Determination of the Phloem Path and Destination of the Photosynthates by Autoradiography and Fluorescent Marker Imaging

Abstract Vascular bundle connectivity, which is closely associated with phyllotaxis, plays a crucial role in determine the destination of photosynthates. The correlation between vascular connections and phosphate distribution has been studied in some plants; however, it is far from fully understood. In this study, the phloem paths and destinations of photosynthates in soybean plants was investigated. In soybeans, trifoliate leaves are alternately arranged. To trace the movement of photosynthates, radioisotope-labeled 14C-sucrose was applied to mature leaves, and autoradiography was performed. The results indicated that photosynthates were translocated from the first trifoliate (L1) or second trifoliate (L2) to the immature third trifoliate (L3) and fourth trifoliate (L4) leaves, which are located on both side of the stem. When 14C -sucrose was added to one side of the source leaves, the 14C signal was detected on the corresponding side of the sink leaves. The photosynthates were translocated from the left side of L1 to the right side of L2 and the left side of L3. The phloem paths were traced using carboxyfluorescein (CF), which is known a phloem marker, and 14C-sucrose. The photosynthates were translocated via different phloem paths from both L1 and L2, as well as from the left and right side of the trifoliate. These results indicate that photosynthates from a single trifoliate are broadly distribute, while the underlying translocation paths are finely subdivided.