Journal of Agricultural Science (2002), 138, 255-260. @ 2002 Cambridge University Press DOl: 1O.1017jS0021859602001934 Printed in the United Kingdom 255 " Role of gibberellic acid in cotton fibre development S. J. GOKANI AND V. S. THAKER * .' Department of Biosciences, Saurashtra University, Rajkot 360005, India (Revised MS received 22 August 2001) SUMMARY Fibres of three cotton cultivars (Gossypium hirsutum H-4, H-8 and G. arboreum G. Cot-IS) were analysed for growth in terms of fibre length and dry weight and endogenous gibberellic acid (GA3) content thrice during 1997-2000, at Rajkot. The development of cotton fibre was divided into four distinct growth phases but overlap between elongation and secondary thickening was considerable which suggests that both these phases are independent of each other. During fibre elongation, GA3 content remained low and increased after a decrease in the rate of fibre elongation in all three genotypes. The long staple cultivar (H-4) showed highest endogenous GA3 content followed by the middle one (H-8) and the short staple cultivar (G. Cot-IS). In in vitro studies when GA3, NAA or GA3 + NAA was supplemented to the media, increase in fibre length of the short staple cultivar was maximum,followedby the middle one and the long staple cultivar. Both in vivo and in vitrofindings suggest that GA3 is one of the important factors that determine fibre length. INTRODUCTION , Gibberellins have numerous physiological roles in developing plants that include seed germination, starch metabolism (Derkx et at. 1994), cell elongation (Evans et at. 1994) and induction of flowering in photoperiodic plants (Pharis & King 1985). Gib- berellic acid (GA3) can reverse the inhibition of stem elongation caused by light in many plant species (Vince 1967; Silk & Jones 1975; Thomas et at. 1980). However, stem sections are composed of a number of cell types, which respond differently to exogenous gibberellins. Many studies have been conducted to examine the effect of GA3 in a relatively simple biological system consisting of one cell type but the results have been conflicting. For example, Fry & Street (1980) attempted to define conditions, which would promote gibberellin-induced cell elongation in suspension cultures. Application of exogenous GA3 showed expansion in Rosa cells and Spinacia cells, while cultured. cells of Daucus, Nicotiana, Vinca and Catharanthus were not responsive to it (Grossman 1988). Thus, with any experiment that can be responsive to exogenous supply of a hormone, a number of questions have to be addressed before it can be accepted that the observed hormone response is physiological (Trewavas 1991). This requires an isolated system to compare and study the response of ., * To whom all correspondence should be addressed. Email: vsthakar@hotmail.com endogenous or exogenous hormonal concentrations and physiological response. Cotton fibre offers many advantages for studies on hormonal response in an isolated system including: (i) it is a single-cell epidermal trichome on the seed coat and provides large populations of nearly synchro- nously growing cells; (ii) cotton genotypes show wide variation in the staple length which can help us to understand the possible correlation with endogenous concentration of hormone and cell elongation; and (iii) whether this difference is hormone-induced or due to some physiologicalfactor(s) can be tested in vitro. Therefore, in the present study, three cotton genotypes with different staple length were studied for the endogenous GAa concentration and its probable correlation with fibre length, in vitro. MATERIALS AND METHODS Seeds of three cotton cultivars, Gossypium hirsutum Hybrid-4 (H-4), Hybrid-8 (H-8) and G. arboreum G. Cot-IS were grown in the field thrice during 1997- 2000. Cultural practices including irrigation, appli- cation of fertilizers and insecticides etc. were con- ducted to maximize the lint yield. On the day of anthesis, each individual flower was tagged through- out the growing season. To minimize environmental variations, bolls were collected during as narrow a period as possible, i.e. 3-day intervals. These healthy bolls were used for analysis of endogenous gibberellic acid content and growth in terms of fibre length and dry weight. For in vitro experimentsovulesfrom one-