The challenge of quantitative determination of a chemical present in such minute concentrations (10-7 or 10-8 M) was first met by Went, using the Avena curvature test. A source of unknown concentration placed asymmetrically on an oat, maize, or wheat coleoptile stump (Weaver 1972) resulted in differential growth and curvature proportional to the concentration. The angle of the new growth in relation to normal is an index of the concentration.
The Avena coleoptile straight growth test is another bioassay, also based on cell expansion. This involves determining growth response in terms of the increase in length of young, etiolated, decapitated shoot segments in a solution of the test growth substance. Chromatography has added a new dimension by providing a method of effective separation of hormones and analogs. Light and mass spectroscopy is effective tools for identification and quantification by chemical methods.
Responses to auxins range from influences on cellular metabolism to coordination of plant morphogenesis, including abscission and senescence. Cellular effects included,
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(1). Increases in the nucleotides DNA and RNA, and protein and enzyme synthesis
(2). Increases in proton exchange, membrane charge, and potassium uptake
(3). Influences on the phytochrome reaction with red and far-red light
Auxin response is related to concentration. A high concentration is inhibitory, which has been explained as competition for attachment on receptor sites, that is, increasing the concentration increases the probability of partially attached molecules occupying receptor sites, rendering the complex less effective. Also, responses vary greatly, depending on sensitivity of plant organ. Stems respond to a wide range of auxin concentrations. Roots are essentially inhibited over most of the hormone range.
Until recently geotropic and phototropic responses have been explained, respectively, by asymmetric, gravity-induced shoot levels due to redistribution of auxin and asymmetric levels due to light destruction of auxin on the lighted side (Audus 1972). In geotropic or gravitropic responses, auxin moves to the cells on the lower side of a horizontal organ, stimulating cell elongation and curvature asymmetrically; this is known as the classic Cholodny-Went theory. It is theorized that movement of auxin to the lower side of a root inhibits growth on that side, with resultant downward curvature.
A number of physiologists have raised doubts as to the validity of this theory. It has been suggested that the root cap rather than the growing point is the gravity-sensing tissue, and movement of abscisic acid (inhibitor) acropetally and to the lower side may explain the tropic root response. Likewise the Cholodny-Went theory has been questioned because of observations that suggest that ethylene diffusing upward and inhibiting the upper part of a horizontally placed stem is the cause of upward bending. IAA seems to move too slowly to initiate geotropism and may be only incidentally associated with it, rather than the causative factor.
Whatever the causative factor, one-sided expansion of the stem or root was associated with cell wall extensibility, which appears to result from loosening of the polysaccharide matrix (Masuda 1977). Auxins bind to the plasma- lemma, particularly to lecithin, inducing increased respiration and potassium uptake. These effects may explain the plastic expansion of the cell wall by the deposit of additional polysaccharides in the loosened matrix.
Auxins were necessary for callus growth, whether in tissue culture or in gall and nodule tissues. Auxin was believed to induce curl of root hairs, a prerequisite for Rhizobium infection (Allen 1973). Auxins coordinate plant processes in morphogenesis.
For example, lateral bud and root growth are inhibited by auxins, but new root initials are promoted on callus tissue formed on cuttings. On hard-to-root species or cultivars, an exogenous source of auxin was nearly always essential (Hart and Carlson 1967). Callus tissue forms first on the cutting, and roots are differentiated from the callus. Cuttings of numerous species rooted readily only if active bud or young leaf tissue was left on the cutting (often referred to as the leaf factor) (Weaver 1972).
Auxins delay leaf and fruit abscission. They induced parthenocarpy (seed- lessness) in fruits; for example, strawberry fruits grow without seeds if treated with naphthaleneacetic acid (NAA). Normally the presence of seeds or an exogenous source of auxin is necessary for fruit growth.
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Excessive concentrations of auxins cause abnormalities, such as epinasty (malformation of leaf due to differential growth of upper and lower leaf midvein), onion leaf, fused brace roots, and brittle grass stalks. Even vapors from a distant source can cause epinasty in sensitive species like tomato or grape. Supraoptimal concentration of auxins may kill certain species and not affect others; thus auxins are used as selective herbicides. The reasons for such a high degree of selectivity have not been completely resolved.
AGRICULTURAL USES OF AUXINS
Some of the most valuable and widely used selective herbicides in the weed control arsenal are auxins, particularly the phenoxyacetic acid analogs (e.g. 2,4-D, 2,4,5-T, and MCPA). One of the earliest selective herbicides, 2,4-D probably still ranks as the single most important one. It is highly selective, noncorrosive, effective at low concentrations, safe to handle, relatively easy to formulate, and economical to use. Several benzoic acid analogs (e.g. dicamba, chloramben, and substituted picolinic acid, picloram) are also important herbicides.
Auxins have other important commercial uses, as reviewed extensively by Weaver (1972). Using the principle of inhibition of abscission layer formation, auxins (e.g., NAA or 2,4-D) are effective in prevention of fruit drop in apple and pear. Auxins, including 2,4-D, induced ethylene formation and fruit set in pineapple.
Biennial bearing (light and heavy crops in alternate years) is common to many tree crops. This problem can be corrected by thinning in the heavy years by a timely application of NAA or other auxins (Luckwill 1976).
Commercial preparations of rooting compounds are available that promote callus and root formation, which can improve establishment from cuttings. Species and cultivars difficult to root are enhanced by dipping the cut surface into a rooting compound. Commercial nurserymen also recognize the importance of selecting cuttings with some active bud development to supply endogenous auxin. Auxins were also effective in the prevention of sprouting of stored potatoes (Mitchell and Marth 1947). The potato may be dipped in the auxin solution (such as NAA), sprinkled with talc or fuller's earth containing an auxin, or stored with strips of paper impregnated with an auxin solution. Newer and more effective PGRs are now available for this purpose.