SH38/Fuji apples as test material,and SH40,T337,M9,M26 and Pajam2 interstem rootstock of Fuji apples as test materials to clarify the differences in calcium transport among the five interstem rootstocks in order to provide a theoretical basis for the rational application of nitrogen fertilizer and the selection of calcium transport efficient intermediate rootstocks in production.The main results were as follows.1.During apple fruit development,total fruit calcium concentration,water-soluble calcium（H₂O-Ca）,pectin calcium（Na Cl-Ca）,calcium phosphate（HAC-Ca）and calcium oxalate（HCl-Ca）showed an overall decreasing trend,with a slight upward fluctuation in H₂O-Ca and Na Cl-Ca at 150 d after bloom.Nitrogen application increased the total calcium concentration and the four forms of calcium concentration in the fruit compared to the control,with the highest calcium concentration at 200kg/hm² of nitrogen application.Nitrogen application changed the composition ratio of different forms of calcium.Nitrogen application changed the composition ratio of different forms of calcium.In the peel,moderate application of nitrogen reduced the proportion of H₂O-Ca to total calcium and increased the proportion of Na Cl-Ca.Nitrogen application changed the composition of different forms of calcium.In the fruit peel,moderate application of nitrogen can decrease the proportion of H₂O-Ca to total calcium and increase the proportion of Na Cl-Ca.The percentages of both H₂O-Ca and Na Cl-Ca in fruit hearts increased and then decreased with the increase of N application.The percentage of HAC-Ca and HCl-Ca in different parts of the fruit showed a trend of increasing and then decreasing with the increase of N application.2.During fruit development,both protopectin（PP）and cellulose（CLL）contents decreased,and soluble pectin（WSP）content increased and then decreased.Cellulase（Cx）,pectin methyl esterase（PME）and polygalacturonase（PG）activities increased gradually with fruit development.Compared with the control,all N treatments inhibited the degradation of fruit PP and CLL,reduced WSP formation,and significantly decreased PG,PME,and Cx activities,with the most significant effect at 200 kg/hm2of N application.In addition,genes such as Md CAX1,Md CAX2 and Ca M4 in different fruit parts could respond to different levels of N treatment.3.The nitrogen application treatment significantly increased calcium concentration in roots,leaves,new shoots and lateral branches and decreased calcium concentration in the main trunk,interstem rootstock and rootstock compared to the control.The highest calcium accumulation in the tree was observed at 200 kg/hm² of N application.With increasing N application,calcium accumulation increased in lateral roots,increased and then decreased in lateral branches,new shoots,leaves and fruits,and decreased and then increased in calcium accumulation in grafting rootstock.Compared with the control,nitrogen application increased the proportion of calcium allocated to roots,lateral branches,new shoots,leaves and fruits and decreased the proportion allocated to basal rootstock,interstem rootstock and main stem.With the increase of N application,the fruit quality indexes such as fruit shape index,skin brightness,red-green color difference,fruit hardness,soluble solids,soluble sugar and vitamin C content showed a trend of increasing and then decreasing,while yellow-blue color difference and titratable acid content showed a trend of decreasing and then increasing.The results of the combined weighted Topsis method for fruit quality showed that the best fruit quality was achieved at200 kg/hm² of N application.4.By comparing the calcium uptake and accumulation in Fuji fruits of five interstem rootstocks,we found that the highest calcium concentration was found in Fuji apple fruits with’SH40’as the interstem rootstock,and the highest calcium accumulation was found in Fuji apple fruits with’T337’as the interstem rootstock,reaching 1.27 g.In both peel and flesh,the highest percentage of Na Cl-Ca was found in’T337’as the intermediate rootstock with 54.71%and 21.70%,respectively.The analysis of fruit cell wall components and metabolic enzymes revealed that CLL and PP were higher in’SH40’and’Pajam2’as interstem rootstocks,and’M9’as interstem rootstock.WSP content was higher,but PG,PME and Cx activities with’Pajam2’and’M9’as the rootstock were also higher.5.Interstem rootstocks significantly affected the uptake and translocation of calcium by the trees.Interstem rootstock’Pajam2’Fuji had the highest calcium accumulation of 30.00 g.Interstem rootstock’T337’Fuji had the highest calcium accumulation and distribution percentage of 1.27 g and 4.56%in fruit,respectively.The overall fruit quality of the five interstem rootstocks of Fuji was analyzed according to the combined weighted Topsis method and ranked as’T337’>’M9’>’M26’>’Pajam2’>’SH40’.