| Granulation is an important postharvest physiological disease of citrus fruits.Water mutual migrating between the peel and the pulp is one of the causes.In plant,water transport among tissues mainly depends on aquaporins(AQPs).However,the AQPs involved citrus granulation is still unclear.Hence,two types of citrus fruits were selected as materials,one is Ponkan(Citrus reticulata Blanco cv.)which is prone to granulation and puffing,and another is navel orange(Citrus sinensis Osbeck L.Newhall)which is prone to granulation but not puffing.Firstly,the physiological changes in peel(including flavedo and albedo)and pulp were assayed,such as weight loss,water contents,as well as osmotic regulators like soluble proteins,soluble sugars,organic acids,and mineral elements.And secondly,time and space differences of AQPs gene expressions between peel and pulp during the progress of granulation were further analyzed.All aimed to underline the impellin force and the channels of water migration.The results as follows.Physiological changes:1.The weight loss rate of both Ponkan and Navel orange fruits continued to increase.At earlier stage of granulation(0 d to 90 d for Ponkan,and 0 d to 75 d for orange),water loss rates were relatively low,but at later stage of granulation water loss rates sharply increased.2.Water contents of pulp were higher than that of peel for both ponkan and orange.To ponkan,water content in flavedo was higher than that in albedo,but there was no difference between orange flavedo and albedo.peel water content of both ponkan and orange persistently increased during granulation,even in ponkan pulp.3.For both types of fruits,contents of soluble protein in peel were 5~10 times more than that in pulp,and it was higher in flavedo than that in albedo.During granulation,the soluble protein content in the flavedo and pulp increased significantly,while in the albedo it showed a trend of increasing first and then decreasing after 45 days.4.The soluble sugar in the pulp is higher than that in flavedo and albedo for both two type fruits;soluble sugar content in flavedo of Ponkan is higher than that in albedo,what is on the contrary in orange.During granulation,soluble sugar decreased in peel.Soluble sugar in pulp of ponkan increased,but decreased in orange.The change trends of soluble sugar in pulp and flavedo of ponkan was opposite,and the same between pulp and albedo of orange.The changes of fructose,glucose and sucrose in the tissues of the two type fruits were consistent with the total soluble sugar.5.The contents of total organic acid and citric acid in pulp of both ponkan and orange decreased significantly.Contents of total organic acid,citric acid and quinic acid increased in flavedo of ponkan,but decreased in albedo.Organic acids in flavedo of orange decreased within 60 d and then increased thereafter,in albedo,it increased significantly during granulation.6.There were no differences of relative contents mineral elements among the flavedo,albedo and pulp of ponkan,but in orange fruit it was highest in flavedo and lowest in albedo.During granulation,relative contents mineral elements especially K,Ca,and Mg obviously increased in albedo of ponkan and orange,and decreased in pulp of ponkan and flavedo of orange.AQPs gene expression changes:Spatial and temporal expression changes of the total 34 AQPs genes were detected during granulation,29 of them were found finally to be differentially expressed.Interestingly,vacuolar membrane intrinsic proteins(TIPs)showed dozens of times higher expression during granulation.Cluster analyzation of differential expression genes found that,the temporal model of AQPs genes expression could to be classified to three periods: 30 d to 60 d,90 d and 105 d for ponkan,and 90 d to 105 d,60 d and 30 d.The AQPs gene expressions were mainly up-regulated in peel(except 60 d)and pulp of ponkan.To orange in peel,AQPs gene were mainly up-regulated at 30 days,but down-regulated at 90 ~ 105 days;in pulp,they were up-regulated from 0 to 90 days,and down-regulated at 105 d during granulation.In summary,water transportation among different fruit tissues is regulated by osmotic substances and AQPs.During granulation,contents of osmotic substances like organic acids and mineral elements decreased in pulp,soluble proteins,organic acids and mineral elements increased in peel,which lead to a water potential difference between peel and pulp,then water transported from pulp to potential.The osmotic substances in orange changes obviously,within 2 weeks before or after 60 d postharvest,which is the key period for the occurrence of granulation of navel orange pulp.The AQPs continuously highly expressed in the pulp of Ponkan,is a necessary condition for moisture output from pulp to peel and fruit the puffing.The down – regulation of AQPs in pulp of orange at later stage inhibits moisture output from pulp is an important reason for orange granulation but not puffing.To both types of granulation fruits,TIPs showed more than ten times higher expression,which might be the key channels for water transportion during fruit granulation. |
