Plant Physiology Prof.(Dr.) Punam Jeswal Head B.Sc (Hons.) Part III Botany Department AUXIN Auxin Translocation Went (1928) reported that auxin is transported basipetally, i.e. it moves from apical to basal end. The movement is quite fast, about 1 to 1.5 cm/h ( in roots 0.1 to 0.2 cm/h). McCready and Jacobs (1963) working on petiole segments of Phaseolus vulgaris observed acropetal movement of auxin. Acropetal movement of auxin consumes metabolic energy whereas basipetal movement is purely a physical process which goes on even under anaerobic condition. Recent studies have indicated that auxin transportation in plant is an overall active transport system. Some points in favour of it may be summarized as fellows :- (i) The velocity of auxin transport (1 to 1.5 cm/h) in stems and coleoptiles is about ten times faster than diffusion. (ii) Metabolic inhibitors are less effective on auxin transportation as compared to poisons. 2,3,5- triiodobenzoic acid and naphthylthalamic acid are highly effective. (iii) Auxin transportation depends upon aerobic metabolism. (iv) Auxins are able to move against a concentration gradient. (v) It has been observed that there is good correlation between structural requirements for auxin activity and the auxin transport. (vi) The saturation effect of transport site being in between 0.1 and 1 µM per tissue volume also supports it. Collectively these findings confirm that the polar movement of auxin is an active transport. In general, transport of auxin is effected by temperature, oxygen, gravity, age, applied chemicals etc. Auxin is transported through the living cells including phloem parenchyma and parenchyma cells that surrounds the vascular bundle. The transport is inhibited by antiauxins, e.g. 2,3,5- triiodobenzoic acid (TIBA), naphthylthalamic acid (NTA) and ethylene chlorohydrin. DCA (dichloroanisole) is anti-auxin effective against 2,4-D. Fig :- The Standard method for measuring polar auxin transport. The polarity of transport is independent of orientation with respect to gravity. Physiological Responses of Auxins ( Role of Auxins) Some important roles of auxin are as fellow :- 1) Apical Dominance. 2) Cell Division and Cell Enlargement. 3) Shoot and Root Growth. 4) Xylem Differentiation. 5) Nucleic Acid Activities. 6) Manifold Activities. Apical Dominance :- Thimann and Skoog (1933) found that on removal of apical buds, the lateral buds begin to sprout and a large number of lateral buds are formed. They concluded that auxin inhibits lateral bud formation and since auxins are synthesised in apex, they referred this phenomenon as apical bud dominance. This fact was proved in almost all cases. They further observed that with the application of ethylene chlorohydrin (this substances destroys auxins) on potato tuber, both apical as well as lateral buds sprouted. In normal cases, only apical bud sprouts. Fig :- Auxin suppresses the growth of axillary buds in bean. (A) The axillary buds are suppressed in the intact plant because of apical dominance. (B) Removal of the terminal bud releases the axillary buds from apical dominance. (C) Applying IAA to cut surface prevents the outgrowth of the axillary buds Cell Division and Cell Enlargement :- Auxin stimulates cell divisions, cell enlargement and cell elongation in the apical region. The primary physiological effects of auxin is to promote the elongation of cells. Role of auxin in cell elongation has been explained by Strafford (1963, 73). Auxin stimulates cell enlargement by increases the (i) amylases activity, (ii) permeability, (iii) formation of energy rich phosphates (ATP) and (iv) cell wall plasticity while decreases viscosity and wall pressure. According to acid-growth hypothesis, auxins by lowering pH, cause loosening of cell wall and thereby growth. Shoot and Root Growth :- Auxin promotes growth of shoots at a relatively higher concentration and that of root at a very low concentration. Xylem Differentiation :- The phenomenon of auxin promoting xylem element differentiation is well established and is now taken as a measure of auxin bioassays. It is because of this property, auxins have been commonly used in bud grafting into a callus. Callus is a mass of undifferentiated parenchymatous cells and when treated with auxin, xylem elements get differentiated into the cells tissue. Auxin thus helps established contact between vascular tissue of the callus and that of bud and makes it possible for the bud to grow properly in callus. By adding auxin and sugar, continued growth of callus may be obtained and new shoots and even new plants can be produced from callus. Fig :- IAA induced xylem regeneration around the wound in cucumber Nucleic Acid Activities :- Auxin promotes activities in growth. Satterfield (1963) reported that IAA increases total RNA synthesis particularly a messenger type of RNA and the synthesis of specific enzymes. It is just believed that role of auxin is to release DNA template from nucleohistone for mRNA synthesis. By synthesis of specific enzymes, the insertion of new material into cell wall occurs which causes cell wall extension. Manifold Activities :- Auxin affect a life of a plant in manifold ways. Auxins also play specific roles in seed germination, growth, rooting, flowering (reproductive process), abscission, parthenocarpy, tissue culture etc. Sircar (1971) wrote a monograph entitled " Plant Hormone Researches in India". While discussing the different aspects on mechanism of auxin action, he proposed that auxin is the key substance of a plant life. .