Sieve-tube regeneration

Fig. 4.6. A portion of a cleared and stained phloem strip, showing regenerated sieve tubes cutting diagonally across the interfascicular tissues. The scale represents 0.1 mm. (LaMotte and Jacobs 1962)...

A major advance made since the 1965 edition of the volumes lA-16 on developmental physiology in the first series of this Encyclopedia was prepared has been the quantitative elucidation of hormonal control of sieve-tube differentiation during regeneration in stems. In addition, the earlier quantitative studies of tracheary differentiation have been extended and amplified. An unexpectedly close connection between sieve tubes and tracheary cells in their regeneration and normal differentiation has been revealed and a unifying picture of vascular differentiation in the shoot has resulted. A start has recently been made in unraveling the controlling factors for fiber differentiation. These advances will be described and discussed below. 

Table 4.1. Effects on sieve tube regeneration of lAA and sucrose substituted for distal shoot organs of Coleus blumei. (Data from LaMotte and Jacobs 1963)...

Fig. 4.8. The time-course of the regeneration of sieve tube cells around a slit wound in older internodes of Coleus, showing the effectiveness of lAA in replacing the leaves and shoots above the wound. (Graph and regression calculations from Jacobs 1979, other data from Thompson 1967)...

Fig. 4.9. The dose-response curve to lAA (added in lanolin to the apical end of excised older internodes of Coleus) of regenerating tracheary cells solid circles) and sieve tube strands open circles). Asterisks mark the interpolated values from intact controls (as in Fig. 4.4) (Thompson and Jacobs 1966). Note that each sieve tube strand contains an average of ten cells (LaMotte and Jacobs 1962) hence, the two curves directly reflect the number of cells that regenerated...

In other experiments, internodes were cut free of the entire rest of the plant and regeneration tested in these isolated stem pieces. lAA added to the apical end of such an excised older internode increased sieve-tube regeneration over the controls (LaMotte and Jacobs 1963, Thompson and Jacobs 1966). The dose-response curve of apically applied lAA, as it affected both tracheary and sieve cells, is shown in Fig. 4.9 with no externally added lAA, a small number of sieve-tube cells but no tracheary cells regenerated as the lAA level was increased, more vascular cells of both types regenerated (Thompson and Jacobs... 

Plants intact except for the transverse wound regenerated numbers of vascular cells equivalent to those formed by about 0.05% lAA added to the isolated internode (asterisks in Fig. 4.9). Hence, 0.05% lAA in lanolin exactly replaced the root system and the rest of the shoot system in their effects on the regeneration of both sieve tubes and tracheary cells. At any one lAA level, more sieve cells than tracheary cells regenerated. 

There are only a few other papers on hormonal effects on sieve-tube regeneration aside from those on cultured material. Benayoun et al. (1975) reported that lAA could replace distal leaves of Cucumis or Coleus in causing sieve-tube regeneration but provided no data on the point. 

The smaller, xylem-less strands, whose sieve-tube regeneration was studied by Houck and LaMotte (1977), increased sieve-tube regeneration significantly when L-glutamic acid was added basally to excised internodes treated apically with lAA, but various other chemicals (L-proline, kinetin, GA3, sucrose, and L-glutamine) were without effect. 

In his elegant study of hormonal control of the normal differentiation of phloem fibers (described below), Aloni (1976) pointed out that the fibers did not differentiate from the parenchymatous cells of the wound callus, even though sieve tubes and tracheary cells did. The differentiation of phloem fibers was always limited to the longitudinal vascular strands, never being found in the intervening parenchyma. In that sense, one could say that they do not regenerate however, their regeneration has been seldom studied. 

Aloni R, Jacobs WP (1977 b) The time course of sieve tube and vessel regeneration and their relation to phloem anastomoses in mature internodes of Coleus. Am J Bot 64 615-621... 

Jacobs WP (1970) Regeneration and differentiation of sieve-tube elements. Int Rev Cytol 28 239-273... 

Thompson NP (1965) The influence of auxin on regeneration of xylem and sieve tubes around a stem wound. PhD Thesis, Princeton Univ. (No. 66-4017 from Univ Microfilms, Ann Arbor, Mich... 

Thompson NP, Jacobs WP (1966) Polarity of lAA effect on sieve-tube and xylem regeneration in Coleus and tomato stems. Plant Physiol 41 673-682 Torrey JG (1953) The effect of certain metabolic inhibitors on vascular tissue differentiation in isolated pea roots. Am J Bot 40 525-533 Torrey JG, Loomis RS (1967) Auxin-cytokinin control of secondary vascular tissue formation in isolated roots of Raphanus. Am J Bot 54 1098-1106 Torrey JG, Fosket DE, Hepler PK (1971) Xylem formation A paradigm of cyto-differen-tiation in higher plants. Am Sci 59 338-352... 

It seems that fluid-bed cracking reactor (thermal or catalytic) is the best solution for industrial scale. However, regeneration and circulation of so-called equilibrium cracking catalyst is possible for relatively pure feeds, for instance crude oil derived from vacuum gas oils. Municipal waste plastics contain different mineral impurities, trace of products and additives that can quickly deactivate the catalyst. In many cases regeneration of catalyst can be impossible. Therefore in waste plastics cracking cheap, disposable catalysts should be preferably applied. Expensive and sophisticated zeolite and other molecular sieves or noble-metal-based catalysts will find presumably limited application in this kind of process. The other solution is thermal process, with inert fluidization agent and a coke removal section or multi-tube reactor with internal mixers for smaller plants. 

Special care was required to eliminate a number of undesired effects uncovered in preliminary experiments. These included sample contamination from impurities in the ethane, which was eliminated by placing a small amount (about 200 mg) of 4A sieve in the base of the sample tube. This material, which was regenerated automatically each time along with... 

In addition, the water content of the regeneration gas entering the molectdar sieve adsorber is analyzed. Due to this dew point analysis the leak-proofness of the pressurized tubes of the regeneration gas heat exchanger conducting superheated steam is guaranteed. In normal operation, a dew point of-70 °C is indicated. 

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