Transport sieve tubes

The main components of inner bark are sieve elements, parenchyma cells, and sclerenchymatous cells. Sieve elements perform the function for transportation of liquids and nutrients. More specifically and according to their shape the sieve elements are divided into sieve cells and sieve tubes. The former types are present in gym nosperms, the latter in angiosperms. The sieve elements are arranged in longitudinal cell rows which are connected through sieve areas. The sieve cells are comparatively narrow with tapering ends, whereas the sieve tubes are thicker and cylindrical. After 1 -2 years, or after a longer time in the monocotyledons, the activity of the sieve elements ceases and they are replaced by new elements. 

Path and Sink Features. In the translocation path (e.g., stems and petioles), assimilates and solutes move in mass flow through the cylindrical sieve tubes which have open sieve pores. The ability of a chemical to leak across the membrane from the sieve tube during transit will affect its ability to be transported through the entire pathway. 

Christmann, J., Kreis, W. and Reinhard, E. (1993) Uptake, transport and storage of cardenolides in foxglove cardenolide sinks and occurrence of cardenolides in the sieve tubes of Digitalis lanata. Bot. Acta, 106, 419-27. 

This approach, while reasonable from the point of view of bringing about a unification of cellular and long-distance transport, can be further justified. Giaquinta has given an account of experimental findings in support of an H efflux mediated uptake of sucrose by the sieve tube-corn-... 

The transport system of the sieve tubes is also a multiplet of generalized transport units (pTU) with the sieve plate pores as organismal capillaries. Though the sieve tubes are thin as such, the dependence of the hydraulic conductivity, on the fourth power of the radius, makes the distinction between them and the pores essential. The effects of occlusion on gw are difficult to calculate but the general features of the sieve tube system permit one to surmise that it has been made to cope with adverse hydrostatic gradients of variable but often large magnitudes. 

As in the case of the xylem conduits, the sieve tubes communicate with the cells and tissues around them. Lateral transport systems, including the pathways for the exchange of substances between them and the xylem conduits, may be visualized as lateral branches of the long distance pathways (xTU and pTU). These offshoots (ITUs) link all regions of the cross section of the root and the shoot with the xTU and pTU. 

Jacobs WP, McCready CC (1967) Polar transport of growth regulators in pith and vascular tissues of Coleus stems. Am J Bot 54 1035-1040 Jacobs WP, Morrow IB (1957) A quantitative study of xylem development in the vegetative shoot apex of Coleus. Am J Bot 44 823-842 Jacobs WP, Morrow IB (1967) A quantitative study of sieve tube differentiation in vegetative shoot apices of Coleus. Am J Bot 54 425 31 Jacobs WP, Danielson J, Hurst V, Adams P (1959) What substance normally controls a given biological process II. The relation of auxin to apical dominance. Dev Biol 1 534-554... 

The mechanism of transport in the sieve tubes has still not been elucidated. Nonetheless most of the presently known findings argue in favor of the correctness of the mass flow hypothesis put forward by Munch in 1926. According to it, convection or mass flow is responsible for the transport in the sieve tubes just as for transport in the xylem or in the blood vessel system of animals. The driving force of this mass flow in the sieve tubes is a concentration gradient of osmotically active substances decreasing in the direction of transport. 

Up to now we have postulated what conditions must hold. Now we must point out something that may not happen if the mass flow hypothesis is to have validity, a bidirectional transport in one and the same sieve tube. 

Bidirectional transport in one and the same plant is completely compatible with the mass flow hypothesis and has also been demonstrated. Thus, assimilated materials from leaves can be conducted both upward into the shoot meristem and also downwards in the direction of the root. However, bidirectional transport in one and the same sieve tube would contradict the reality of the mass flow hypothesis. 

Transport in both the xylem and the phloem is a long distance transport. We have just mentioned that the sieve plates still raise problems for the mass flow hypothesis. According to some hypotheses transport over the short distance through the sieve plates is not accomplished by mass flow as in the sieve tubes but by an active transport. This active transport is over a short distance and its essential characteristic is that it requires the expenditure of energy. 

Fig. 226. Apparent two-directional transport in one sieve tube. A Vida faba plant was laid horizontal and fluorescein was supplied to a lower leaf and C " -urea to an upper. Aphids were applied between the two leaves and their honey dew (cf. Fig. 222) was collected in a collector rotating beneath them. Both substances could be detected in the honey dew of one aphid. Since one aphid punctures only one sieve tube this result at first suggested a two-directional transport in one sieve tube (from Eschrich 1967).

Fig. 227. Explanation of the result of the experiment shown in Fig. 226 without assuming a two-directional transport the materials escaped into neighboring sieve tubes via transverse connections and are there carried along by mass flow in the opposite direction (from Eschrich 1967).

Transport of water into sieve tube cells by osmosis... 

Relatively hydrophilic pesticides are especially interesting insofar as they can penetrate into sieve tubes and be submitted to phloem transport (organophosphorus insecticides, glyphosate...). 

The crux of the problem of the relationship of form to function in sieve tubes relates to the disputed nature of the interconnections between the sieve tube units via the sieve plates and in a recent review it was concluded that at least four or five possible arrangements could not be excluded by the structural evidence (Fig. 7.7 a, b, c, d). These structure models can be considered in relation to hypotheses of the mechanism of solute transport in the sieve tubes. 

This discussion has indicated that the companion cells of the phloem may be involved in supplying energy to the sieve tubes both for the initial transport of sucrose into and out of sieve tubes and for its transport within the tubes. They may also be involved in the polarised... 

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