Geotropic responses

In a recent review (16) of its physiological functions, evidence was presented that ABA plays a role in the geotropic response of roots and in tuber formation. It may have a role in other growth and developmental processes as well, but so far the evidence is inconclusive. 

Inhibition of protein synthesis and root growth. Inhibition geotropic responses. Root cell disorganisation and multinucleation. 

Auxin-like responses, exudates, and initiation of adventitious roots affected. Cell elongation and division promoted or inhibited. Geotropic response lost. 

Suzuki, T. and Fujii, T., Spectral dependence of the Hght-induced geotropic response in Zea roots, Planta, 142, 275, 1978. 

Fig. 15. Specific chemical inhibition of physiological bluelight responses 04) Swimming, inverse phobic response and direct photophobic response of Euglena as a function of iodide concentration118. (B) Phototropic and geotropic curvature of the avena coleoptile as function of azide concentration1S41...

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]. 

Geotropism is the response of protoplasm to the stimulus of gravity. Roots of Pteridophytes and seed plants invariably grow downward toward the center of gravity and so are positively geotropic. The... 

Etiolated maize seedling mesocotyls elicited two responses to higher concentrations of brassinolide (10 1-10° ppm). The first response was transverse geotropism and the second was twining (Figure 3). The seedlings in light culture treated with brassinolide at 10° ppm had the same... 

Transverse geotropism in stems is a characteristic response to ethylene. It has been reported that treatments with epibrassinolide at concentrations of 1-10 ppm accelerated ethylene production in etiolated mung bean hypocotyl segments by increasing ACC (1-aminocyclopropane-l-carboxylic acid) (7). Thus it is possible that the transverse geotropism and even the twining growth response evoked by brassinolide at 10 1-10° ppm were related to the production of ethylene. 

Auxins are responsible for bending toward a light source (phototropism), downward root growth in response to gravity (geotropism), promotion of apical dominance (the tendency of an apical bud to produce hormones that suppress growth of the buds below it on the stem), flower formation, fruit set and growth, formation of adventitious roots. 

Geotropism is the name given to the familiar response in plants in which roots grow down and shoots grow up. Were it not so, pity the poor farmer or gardener who would have to make sure that his seeds were always planted in the soil "right side up. A familiar explanation for roots growing... 

Dayanadan P, Hebard FV, Kaufman PB (1976) Cell elongation in the grass pulvinus in response to geotropic stimulation and auxin application. Planta 131 245-252 Dayanadan P, Hebard FV, Kaufman PB (1977) Role of indole-3-acetic acid and gibberel-... 

Gillespie B, Thimann KV (1963) Transport and distribution of auxin during tropistic response. I. The lateral migration of auxin in geotropism. Plant Physiol 38 214-225 Goldsmith MHM (1959) Characteristics of the translocation of indoleacetic acid in the coleoptile of Avena. PhD Thesis, Radcliffe Coll, Cambridge, Mass Goldsmith MHM (1966 a) Movement of indoleacetic acid in coleoptiles of Avena sativa L. II. Suspension of polarity by total inhibition of the basipetal transport. Plant Physiol 41 15-27... 

---