Auxins functions

Dicamba (3,6-dichloro-2-methoxybenzoic acid) is primarily used as a postemergence broadleaf herbicide, which interferes with normal plant auxin function, subsequently causing uncontrolled growth and the inhibition of the phototropic and geotropic function. Cumulative response results in plant death. The success of auxinic analogues such as Dicamba and 2,4-dichloropheno-xyacetic acid in weed control has led to widespread manufacturing and use. Estimated U.S. production for Dicamba was 5 million kg in 1990 [391]. 

Microscale measurements of lAA levels in tissue sections through the vascular cambium of both Pinus and Populus [199,200] have revealed the existence of a steep gradient of lAA levels across the cambium. The existence of these gradients supports the idea that auxin functions in the plant as a positional signal which can specify developmental... 

Antiauxin action involves a competition with the active auxin on the primary active site. The auxin molecules functioning in a biological system will, however, not be in a dynamic equilibrium with these sites exclusively, but certainly may be interacting at sites of more secondary importance for the total function, or be adsorbed on sites which result in eliminating or enzymically destroying the auxin. With respect to the auxin function, in all of these cases we might speak of sites of loss . 

Other auxin-like herbicides (2,48) include the chlorobenzoic acids, eg, dicamba and chloramben, and miscellaneous compounds such as picloram, a substituted picolinic acid, and naptalam (see Table 1). Naptalam is not halogenated and is reported to function as an antiauxin, competitively blocking lAA action (199). TIBA is an antiauxin used in receptor site and other plant growth studies at the molecular level (201). Diclofop-methyl and diclofop are also potent, rapid inhibitors of auxin-stimulated response in monocots (93,94). Diclofop is reported to act as a proton ionophore, dissipating cell membrane potential and perturbing membrane functions. 

In addition to inhibitory chemicals which enter the plant from the external environment, many endogenous inhibitors appear to function as regulators of seed germination and plant growth and development. The interrelationships between endogenous inhibitors and growth promoters such as the auxins, gibberellins, and kinins remain to be elucidated. 

Fig. 12. Curvature responses of decapitated avena coleoptiles as a function of auxin concentration. (A) curvature obtained in 90min, auxin applied asymmetrically (standard auxin test). (B) curvature produced in 60 min by symmetrical application of auxin upon succeeding transverse electrical stimulation. Bending occurs towards the positive pole, as sketched187 ...

As before, the DOS and COOP functions calculated for Na2 Auxins [84] exhibit a pseudogap and empty Au-In and In-In bonding states above F (Fig. 14). The... 

Figure 8.4 Glucosinolate and IAA pathways. IAA indole-3-acetic acid IAN indole-3-acetonitrile IAOx indole-3-acetaldoxime. A loss of function of CYP83B drives the IAOx to IAA and an auxin-overproducing phenotype is the result.

Sections of non-photosynthetic oat-tissue fed with radioactive substrates (acetate and sucrose) incorporated the label into the soluble carbohydrates and lipides, with no apparent differences between auxin-treated segments and untreated controls. 7 The tissue was apparently grown with an inadequate substrate of carbohydrate, which may explain why indole-3-acetic acid at 1 mg. per liter increased the utilization of sucrose, lipides, and organic acids by pea-stem segments in other work. None of the available investigations on cellular carbohydrates have provided very much promising information concerning the nature of the metabolic functions directly affected by the auxins. 

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