Dr. Dennis Walker,   Department of Biological Sciences,   Humboldt State University


Helianthus Vessel Member Ultrastructure

Walker, D. K. 1966. The Ultrastructure of Differentiating Vessel Members in Helianthus annua L. "Mammoth Russian".
      Ph.D. dissertation at University of California, Davis
      [major professor: Ernest M. Gifford]


      A study of the ontogenetic development of vessel members in the primary xylem of Helianthus annua. "Sunflower" seedlings were grown in the greenhouse and specimens were prepared and examined with the transmission electron microscope in the Department of Botany at the University of California at Davis. Vessel members in the primary xylem of this plant's petiole differentiate in a sequential order that makes it possible to predict which procambial cells are destined to become vessel members. Thus, it was possible to examine the earliest stages of differentiation with a transmission electron microscope before any cytological changes were recognizable by light microscopy.

      The first evidence of changes in ultrastructure during the transformation of a procambial cell into a vessel member occur in plastids when grana expand to form a single large "postlamellar body" in the center of the organelle. As the vacuole increases in size with overall cell enlargement, other cytoplasmic organelles such as dictyosomes and mitochondria increase numbers. Deposition of secondary wall thickenings in the characteristic patterns of the protoxylem and metaxylem are predetermined by the endoplasmic reticulum pattern in the parietal layer of the cytoplasm. Secondary wall materials are deposited only where the ER arches far away from the primary wall, and are never deposited where ER is close to the plasmalemma.

      The latter rule also applies to the sites at which the perforations develop through the walls at the ends of the cells. Secondary wall materials are not deposited where the ER is near the end wall. Instead, pectic materials are transported in golgi vesicles from dictyosomes and secreted into the cell wall resulting in a swelling of the primary wall at perforation sites. After lignification of the secondary wall, the tonoplast ruptures and unlignified portiions of the wall such as the dilated perforation sites are digested away. Autolysis of the protoplast completes the maturation of the vessel member.

electron micrograph of a plastid in a differentiating vessel member of Helianthus showing the stroma (S), the plastid envelope (PE), and a postlamellar body (PB) developing from grana (G) electron micrograph showing the cell wall (W) and vacuole (V) of a developing sunflower vessel member including the plastid envelope (PE), postlamellar body (PB), and grana remnants (G) of a plastid electron micrograph of a fully-developed postlamellar body (PB) in an Helianthus vessel member that shows the plastid envelope (PE) and thylakoids (PB) within the stroma (S) ultrastructure of vessel member cytoplasm at the site of secondary wall decomposition showing the vacuole (V), mitochondria (M), endoplasmic reticulum (ER), ribosomes (R), numerous golgi vesicles (GV) from the dictyosomes (D), and plastids including starch grains (S) and postlamellar bodies (PB)
       
early stage in deposition of secondary wall (SW) showing mitochondria (M), a dictyosome (D), and the endoplasmic reticulum (ER) distribution that controls the wall pattern (note the close proximity of the ER to the plasmalemma (P) between the secondary wall thickenings) electron micrograph of an immature vessel member with dictyosomes (D), vacuole (V), mitochondrion (M), and the endoplasmic reticulum (ER) between the developing secondary wall thickenings (SW) adjacent to the vacant lumen (L) and lignified secondary wall thickening (LW) of a mature vessel member electron micrograph of a chloroplast (C), nucleus (N), and vacuole (V) in a xylem parenchyma cell next to a maturing vessel member with endoplasmic reticulum (ER), mitochondrion (M), secondary wall thickenings (SW), and vacuole (V) the site of a future perforation (PS) through the apressed end walls (EW) of developing vessel members and the adjacent cytoplasm including endoplasmic reticulum (ER), dictyosomes (D), mitochondria (M), and vacuole (V)
       
ultrastructural details at the perforation site showing plasmodesmata (P) in the dilated primary wall layers (W) on either side of the middle lamella (ML), and a mitochondrion (M) and endoplasmic reticulum (ER) in the parietal cytoplasm ultrastructural details including plasmodesmata (P), primary wall layers (W), and middle lamella (ML) in the perforation site, and the mitochondria (M), dictyosomes (D) and golgi vesicles (GV), endoplasmic reticulum (ER), and the tonoplast (T) bulging into the vacuole (V) electron micrograph showing the new perforation bordered by secondary wall rims (R) in a maturing Helianthus vessel that still contains remnants of the disintegrating protoplast cytoplasm (C) electron micrograph showing the perforation rim (SW) of a maturing vessel member and its protoplasmic remants including a cytoplasmic membrane (CM) around a plastid's ruptured envelope (X), postlamellar body (PB), starch grain (S), and grana (G)