Polar secretion theory

Leopold 1963, Christie and Leopold 1965a, b. Winter 1968, Vardar 1970, Kaldewey and Kraus 1972, Morris et al. 1973, Gagianas and Berg 1977, Kaldewey et al. 1977, Wangermann and Withers 1978) which thus lead to an intracellular accumulation of auxin (Hertel and Leopold 1963, Christie and Leopold 1965 a, b, Hertel and Flory 1968, Winter 1968, Beyer 1972, Bopp 1972, Davies 1973, 1974, Cande and Ray 1976, see also Hertel 1983). 

The results of such studies are consistent with the concept that the polarity of auxin transport is imposed by the active secretion of auxin molecules out of the cytoplasm through the membrane at the base of the cell and that the molecules then diffuse passively into the adjacent cell below. This model, first developed by Hertel (1962), Hertel and Leopold (1962, 1963), Leopold (1963), assumed that secretion is mediated by metabolically driven carriers which is a feature consistent with the facts that auxin transport depends upon an adequate oxygen supply (e.g., Gregory and Hancock 1955, Brown and Wet- 

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

MORE 1959, Naqvi etal. 1965, Goldsmith 1966a, b, 1967a, b 1968a, Wilkins and Martin 1967, Wilkins and Shaw 1967, Wilkins and Scott 1968, Wilkins and Whyte 1968) and is inhibited by metabolic poisons (e.g., van der Weij 1934, DU Buy and Olson 1940, Niedergang-Kamien and Leopold 1957, Reiff and V. Guttenberg 1961, Hertel 1962, Hertel and Leopold 1963, Christie and Leopold 1965 b, Wilkins and Martin 1967, McCready 1968 a, b, Wilkins and Whyte 1968, Goldsmith and Ray 1973, Cande and Ray 1976, da Cruz and Audus 1978). 

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A principal difference between the chemiosmotic and the secretion theory is the involvement of metabolic energy in the process of the polar transport. According to the secretion theory, energy is directly coupled to carriers, in those cases, where auxin transport appears to be uphill whereas the new theory postulates that auxin transport is thermodynamically downhill, but with metabolic energy expended to maintain the pH and electrical gradients in addition to maintaining an ordered membrane with asymmetric permeability to lAA" (Goldsmith 1977, pp 446 and 491). 

This new theory has been extensively discussed in the review of Goldsmith in 1977 (see also Rubery 1980), and her introductory sentences (p 441) outline the basic concept. Cells, being more permeable to undissociated auxin molecules than to auxin anions, can accumulate auxin when the pH of the cytoplasm is above that of the walls... Carriers may be but are not necessarily involved in passage across the cell membrane... Unlike the conventional theory of polar secretion, which presumes active transport of auxin with direct coupling of energy to carriers in those cases when auxin transport appears to be uphill. 

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