| Citrus fruit mastication is an important indicator of fruit quality,and in many countries where fresh citrus fruits are mainly consumed,fruit mastication is the first concern from consumers.Poor citrus mastication is mainly manifested by the rough taste of in flesh chewing,in which a lot of fiber residues are lest after repeated chewing.Inferior mastication seriously affects the commercial performance of citrus fruit.Segment is the main edible part of citrus fruit,which consists of segment membranes and juice cells.The texture of segment membrane directly affects the mastication trait of citrus fruits.Therefore,it is very important to reveal the development pattern of citrus segment membrane and the mastication trait.In this study,using different types of citrus fruits,roughing disorder citrus fruits and drought-stressed citrus fruits,the developmental characteristics and key factors affecting the performance of segment membrane mastication were investigated through ultrastructural observation,analysis of cell wall components and related gene expression.This study hopes to provide a theoretical basis for improving citrus fruit quality.The main results are as follows.1.Significant differences were found in segment membrane structure and cell wall components between different citrus types.The segment membrane side which adjoins each other between segments was defined as the outside,and the other side which the juice cells grow on was defined as the inside.In varieties of hybrid citrus(Ehime No.30((C.unshiu ×C.clementina)×(((C.unshiu × C.sinensis)× C.unshiu)×((C.erythrose × C.paradisi)× C.reticulata)))、Nova(C.reticulata × C.grandis Osbeck)),Satsuma mandarin(Okitus wase(Citrus unshiu Marc.)),Navel orange(Yuanfeng(Citrus sinensis Osbeck)、Newhall(Citrus sinensis Osbeck))and pomelo(Anjiang pomelo(Citrus grandis Osbeck)、Jiangyong pomelo(Citrus grandis Osbeck)),the outsides of segments membrane all exhibited roughness and unevenness,with textured structure,the subcellular layers are irregularly arranged and adherent to each other;while the inside showed smoothness with obvious wax layer.Under the wax layer,the top 1～2 cell layers were neatly arranged and the cell wall was thickened.Cell density in segment membrane increased gradually from outside to inside.Among them,segment membranes in hybrid citrus,Satsuma mandarin and Navel orange fruits exposed thin membrane,less cell layers and low water-soluble pectin content,which showed better segment membrane mastication.Pomelos fruits had the thickest segment membrane,more cell layers,highest water-soluble pectin content and the worst segment membrane mastication.2.During citrus fruit development,developmental patterns differed of in segment membrane structure and cell wall composition between cultivars with different mastication trait.During fruit development,segment membranes of ‘Shatian’ pomelo with the most difficult chewing texture had the longest maturation period,the thickest cell walls,the most segment membrane cell layers,and the highest pectin content.Segment membranes of‘Yuanfeng’ Navel orange with a medium mastication trait had long maturation period,thick cell walls,high number of cell layers,and medium pectin content.Segment membranes of‘Juxiangzao’ Satsuma mandarin with the best mastication trait had the shortest maturation period,the thinnest cell wall thickness,the least cell layers and the lowest pectin content.The protopectin contents in the segment membrane of three varieties all decreased with fruit development,while the water-soluble pectin content showed an increasing trend.Comparative analysis for these three varieties revealed that the pectin content in the segment membrane of‘Shatian’ pomelo was significantly higher than that in the other two varieties,followed by‘Yuanfeng’ Navel orange.Segment membranes of ‘Juxiangzao’ Satsuma mandarin contained the lowest pectin content.In October,segment membrane protopectin in ‘Shatian’ pomelo was74.7% and 120.2% higher than that of ‘Yuanfeng’ Navel orange and ‘Juxiangzao’ Satsuma mandarin respectively,and the water-soluble pectin increased by 289% and 153%,respectively.In November,the protopectin content of ‘Shatian’ pomelo increased by 56% compared to‘Yuanfeng’ Navel orange,while the water-soluble pectin content increased by 399%.3.Inferior mastication performance with found in roughing-disordered Satsuma mandarin fruit,in which structural changes play important roles in this process.In ‘Juxiangzao’and ‘Miyamoto Wase’ Satsuma mandarins,segment shear force of roughing-disordered fruit was significantly enhanced relative to that of normal fruit(CK),and the number of cell layers and cell wall thickness increased as well.Segment membrane vessels with 200-300 μm in length and 5-15 μm in diameter were predominant in the roughing-disordered fruit,which showed 13%-16.5% expansion compared to CK.Analysis for cell wall composition showed that the protopectin content in segment membrane decreased with fruit development in both roughing-disordered fruits and CK fruits,while the water-soluble pectin content increased in that.The protopectin in segment membranes of roughing-disordered fruit increased significantly throughout fruit development,while the water-soluble pectin content increased at the late fruit development stage.Comparing the inner-ring with the outer-ring portion of one segment membrane in roughing-disordered fruit,it was found that the cells in the innerring portion arranged densely and had higher water-soluble pectin.Expression of Cu PME21 gene,which is involved in pectin hydrolysis,was consistently up-regulated in the segment membranes of roughing-disordered fruit throughout development.4.Drought stress significantly affects citrus fruit quality including segment membrane mastication.The results of drought stress treatment in two consecutive years showed that the temperatures of canopy,leaf and fruit increased significantly after drought treatment.Fruit size and coloration decreased significantly after drought stress,but total soluble solids(TSS)and titratable acid(TA)increased significantly after drought.The observation of segment membrane anatomy showed that the drought-treated fruits had thicker segment membranes,accompanied by features including rough and uneven outside but smooth and flat inside of segment membranes,increased segment membrane cell layers and thickened the cell wall.Analysis for cell wall composition showed that water-soluble pectin increased significantly in the segment membranes of drought fruits.Further analysis showed that expression of Cu PMEI3,Cu PMEI3.1 and Cu PMEI28 genes were all significantly up-regulated in segment membranes under drought stress. |
