The other characteristics of auxin transport are that it is metabolically dependent and moves basipo-
larly with a velocity generally ranging from 10 to 20 mm/hr, depending on the plant species tested. In
short-term (4 hr or so) experiments, it moved unchanged with similar velocity in intact and isolated seg-
ments. It could also move laterally following tropic stimulation [27,31,32].
In intact plants, auxin moves in two distinctly different systems. From the apex it moves toward
the roots, and movement from the young leaves and meristematic regions of shoots resembles polar
transport. This transport requires living cells and is interferred with by inhibitors such as triiodobenzoic
acid (TIBA) and abscisic acid (ABA), anoxia, and low temperature. Auxin was not usually translocated
through the epidermis, cortex, pith, or vascular bundles but instead through parenchyma cells in con-
tact with vascular bundles [33]. Cell-to-cell auxin transport across the tissues is now considered by
some investigators to be chemiosmotically coupled to the electrochemical potential of auxin and pro-
ton [7]. The second mode of transport is along with the assimilates exported from the leaves. This
movement lacks polarity and the auxin can move in any direction with a velocity of 100 to 240 mm /hr,
depending on the location of the metabolic sink and the water status of plants. This indicates that auxin
supplied to or from the mature leaves enters the sieve tubes and is transported rapidly with assimilates.
The physiological importance of this system for delivering endogenous auxin over long distances has
not been investigated.
D. Biological Activity
Biological activities of the applied auxin are so diverse that compiling a complete list is quite difficult. A
number of responses at the molecular, cellular, organ, and whole plant levels have been described which
are known to be influenced by the exogenous application of IAA. But to what extent these are under the
control of endogenous IAA has not been established unequivocally. However, there are a few examples,
such as control of the elongation of stamen filament of Gaillardia grandifloraand the photoinhibition of
mesocotyl growth, that correlate well with the endogenous IAA levels [32].
Auxin response is related to concentration, which is normally extremely low. In plants, free IAA
is on the order of 10^8 g/kg fresh weight. The endogenous level of auxin is important in determining
the course of development [34] (Figure 2). Changing concentration can convert root meristem to shoot
meristem and vice versa [35]. A high concentration is inhibitory, while a low concentration is stimula-
tory, and both are important. Commonly, the highest concentration of auxin is found in the meristem-
atic regions.
PLANT GROWTH HORMONES 505
Figure 2 Growth responses of plant organs to various concentrations of auxin. (From Ref. 34.)