DEVELOPMENT AND STRUCTURE OF THE VASCULAR CONNECTION BETWEEN THE PRIMARY AND LATERAL ROOT OF LYCOPERSICON ESCULENTUM

The primary xylem connection between the diarch parent root and the diarch lateral root was derived from the pericycle and stelar parenchyma. Early in lateral root development stelar parenchyma that was positioned between the parent xylem and the primordium divided transversely. These transverse divisions produced a plate of cells, most of which subsequently differentiated into vessel element connectors. After emergence of the lateral root, xylem maturation began in the stelar vessel element connectors and maturation proceeded acropetally into the lateral root. Protoxylem of the lateral root was connected to the metaxylem of the parent root via stelar vessel element connectors. The circular phloem connection was pericyclic in origin. Axial phloem connections which vascularized the lateral root were established with sieve tube elements of both parent phloem poles. Maturation of the phloem connection occurred prior to lateral root emergence. Transaxial phloem, positioned in arches above and below the lateral root vascular cylinder, was derived from the pericycle; and each arch consisted of three to four sieve tube elements. No transfer cells were found in the transaxial phloem. BOTH THE DEVELOPMENT and the structure of a root vascular connection between the primary and secondary root were first described in detail for soybean (Byrne, Pesacreta and Fox, 1977a). We reported three major developmental and structural features of the soybean vascular connection. First, the primary xylem connection was derived from stelar parenchyma rather than the pericycle (Esau, 1965). Second, primary xylem maturation began within the parent root and proceeded acropetally into the lateral root. Third, the phloem connection was circular and consisted of two parts developmentally: the axial connection which was derived from the pericycle, and the transaxial connection which was derived from stelar parenchyma. The major purpose of this investigation was to compare the vascular connection of the diarch tomato root with that of the tetrarch soybean root. From previous experience and the literature (Hayward, 1938; Byrne et al., 1975; Aung, Bryan and Byrne, 1975), we knew that lateral roots of tomato originate between a xylem and phloem pole. Also, unlike soybean, outer stelar parenchyma is absent be1 Received for publication 29 October 1980; revision accepted 3 April 1981. Part of this paper represents research done for a B.S. Honors degree in Biology. 3 Present address: 21567 South Park Dr., Fairview Park, OH 44126. tween the protoxylem and pericycle in the tomato root. Although there are at least three modern reports concerning the development of lateral roots from diarch parent roots, various aspects of lateral root vascularization have not been adequately studied (Esau, 1965, Byrne et al., 1977a). In addition, if transaxial phloem existed in the tomato root, we hoped transmission electron microscopy (TEM) would shed some light on the proposed functional relationship (Byrne et al., 1977a) between the transaxial phloem and the vascular cambium. MATERIALS AND METHODs-Seedlings of Lycopersicon esculentum Mill. cv Fireball were grown in Seed-Pack Growth Pouches (Scientific Products, Evanston, Illinois; Haas and Carothers, 1975). The seedlings were maintained on 0.5% Hoagland's Solution in the greenhouse. Histological methods reported previously (Byrne et al., 1977a, b) were used to investigate the root material taken from 10to 14-day-old seedlings. Material for TEM was fixed in cold, buffered (pH 7.2), 3% glutaraldehyde. Specimens were trimmed to size while submerged in the fixative. Specimens were post-fixed in 2% OSO4 for 1 hr, dehydrated in a graded series of ethanol and block stained with 0.5% uranyl acetate. The tissue was embedded in Spurr's Low Viscosity Resin and sectioned on a Sorvall M1 ultramicrotome. Sections were post-stained