The Ziziphus Jujuba Mill Fruit Development And Its Relationship With Fruit Cracking

Jujube fruit(Ziziphus jujuba Mill)is an endemic species in China with wide international competitive advantage.Widely distributed and cultivated in China,it has become the main economic tree species and pillar industries in China's northern mountain,sand,alkali and dry areas.Jujube fruit cracking has occurred in many areas of China and caused huge economic loss to production after the 21 st century.However,due to that the mechanism of jujube fruit cracking is unclear,the control technology has a certain effect but cannot effectively control the occurrence of cracking fruit.For these reasons,this paper,with easy-cracking fruit ‘Hupingzao',‘Pozao' and anti-cracking fruit ‘Popozao' as test materials and based on the analysis of fruit epicarp,sarcocarp and surface structure,focused on the changes of cell wall characteristics affecting the rigid structure of fruits.At the same time,the method of Low-field nuclear magnetic resonance imaging(LF-NMR)was used to analyze the water absorption and water distribution of the fruit ripening.The reason of the fruit cracking caused by water absorption was discussed from the perspective of biomechanics,in order to provide the basis for further exploring the cracking mechanism and effective methods to control the occurrence of fruit cracking.1.During the development and mature process of jujube,the stomata gradually suberized to form the lenticels and part of the lenticels gradually clogged to form a swollen corky fruit dots.The clogged fruit dots were prone to breakage and format micro fracture in the process of further maturation and coloring.Most of the lenticels remained open after fruit coloring and part of the lenticels formed microcracking along the longitudinal direction in the late stages of fruit development.Dye traces showed that water could enter the fruit through open stomata.Low Field NMR imaging analysis showed that micro fracture was an important way for a large amount of water to enter the fruit and it could also cause an unusual rise of water amount in part of the fruit.It was generally observed that the macroscopic cracks were connected by two or more fruit dots in the cracking fruit caused by both the natural rainfall and submerging test.The intersection of the cross cracks was the location of the stomata and a lot of microcracking paralleling to the cracks was formed from stomata.It was proved that the surface stomata and the microcracking were the key factors that caused the macroscopic cracking of fruit.2.There were a lot of cavities in the flesh of the jujube fruit,so that the flesh formed a three-dimensional network of sponge-like structure.The middle loose part of the flesh was the network structure of monolayer cells.The single cell layer was distributed radially around the vascular bundle and the cavity area could account for 30.3% of the fruit cross-sectional area.There were a lot of cavities in the part of the mesocarp under sub-epidermis and the outer skin of the jujube fruiting.The flesh and the peel could be clearly distinguished from each other.In many parts the flesh and the peel were often connected only by the monolayer.Besides,there existed a large number of cavity structures beneath the epidermis and around the stomata,and a lot of water distribution in the pulp cavity of fruit cross section was observed after the jujube fruit incubation in water.The experiment results above proved that there was great possibility that water entered the fruit through the “open pores-pulp cavity”.3.It was found in the stereoscope observation that the common longitudinal cracks on the ‘Hupingzao' fruit usually through the eye-shaped scar points on the scar circle of the fruit stem cavity.Further experiments showed that the scar on the ‘Hupingzao' fruit stem cavity is circular scars formed by the abscission of jujube flower stamens and sepals.The 5 eye-shaped scar points evenly distributed on the circumference were the positions where the stamens abscissed.During the fruit ripening process,the eye-shaped scar points were easy to form microcracking.Twelve branches of the main vascular bundles were sprouting from the stem cavity.Among the twelve branches of the main vascular bundles,two of them led to the inside of the fruit stone,the other ten led to the scar points on the fruit stem cavity.And ten of the vascular bundles corresponded to the scar on the five obvious eye-shaped scar points and every two points between the middle of the scar.The inner tube of the fruit was mainly composed of the branches formed by the 10 main vents.The eye-shaped scar points in the stalk had an important role in the formation of the longitudinal cracks.4.The rigid structure of the cell wall of jujube fruit had an important effect on the occurrence of fruit cracking.The ultrastructural observation showed that the degradation of the flesh cell wall and the secondary thickening of the subcellular cortical cell wall were found during the maturation of the jujube fruit.In the easily cracking ‘Hupingzao' and ‘Pozao' fruit ripening process,some of the flesh cell wall and intercellular layer degraded and plasmolysed;sub-epidermis cell walls of the very significant secondary thickened;cells across the fruit surface tangential severely shrank and flattened;the epidermal cells severely deformed;and the cell wall degraded.However,in the crack resistent ‘Popozao' fruit ripening process,both the flesh cell wall and the sub-epidermal cell wall had maintained a good integrity and the cell deformation was small.The epidermal cell wall occurred secondary thickening.The increase of the activity of galacturonidase(PG)and Cx-cellulase in the coloring period of jujube fruit resulted in the degradation of cell wall components,which led to the change of cell wall structure.The content of water-soluble pectin,hemicellulose and cellulose in fruit peel,and the content of water-soluble pectin,ionic soluble pectin,hemicellulose and cellulose in the flesh of the crack resistent ‘Popozao' were significantly higher than those of easily cracking ‘Pozao' and ‘Hupingzao'.Therefore,it was suggested that the three factors including the increase of cell wall degrading enzyme activity,the decrease of cell wall material content,the degradation of cell wall,the decrease of cell wall rigid structure caused by these three factors,and the decrease of fruit toughness caused by severe secondary thickening of subcellular cortical cell wall,all of which were rerequisites for fruit cracking in the rainy condition.5.The relative water uptake rate of the fruit through the vascular bundle system of fruit stem gradually decreased with the maturity of the ‘Hupingzao' fruit,and the rate in coloring stage was similar to that in the white-ripening stage and both of them were significantly higher than that in the full red period.The relative water uptake rate of the ‘Hupingzao' through the fruit surface in the coloring stage was significantly higher than that in the white-ripening period and in the full red period.The results of incubation test showed that the relative water absorption of the ‘Hupingzao' fruit via the surface in the coloring period accounted for 51% ~ 54% of the total relative water absorption.The relative water absorption via the stem cavity was 9% ~ 18% and relative water absorption via fruit stem accounted for 31% to 40%.The relative water uptake via the stem cavity significantly increased during the incubation.Water absorption via fruit stem cavity increased once after 10 hours incubation.The results of the dyestuffs trancer experiment showed that there was a noncastectic pathway for water to enter the fruit via the fruit surface.Low Field NMR imaging experiments showed that during the incubation,water could quickly enter the fruit through the fruit surface.6.The significant difference in the water swelling of fruiting and fruiting cells of jujube fruit resulted in the rapid reversion of the fruit fragments of the jujube,which indicated that the water absorption of the fruit near the pericarp could form a greater pressure on the fruit surface.The water swelling of the fossa cells of the fruit might be the source of the power that caused the fruit to absorb.The puncture test of fruit after submerging showed that the pericarp strength and pericarp toughness of the ‘Hupingzao' fruit decreased after the incubation.The brittleness of the peel increased,especially in the fruit stem-end region where the fruit was prone to cracking and it was significantly positively correlated with the fruit cracking rate.Therefore,the impact of water on the fruit cracking was not only the mesocarp of the water cell swelling and it also led to decreased mechanical strength of pericarp and then induced cracking fruit.