Photoassimilates Transporting Pathway In Phloem Of Tree Peony

The content of sucrose,glucose and fructose was determined in phloem of all parts of tree peony(Paeonia suffruticosa Andr.‘Luoyanghong’)including root,old stem,new stem,leaf and leaf stalk.The organizational structure of phloem was observed systematically using optical microscope.The ultrastructure of phloem and the parenchyma cells surrounding it were investigated using transmission electron microscopy.The pathway of phloem transporting and the direction of nutrient flow were studied using carboxyfluorescein transport.The main results are as follows:1.The soluble sugar in phloem of all parts of tree peony existed mainly in the form of sucrose during both growth period and dormant preiod and its content was far beyond that of glucose and fructose.Especially when it was dormant,there was only sucrose in old stem and bud and glucose and fructose were not detected.When it was dormant,the total content of the three kinds of sugars in root is the highest among all the parts and higher than all the other parts of that in growth period.The relative amount of total content of the three kinds of sugars in bud of the two developmental period changed the most and the amount of there kinds of sugars in growth period(15.53 mg/g)was up to 5.56 times the amount in dormant period(2.79 mg/g).2.The phloem of root,old stem and new stem and the vascular bundle of leaf stalk and main vein of tree peony were located under optical microscope.The boundary of phloem and cortex cells was not obvious while the vascular of leaf stalk and main vein was quite distinct.Moreover,starch grains were increasingly stored in the parenchymal cell of root with the arrival of dormant period.3.The results of ultrastructural investigations showed that plasmodesmata were not found between SE/CC complex and its surrounding phloem parenchymal cells in root phloem at growth period,resulting in the symplastic isolation between them.While plasmodesmata existed between SE and CC,SE/CC complex and phloem parenchymal cells,phloem parenchymal cells and phloem parenchymal cells in old stem,new stem,leaf stalk and leaf main vein.The plasmodesmata density between each type of cell at dormant period increased or decreased compared with that of growth period.Even though some of the plasmodesmata were blocked,plasmodesmata still existed.On the whole,there were widely symplastic links between all parts of phloem in tree peony.4.The results showed that the fluorescence signal in root,old stem and new stem at growth period was brighter than that at dormant period while the leaf stalk had the opposite result.At growth period,the phloem-transported carboxyflurescein diffused into the surrounding phloem parenchyma cells in old stem,new stem and leaf stalk except root.Moreover,the fluorescence signal above the treatment site was brighter than that of below the treatment site,but the the leaf stalk had the opposite result.At growth period,CF was obviously released into the surrounding phloem parenchyma cells in all treatment parts and the fluorescence signal below the treatment site was brighter than that above the treatment site.This suggested that the nutrient mainly results from photosynthate made by leaves and were mainly transported to the top at growth period.While at dormant period,the left nutrient in the body was mainly transported to the fleshy root and stored.In conclusion,nutrient was badly needed to support the growth of the aerial portion at growth period.There were widely symplastic links between phloem cells of all parts except roots in tree peony and photosynthate transporting pathway was mainly symplasmic pathway and supplemented with apoplasmic pathway.Nutrient went back to root at dormant period.There were also widely symplastic links between phloem cells of all parts in tree peony and photosynthate transporting pathway was symplasmic pathway.