The trans‐tissue pathway and chemical fate of ¹⁴C photoassimilate in carrot taproot

Abstract

Axial and radial transport and the accumulation of photoassimilates in carrot taproot were studied using "%C labelling and autoradiography. Axial transport of the "%C labelled assimilates inside the taproot was rapid and occurred mainly in the young phloem found in rows radiating from the cambium. The radial transport of the assimilate inward (to cambium, xylem zone and pith) and outward (to phloem zone and periderm) from the conducting phloem was an order of magnitude slower than the longitudinal transport and was probably mainly diffusive. The cambial zone of the taproot presented a partial barrier in the inward path of the assimilate to the xylem zone. We suggest that this is due to the cambium comprising a strong sink for the assimilate on the basis that our previous work has shown that it contains very low concentrations of free sucrose. By contrast, a high accumulation of nonsoluble "%C was found in the cambium region in good agreement with the active growth of this zone. Autoradiography following the feeding of "%C labelled sugars to excised sections of taproot indicated that only a ring of cells at and\or just within the cambium take up sugars from the apoplast. This indicates that radial movement in the phloem and pith must be symplastic. An apoplastic step between phloem and xylem is possible. The rapid uptake of sugars from the apoplast at this point might represent a mechanism for keeping photoassimilates away from the transpiration stream and re-location back to the leaves.

Suboptimal temperature favors reserve formation in biennial carrot (Daucus carota) plants

“…Studies in carrot taproots have shown that radial movement of 14 C‐assimilate from the active phloem zone was more rapid in the centrifugal direction (i.e., across the phloem region) than in the centripetal one (across the cambium into xylem tissue); this highlights the role of cambium as a barrier for the radial movement of the assimilates into the xylem (Caplin 1973, Korolev et al 2000a). Thus, the increased growth and carbon deposition in the phloem parenchymatic zone at 12°C may be the consequence of both increased assimilate availability and the distinctive transport properties of assimilates within the carrot taproot.…”

Symplasmic, long‐distance transport in xylem and cambial regions in branches of Acer pseudoplatanus (Aceraceae) and Populus tremula × P. tremuloides (Salicaceae)

“…The intensively dividing cambial cells form an important sink (Korolev et al, 2000; Sauter, 2000) for nutrients transported from xylem and phloem regions, preferentially via the rays (Sauter and Kloth, 1986; van Bel, 1990; Korolev et al, 2000; Chaffey and Barlow, 2001). Radial cell‐to‐cell transport in rays may proceed via two different routes: symplasmically (van Bel, 1990; Korolev et al, 2000) or apoplasmically (van der Schoot and van Bel, 1989; Kitin et al, 2009).…”

“…The intensively dividing cambial cells form an important sink (Korolev et al, 2000; Sauter, 2000) for nutrients transported from xylem and phloem regions, preferentially via the rays (Sauter and Kloth, 1986; van Bel, 1990; Korolev et al, 2000; Chaffey and Barlow, 2001). Radial cell‐to‐cell transport in rays may proceed via two different routes: symplasmically (van Bel, 1990; Korolev et al, 2000) or apoplasmically (van der Schoot and van Bel, 1989; Kitin et al, 2009). Abundant plasmodesmata in the tangential walls of ray cells (Farrar, 1995; Fuchs et al, 2010b), the radial arrangement of cytoskeleton elements in xylem and phloem rays (Chaffey and Barlow, 2001), the translocation of 14 C‐labeled assimilates in carrot taproot (Korolev et al, 2000), and the high radial flux rates of sugars through the ray cell tangential area (Sauter and Kloth, 1986; Sauter, 2000) support a symplasmic route of radial transport.…”

“…Radial cell‐to‐cell transport in rays may proceed via two different routes: symplasmically (van Bel, 1990; Korolev et al, 2000) or apoplasmically (van der Schoot and van Bel, 1989; Kitin et al, 2009). Abundant plasmodesmata in the tangential walls of ray cells (Farrar, 1995; Fuchs et al, 2010b), the radial arrangement of cytoskeleton elements in xylem and phloem rays (Chaffey and Barlow, 2001), the translocation of 14 C‐labeled assimilates in carrot taproot (Korolev et al, 2000), and the high radial flux rates of sugars through the ray cell tangential area (Sauter and Kloth, 1986; Sauter, 2000) support a symplasmic route of radial transport. The apoplasmic route of the radial pathway toward the cambium is, on the other hand, reinforced by the architecture of the secondary xylem, e.g., by the presence of pit connections and intercellular spaces (van der Schoot and van Bel, 1989; Kitin et al, 2009).…”

“…The best‐known route of long‐distance, symplasmic transport is a phloem strand (Romberger et al, 1993; van Bel and Ehlers, 2000; van Bel, 2003). However, even in the secondary xylem, where extensive apoplasmic transport of water and ions takes place, symplasmic communication is present (van Bel, 1990; Romberger et al, 1993; Korolev et al, 2000; Barnett, 2006) as realized in a spatial network of longitudinally and radially elongated elements that form two perpendicularly oriented systems of xylem parenchyma cells in angiosperms (van Bel 1990). The long‐lived parenchyma cells, with their directional arrangement of cytoskeleton elements and specific localization of PD, are believed to be involved in directional long‐distance symplasmic transport in wood (Sauter and Kloth, 1986; Chaffey and Barlow, 2001).…”