PHYSIOL. PLANT. 54: 225-229. Copenhagen 1982 Physiological and biochemical changes associated with cotton fibre development , I. Growth kinetics and anxin content S. C. Naithani, N. Rama Rao and Y. D. Singh Naithani, S. C , Rama Rao, N. and Singh, Y. D. 1982. Physiological and biochemieal changes associated with eotton fibre development. 1. Growth kinetics and auxin content. - Physiol. Plant. 54: 225-229. Growth kinetics and levels of auxin substances were studied in three eotton eultivars, designated as long, medium and short staple cultivars. Fibre length and dry weight plotted against boll age showed sigmoidal patterns and were fitted to a logistic curve by computer curvilinear regression analysis. The final length of the fibre in different eultivars was the product of the rate of elongation per day and the total period of elongation. Further, considerable overlap between the elongation and the secondary thickening phases was reeorded. No relationship between auxin substanees and rate of fibre elongation was discerni- ble. The peak levels of auxin substances in all the cultivars were recorded before or about the time when elongation had just started, and it is concluded that the auxin synthesized during the elongation phase is consumed in elongation growth. Thus there is neeessarily no relationship between remaining auxin and growth. Additional key-words: Auxin substances, elongation phase, secondary thickening phase, Gossypium liirsutum, G. herbaceum, S, C, Naithani, N, Rama Rao and Y. D, Singh {reprints). Department of Biosciences, Saurashlrii University, Rajkot 360 005, India, Introduction Morphologically the cotton fibre develops as an exten- sion of an epidermal cell of the ovule. The fibre initia- tion starts a day before up to a day or two after anthesis, and the initials enter into elongation phase immediately. Growth studies on cotton fibres (Hawkins and Serviss 1930, O'Kelley and Carr 1953, Jasdanwala et al. 1977, Rama Rao et al. 1980) have revealed that fibre growth involves two distinct phases, i.e. the elongation is fol- lowed by the secondary thickening phase. Further, it has been shown (Hawkins and Serviss 1930, O'Kelley and Carr 1953, Kerr 1966) that the secondary thickening phase does not begin unless the elongation phase is completed. Contrary to this, in more recent publications (Schubert et al. 1973, Jasdanwala et al. 1977, 1980, Rama Rao et al. 1980) a considerable overlap between the elongation and the secondary thickening phases has been reported. However, the mechanisms regulating the termination of elongation and the initiation of second- ary thickening are still unknown. On the basis of in vitro culture studies of cotton ovules, Beasley and Ting (1973) concluded that the fer- tilized ovules are (i) deficient in their capacity to syn- thesize optimum levels of gibberellins, (ii) sufficient in their production of cytokinins and (iii) mere optimum in production of auxin (IAA). The unfertilized ovules, on the other hand, require addition of IAA to the basal medium in order to produce fibre (Beasley and Ting 1974), and it is suggested that auxin is the major hor- mone produced in response to the process of fertiliza- tion, whereas gibberellins probably derived, for the most part, from sources external to the ovule. Further, Birnbaum et al. (1974) concluded that IAA, whether synthesized endogenously in fertilized ovules or added to the growth medium of unfertilized ovules, may be of Reeeived 5 May, 1981; revised 10 September, 1981 Physiol. Plant. 54, t982 0031-9317/82/020225-05 $03.00/0 © 1982 Physiologia Plantarum 225