ARCHITECTURE OF THE INTERNODAL XYLEM OF TOMATO (SOLANUM LYCOPERSICUM) WITH REFERENCE TO LONGITUDINAL AND LATERAL TRANSFER

Abstract

The anatomical organization of the internodal tomato xylem was studied with the goal to relate xylem architecture, longitudinal flow, and lateral solute displacement in the stem. On account of a vessel member inventory, obtained by maceration, two vessel types were reconstructed distinct in diameter, morphology, and location. Type I: wide vessels (diam 50–160 μm), with a relatively constant diameter and low horizontal perforation rims, the widest (in the main areas) often extending over more than one internode in length. Type II: narrow vessels (diam 10–50 μm), with occasional vessel dichotomy, average length 5 mm, an irregular diameter, and a meandering vessel lumen due to many oblique and side positions of the perforations. The axial symplast units consisting of various intergrading elements, communicate with the vessel system via sheaths of axial contact cells with a high reducing capacity. The radial symplast units communicate with the vessels via radial contact cells at windows in the axial sheaths. The vessel/symplast interface of the axial and radial symplasts is in the proportion 65:35. The excessive metabolic activity and the high tangential pit frequency of the ray cells mark the rays as the conduits for symplastic xylem‐to‐phloem transfer. Radially fused intercellular spaces in the rays suggest that radial apoplastic solute transfer may also occur.