Studies on Cucurbita phloem with special reference to a modified companion cell - sieve tube complex
Two types of sieve elements were identified in Cucurbita: fascicular sieve elements (FSE) of the central phloem, and extrafascicular sieve elements (ESE). ESE appeared in the peripheral cell layers surrounding the inner and outer fascicular phloem and formed an anastomosing system. ESE were accompanied by extrafascicular companion cells (ECC). ECC were often the same or bigger than ESE in diameter, whereas fascicular companion cells were much smaller. The EEC stained a vivid orange with acridine orange stain and were nucleate, often short and broad, with dense cytoplasm, and small vacuoles. ECC also contained one or more phloem proteins, including the characteristic phloem lectin. They were ubiquitous throughout the plant body and were structurally similar to intermediary cells (I-cells). Application of 5(6)carboxyfluorescein (5(6)CF) to the abraded leaf of Cucurbita maxima indicated that it was transported in both types of sieve elements. The main channel of longitudinal transport was the external fascicular phloem of the vascular bundles. Lateral transport took place in sieve tube anastomoses which spread dye to every bundle. 5(6)CF was also transported in the ESE within sections being examined by fluorescence microscopy, indicating that ESE and FSE were quite separate, and that ESE were not blocked after sectioning. Microinjection experiments showed that the turgor pressure in the ESE/ECC complex was very low, and that dye movement followed a low resistance pathway. In cucurbits, both sucrose and stachyose are transported. The results support the suggestion that stachyose is made in I-cells (proposed by Turgeon, 1991) and transported in ESE, while sucrose is symplastically loaded and transported in FSE.