Transport in Plants I

Volume 1 of the series Encyclopedia of Plant Physiology pp 59-100

Nature of Transported Substances

  • H. Ziegler

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A detailed knowledge of the chemical nature of the substances transported in the sieve tubes of angiosperms or their analogs in other groups of the plant kingdom is of special interest in several respects:
  1. 1.

    These transport-substances are the bulk of the organic material which is available for all non-autotrophic cells and tissues in a plant. (A very restricted additional source is the transpiration stream, which usually carries organic compounds in very low concentrations.) The transport substances in the phloem, therefore, can be considered as the skeletons, from which all necessary substances in tissues like cambium, meristems, developing fruits and storage tissues, must be synthesized.

  2. 2.

    A knowledge of the transport substances may also give some indications of the transport mechanism. A simultaneous transport of several substances of very different molecular architecture, would, for example, make the assumption of a carrier transport very improbable, as would the translocation of substances of very different hydrophilic and lipophilic properties for the migration on phase surfaces (van den Honert, 1932; Chapter 15, p. 356). For a streaming solution the transport of water is of course an inevitable prerequisite.

  3. 3.

    A clear idea about the question of what can move in the phloem and what can not is necessary for the pertinent use of applied substances, e.g. herbicides or fertilizers.

  4. 4.

    The content of the assimilate-conducting cells is the food source for a great number of animal species, mainly aphids and coccids. A detailed knowledge of the composition of this diet allows an understanding of the nutrition of these organisms. It is impressive how far the synthetic diet for phloem-sucking aphids (cf. Mittler, 1970) resembles the “standard composition” of the sieve-tube sap, as outlined below.