Loading of the Transport System

Sheldrake (1973 a) demonstrated by separation of the stem tissues of Nico-tiana internodes that the great majority of the strongly basipetal auxin transport took place in cells of the internal phloem and in cells close to the cambium. Very small amounts were transported in bark and pith preparations and none in xylem tissues. Using the intercept method of van der Weij (1932), he estimated velocities of about 5 mm h for the transport of l- C-IAA in complete stem sections, in inner tissue segments containing the internal phloem, and in xylem + cambium + bark section. The transport densities were similar to each other in the two latter preparations. Lower transport densities were found in the bark and in exclusively pith sections having transport velocities of 3.8 and 3.1 mm h S respectively. In all cases, however, a small amount of radioactivity was found in basal receivers considerably in advance of the time intercepts calculated from the linear parts of the auxin arrival curves (Fig. 3.3). Thus, even in homogenous tissues, auxin molecules appear to move at different velocities. 

As mentioned above, differing velocities of hormone molecules will lead to an initial gradual increase in the slope in the arrival curves. This would also occur if there were a gradual generation of transport sites, as has been suggested by DE LA Fuente and Leopold (1970a). In both cases, the time required for linearity of the arrival curves should be independent of the hormone concentra- 

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

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Van der Weij s (1932) comparison of the transport system with a unidirection-ally moving conveyor belt loaded with different amounts of auxin molecules was a first attempt to conceptualize the basipetal auxin transport mechanism. It is noteworthy that this rather mechanistic picture, which has found little experimental basis, has had something of a revival by assuming plasmalemma-somes to be possible vehicles of auxin transport (Wangermann and Withers 1978 see Fig. 3.7). 

The central transport chamber is an 80-cm-diameter stainless steel vessel, and is pumped by a 1000-1/s turbomolecular pump, which is backed by a small (501/s) turbomolecular pump to increase the compression ratio for hydrogen, and by a 16-m /h rotating-vane pump. UHV is obtained after a bake-out at temperatures above 100°C (measured with thermocouples at the outside surface) of the whole system for about a week. A pressure in the low 10 " -mbar range is then obtained. With a residual gas analyzer (quadrupole mass spectrometer, QMS) the partial pressures of various gases can be measured. During use of the system, the pressure in the central chamber is in the low 10 -mbar range due to loading of samples. Water vapor then is the most abundant species in the chamber. 

In order to achieve a complete transportation system a host of other subsystems support the transportation system operations. Loading facilities, pumping and compressor stations, tank farms and metering and control devices are necessary for a complete transportation system of liquid or gases hydrocarbon commodities. 

Desolvation systems can provide three potential advantages for ICP-MS higher analyte transport efficiencies, reduced molecular oxide ion signals, and reduced solvent loading of the plasma. Two different approaches have been used for desolvation in ICP-MS. The heated spray chamber/condenser combination has been discussed it is the most commonly used system. The extent of evaporation of the solvent from the aerosol and cooling to reduce vapor loading varies from system to system. The second approach is the use of a membrane separator to remove solvent vapor before it enters the ICP. 

Vascular Anatomy. One aspect of the translocation system that is often overlooked is the influence of the structural features of the plant s vascular system on solute transport. For example, although much of what is known about phloem loading has been derived from a few dicotyledon leaves, all dicotyledon leaves are not similar. A notable example is the soybean leaf. Soybean leaves are specialized in that they have a unique cell type called the paraveinal mesophyll... 

The physical factors (precipitation, soil texture and depth) regulate the flow of water as the transport media for nitrate and the contact time of the root system with inorganic nitrogen species. It is clear that the atmospheric loading is important. 

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