Flexibility of the Transport System

3 Transport and Other Modes of Movement of Hormones (Mainly Auxins) 

Lateral redistribution of auxin in epinastic main and side shoots of Coleus and petioles of different species as postulated by Lyon (1963 a, b, 1965 a, b) would be the equivalent of a supplementary auxin source for the receiving half of the axis and it may cause an increase in the transport intensity in that half However, this is not a suitable explanation in the case of Fritillaria, since regions of increasing transport intensity do not coincide in upper and lower halves, in spite of the fact that overall intensity does increase basipe-tally initially. Thus an alternative explanation for the intensity alterations might be that an exchange may exist between the mobile and immobile auxin fractions, and that this may take place between the cytoplasm and the vacuoles. 

The examples mentioned so far point to considerable flexibility of the auxin transport system. This view is also supported by the observation of oscillations of electric potential moving down Avena coleoptiles after illumination or after the supply of auxin (Newman 1959, 1963), by the report of fluctuations of lAA movement in segments of oat coleoptiles after blue light illumination (Thornton and Thimann 1967), and in individual plant parts, by the demonstration of even more pronounced oscillations of the export rate of radiocarbon from auxin-depleted segments of oat and corn coleoptiles, supplied with labeled lAA (Hertel and Flory 1968). It is further supported by previously mentioned (see Sect. 3.3.3.4) experiments of Shen-Miller (1973a), where rhythmic fluctuations of the lAA transport intensity in intact coleoptiles of oat and corn were observed, moreover the rhythmicity was out of phase between the upper and lower halves of geostimulated coleoptiles (Shen-Miller 1973b, p 169). 

In summary, then, the concept of a continuous stream of auxin molecules moving at a uniform and a constant velocity and density has been based mainly on, and derived, from studies of the time course of auxin transport through sections which were similar to the classic experiments of van der Weij (1932, 1934). It is now clear that this concept has not only constrained the thinking about auxin transport for the best part of half a century (Goldsmith 1977, p 454), but that it has no basis in reality. It is unfortunate that there is as yet no generally accepted substitute for this too inflexible picture of the auxin transport system. 

Any model of the mode of action for a transport system of a polarly transported hormone must account for the preferential direction of movement. For auxins, the direction of movement is predominantly basipetally in shoots and acrope-tally, i.e., toward the tip, in roots. Basipetal auxin transport has been shown to be established, to a certain degree, in hepatics (Maravolo 1976) and in embryonic axes of vascular plants (Greenwood and Goldsmith 1970, Fry and Wangermann 1976). This polarity develops gradually up to a maximum with increasing age of the plant (e.g., Jacobs 1950, Smith and Jacobs 1968, 1969), and then declines, either because of a decrease in basipetal transport 

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As illustrated in Figure 16.52, the audio subsystem comprises the audio encoding/decoding function and resides between the audio inputs/outputs and the transport subsystem. The audio encoder(s) is (are) responsible for generating the audio elementary stream (s), which are encoded representations of the baseband audio input signals. The flexibility of the transport system allows multiple audio elementary... 

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