Physiological and Developmental Roles in Early Events of Fiber Development
Much of our current understanding about the role of phytohormones in fiber growth and development comes from studies of isolated ovules in culture (Beasley and Ting, 1974). Fiber initiation in unfertilized ovules is promoted by the addition of auxin and gibberellic acid. Either hormone can promote fiber initiation in unfertilized ovules and the effect is additive when both hormones are present. Ovule epidermal cells are competent to respond to phytohormones about two days prior to anthesis (Graves and Stewart, 1988). In some cotton varieties, although not in TM1 or the isogenic mutants, fiber initials are evident prior to anthesis (Gialvalis and Seagull, 2001) (Triplett, unpublished). This condition seems to be a property of newer cotton varieties such as G. hirsutum DeltaPine 90 & Maxxa, with older varieties such as TM-1 & Acala SJ-2 delaying visible signs of fiber initiation until after anthesis (Beasley and Ting, 1974; Gialvalis and Seagull, 2001). This observation suggests that efforts to increase fiber yield in modern commercial varieties may have inadvertently selected for ovules that are less influenced by physiological events occurring at anthesis. Regardless of whether fiber cells are visible pre-anthesis or not, exogenous application of IAA and GA3 to flower buds in planta and unfertilized ovules in vitro results in increased fiber numbers (Gialvalis and Seagull, 2001) (Triplett, unpublished).
