Mechanisms Of Photoassimilate Transportation & Partitioning In Nectarine Tree (Prunus Persica L. Var. Nectarine Ait) Under Weak Light Environment

The mechanisms of photoassimilate transportation and partitioning of nectarine trees (Prunus persica L. var. nectarina Ait.) were studied under weak light environment using the three-year-old 'Zao Hong Van' nectarine trees cultivated in solar greenhouse.Firstly, the ultrastructural characteristics of phloem tissues of source leaves and fruits were observed and compared in normal and weak light intensities using the transmission electron microscope focusing on the response of loading zone and unloading zone ultrastructural to weak light. Results showed that the average diameters of companion cells (CC) and sieve elements (SE) of all kind of veins of source leaves were bigger in normal than in weak light intensity (WLI), dense cytoplasm with abundant mitochondria, endoplasmic reticulums, multivesicular bodies, vesicles and plastids were observed in normal light intensity both in source leaves and fruits. On the contrary, CC with small vacuolar structures and few mitochondrias, endoplasmic reticulums were shown in weak light source leaves and fruits. Misalignment of grana thylakoid margins of nectarine leaves also was seen in weak light. The sieve pores of SEs were obstructed in weak light. Chloroplasts with numerous starch grains and few mitochondrias were noticed in the mesophyll cell (MES) surrounding the bundle sheath in weak light source leaves. The size of SE was smaller, the cavity of SE was narrow, cytoskeleton of CC was seriously destroyed in weak light fruit at the endocarp hardening stage. Many starch grains were accumulated in young fruit stage CCs, PPs and mature stage SEs in normal light intensity. Plasmodesmal densities between SE/CC, CC/PP (phloem parenchyma cell), PP/PP and PP/BSC (bundle-sheath cell) decreased in weak light source leaves and fruits. These results indicated that weak light might influence the phloem tissue metabolism viability, photoassimilates translocation ability, loading and unloading pathways in source leaves and sink fruits.Secondly, the effects of weak light to the sugar content and sucrose synthase (SS) regulation in source leaves and fruits of nectarine trees were studied via immunogold electron-microscopy localization and Western blotting. The experiment showed that the sugar content and SS activity were decreased in weak light source leaves. The fruit sucrose, glucose and fructose contents were not significantly changed by weak light during young fruit stage, but the contents declined during endorcarp hardening and mature stages. The SS activity was lower during young fruit stage, could hardly be detected at endorcarp hardening stage, but increased during mature stage. A 87kD SS isozyme protein band was detected in source leaves and two SS isozymes protein bands 65kD (SSI) and 87kD (SSII) were detected in nectarine fruit during mature stage via SDS-PAGE and western blotting. This showed that the SS was expressed with organ specialty. The results of immunogold electron-microscopy localization showed that SS was mainly localized in cytoplasm and vacuole of PPs of source leaves and FPs of fruits. At the same time, SS immunogold particles also localized on the nacreous cell walls (NCW) of nacreous-walled sieve element (NS) both in source leaves and fruit. These results suggested that SS might be the key enzyme in providing energy for the sugar loading in source leaves,providing UDPG-glucose for the NCW construction of NS, and producing sucrose concentrationgradient for the sucrose accumulation in nectarine sink fruit. Weak light did not change the quantity ofisozyme protein, while decreased the activity of SS. The results suggested that weak light signal inducedpost-translated inhibitory regulation to SS in nectarine source leaves and fruit.