Intercept Method of van der Weij

In early auxin transport studies, the straight line was drawn by eye and the time intercept determined from graphical extrapolation. This subjective method has occasionally led to misinterpretations. For instance, from data thus treated, van der Weij (1932) concluded that the transport velocity remained constant in varying temperatures, and that the observed increase in transport intensity was due to an increase in transport density. However, statistical treat- 

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

The transport intensity corresponds to the slope of the straight line, thus 

The intercept of the straight line with the x-axis corresponds to the time intercept, called time delay (t ) by Kaldewey (1968a), and allows to calculate the transport velocity by 

Since the x-intercept results from setting Eq. (3) equal to zero 

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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. 

By use of the intercept method of van der Weij (1932) the transport velocities in sunflower hypocotyl sections (Fig. 3.4), as estimated from the intersections with the time axis of the extrapolated linear regression lines, amount to 5.9 mm h" and 3.7 mm h at the higher and lower donor concentrations, respectively. Similar estimated relationships between donor concentration and transport velocity estimations have been found for basipetal lAA transport in Coleus stems (Naqvi 1963), Zea coleoptiles (Naqvi and Gordon 1964, Naqvi 1976), and Avena coleoptiles (Newman 1963, 1970) and may be deduced from the shape of the arrival curves derived mathematically by Newman (1974) from his experimental data. On the other hand, velocities estimated by the intercept method have been reported not to be significantly different at different auxin concentrations in the donor, though the calculated values tended to be higher at increased donor concentration (e.g., van der Weij 1932 in Avena coleoptiles ... 

Van der Weij introduced the intercept method (see Sect. 3.3.2) and calculated the velocity (Transportgeschwindigkeit) of the auxin stream and its intensity (Transportintensitat), i.e., the amount of auxin passing the cross-section of... 

Short-term application of auxin to the apical cut surface of coleoptile sections, combined with an estimation of auxin accumulation with time in basal receivers which were replaced at brief intervals, was demonstrated by van der Weij (1932, p442ff) to be a means of calculating transport velocity. He observed that the auxin export rate (i.e., the transport intensity) increased initially to a maximum and then decreased. He assumed that the arrival time of the peak of transport intensity was the period of time needed by the auxin stream to traverse the segment. The velocity thus estimated (8mmh" ) was similar to the values of about 10mmh obtained with the intercept method. When labeled hormones became available, such pulse experiments were refined and modified. The duration of the pulse application could be reduced to 60 s (Shen-Miller 1973 a, b) and the receivers could be changed with great frequency to improve the estimation of the peak. 

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