Analysis Of Tissue Structure And Cell Wall Components Of Different Hardness Chinese Olive (Canarium Album L.) Fruits

Chinese olive(Canarium album(Lour.)Raeusch.)is rich in nutrients and is a medicinal and edible fruit announced by the National Health and Family Planning Commission.Chewing Chinese olive fruit is beneficial to health.Hardness affects the taste and storage time of fruits.It is another important fruit quality index besides color,aroma and flavor,and also one of the important factors determining the economic value of fruits.Therefore,it is of great significance to analyze the hardness difference of Chinese olive for the improvement of fresh quality and postharvest storage.As a macroscopic physical property,hardness is determined by the internal tissue structure and chemical composition.Previous studies have mainly focused on cell wall metabolism of Chinese olive,but the cell wall components that affect the hardness of Chinese olive are still unclear,the relationship between the difference of hardness and tissue structure remains to be clarified.Therefore,this study focused on the hardness,tissue structure and cell wall components of Chinese olive,analyzed the relationship between tissue structure,cell wall components and hardness,determined the characteristics of tissue structure and cell wall components related to the difference in hardness of Chinese olive,and explored the changes of hardness,cell wall components and related enzyme activities during the development and maturation,in order to analyze the mechanism of hardness difference of Chinese olive from the perspective of histology and physiological biochemistry,aiming to provide theoretical basis and research basis for revealing the essence of hardness difference of Chinese olive.The main results were as follows:1.The hardness,tissue structure and cell wall components of 10varieties(strains)of Chinese olive were observed and compared.The results showed that,there were significant differences in hardness,tissue structure and cell wall components of 10 varieties(strains)of Chinese olive.The results of cluster analysis showed that,when the Euclidean distance was 5,10 varieties(strains)of Chinese olive were divided into four clusters according to hardness:cluster I included LF,QB,TT and PS;cluster II included FL,RA and PT;cluster III included QL and ZP;cluster IV included ZL.According to the hardness,the order of these clusters was:IV>II>I>III.The results of correlation analysis showed that,hardness was significant negative correlation with the percentage of A_C to A_S(A_C/A_S)(P<0.05),significantly positively correlated with the percentage of A_W to A_S(A_W/A_S)(P<0.05),and significantly positively correlated with the content of ionic binding pectin(ISP)and lignin(LIG)(P<0.05).The results of principal component analysis showed that the sum of the weights of ISP and LIG accounted for 44.56%,indicating that ISP and LIG were the key cell wall components affecting the hardness of Chinese olive.2.Based on the results of cluster analysis,QL,LF and FL belonging to different hardness clusters were selected as test materials to observe the tissue structure of exocarp and mesocarp.The results showed that,the subepidermal cell layer of FL was the thickest(P<0.05),aspect ratio was the smallest(P<0.05),and the roundness was the largest(P<0.05).The results of difference statistical analysis of mesocarp tissue structure showed that the area and perimeter of large parenchyma cells of LF and FL were smaller(P<0.05),the aspect ratio was smaller(P<0.05),and the roundness was larger(P<0.05),the vascular bundle area was larger(P<0.05),and the number of vessels was more(P<0.05).The ultrastructure of parenchyma cells showed that the microfibrils of the primary wall of QL were loose,the electron density of the middle lamella was low,and the boundary with the primary cell wall was not obvious.The cell wall structure of LF and FL were denser,the electron density of the middle lamella at the junction of three cells of LF was higher;the middle lamella of adjacent cells of FL was thicker and the electron density was higher.In summary,parenchyma cell and vascular bundle were the key tissue structures affecting the hardness of Chinese olive.3.During the development and maturation of Chinese olive,the different hardness increasing amplitude of QL,LF and FL led to the difference of hardness,the order of which was FL>LF>QL.The results of correlation analysis showed that,during the development and maturation of Chinese olive,the increase of hardness was positively correlated with the changes of ISP and LIG content(P<0.05).At the late stage of maturation,firmness and ISP were significantly positively correlated with pectin methylesterase(PME)(P<0.05).Therefore,it was speculated that PME may promote the increase of hardness and ISP content in the late stage of maturation.PME was at the activity peak or the PME activity was greatly increased to reach the activity peak,which may be related to the higher ISP increasing amplitude and content,so that promote the formation of higher fruit hardness.In summary,ISP and LIG were the reasons for the difference in hardness of Chinese olive fruit.The Chinese olive with higher ISP and LIG content had tight parenchyma cell wall structure,higher middle lamella density,more vessels,more developed vascular bundles,which could maintain the smaller and rounder cells morphology,and contribute to stronger compressive ability,that is,higher hardness.During development and maturation,higher PME activity and increasing amplitude may promote the increase of ISP content,so that promote the formation of higher hardness in the later stage of maturation.