The Pathway And Physiological Mechanisms Of Photoassimilates Unloading In Populus×Euramericana Cv. 'Nanlin95'

The pathway and physiological mechanisms of photoassimilates unloading was studied in Populus×euramericana cv.'Nanlin95' using a combination of light microscopy,transmission electron microscopy,cytochemical Localization,carboxyfluorescein transport and biochemical determination.The results are as follows:1.According to the ultrastructure of phloem and its surrounding parenchyma cells in the main vein of sink leaves and stems of Populus×euramericana cv.'Nanlin95',the SE/CC complex and phloem parenchyma cells have plasmodesmata,resulting in symplasmic links between them.There was almost no plasmodesmus between the SE/CC complex and its surrounding phloem parenchyma cells in the lateral vein of source leaves,resulting in the symplasmic isolation between them.2.After the phloem cells were labeled with CF in vivo,The results showed that CF could unload in mesophyll cells,the shoot tip and the root tip cells,but the phloem-translocated carboxyfluorescein remained confined to the phloem in the secondary stems and the secondary roots.It was considered that the symplasmic unloading pathway of the photoassimilates exist in the sink leaves,the shoot tip and the root tip cells,the apoplasmic pathways may exist in the secondary stems and the secondary roots.3.In the SE/CC complex of the leaf minor veins and stems,the H⁺-ATPase and Ca^{2 +}-ATPase activity in the plasma membrane and protoplasm of SEs were higher than CCs and phloem parenchyma cells.The H⁺-ATPase and Ca²⁺-ATPase activity reaction product was present in association with parietal P-protein filaments.Crystalline P-Protein Body were less heavily stained than dispersed filaments.The H⁺-ATPase and Ca²⁺-ATPase were highly active in the plasmodesmata.Ca²⁺concentration in the SEs were far higher than CCs.The crystalline P-Protein Body in the SEs had no "exchangeable cellular calcium" induced precipitates,but it had strong Ca²⁺fluorescence intensity.Crystalloid dispersal was induced by 10 mM EDTA,but coundn't repeat exchanges of 10 mM CaCl₂.The crystalline P-Protein Body might had lost transport functions.Insoluble and soluble acid invertase activities in the active cambial zone and the adjacent region of xylem cell differentiation were higher than other tissues.High insoluble and soluble acid invertase activities improved sink strength to ensure that the vascular cambium could receive adequate photoassimilates.4.We studied seasonal changes of soluble sugar contents and related enzymes activities of Populus×euramericana cv.'Nanlin95'.With the temperature lower,SPS,SS activities and sucrose contents increased,but SAI activities didn't change significantly with the seasonal changes.Soluble acid invertase activities in the active cambial zone and the adjacent region of xylem cell differentiation were higher than other tissues during cambial reactivation.In contrast, soluble acid invertase activities were labeled in the inner phloem,the cortex parenchyma and in some ray cells that contact vessels in the mature xylem during cambial dormancy.5.Water stress regulated the net photosynthetic rate(Pn),transpiration rate(Tr),stomatal conductance(Gs)and intercellular CO₂ concentration(Ci)levels of Populus×euramericana cv. 'Nanlin95' were significantly decreased.The soluble sugar content and related enzymes activities of Populus×euramericana cv.'Nanlin95' changed,which resulted in more photoassimilates flowing to the roots and stems,and reducing the partitioning to the sink leaves. With the increasing of water stress levels,δ¹³C value of in vegetative organs of Populus×euramericana cv.'Nanlin95' showed increased trend.