THE ROLES OF PLANT HORMONES IN THE GROWTH OF THE COROLLA OF GAILLARDIA GRANDIFLORA (ASTERACEAE) RAY FLOWERS

Corolla elongation and the roles of plant hormones in this process in GailZardia grandiMlora Van Houtte ray flowers were examined. The sterile ray flowers elongated during a 2-day period, and corolla growth was accompanied by fresh and dry weight increases and epidermal cell elongation (greatest near the base of the corolla) but not by cell division. Corollas excised from young ray flowers were measured during treatment in vitro with solutions of plant growth regulators. They elongated in response to gibberellins and fusicoccin but did not respond to auxins, cytokinins, abscisic acid, ethylene, or inhibitors of ethylene biosynthesis. Sequential and simultaneous hormone applications indicated no additive or synergistic effiects between hormones, but auxin did reduce gibberellin-promoted growth. Analyses of endogenous auxins showed no significant variation, and ethylene production decreased prior to elongation, while a 20-fold increase in endogenous gibberellin activity was observed just prior to rapid corolla elongation. It appears that corolla growth in Gaillardia is accomplished by an increase in gibberellin activity alone, that multiple hormone interactions are not important in the control of corolla growth, and that part of the mode of action of gibberellin is acid-induced growth. THE FLOWERING PROCESS has at least five sequential phases of development: flower induction, flower primordia differentiation, flower opening, anthesis and pollination, and flower senescence. The process of flower opening is in turn a sequence of several elongation and expansion processes which varies from species to species. In the Asteraceae, the inflorescence is usually composed of two flower types: the disc flowers (which are perfect and have a greatly reduced corolla), and the ray flowers (which are sterile and have a much larger corolla). The roles of plant hormones in the flower-opening processes of Gaillardia disc flowers have been described previously (Koning, 1 983a, b). The present work extends the study of the Gaillardia inflorescence by examining the roles of plant hormones in the growth of the corolla of the ray flower. The role of plant hormones in corolla ex' Received for publication 10 September 1982; revision accepted 27 June 1983. 2 Present address: Department of Biological SciencesBotany, Rutgers University, P.O. Box 1059, Piscataway, NJ 08854. This work is part of a doctoral thesis submitted to the Rackham School of Graduate Studies, University of Michigan, Ann Arbor. The author is greatly indebted to Dr. Peter B. Kaufman (University of Michigan, Ann Arbor) for help with the manuscript and for equipment supplied through his NASA Grant NAGW-34. Dr. Jan Zeevaart, Dr. Robert Bandurski, and Dr. Jerry Cohen (Michigan State University, East Lansing) provided valuable advice for the endogenous hormone analyses. pansion has been partially demonstrated for several genera. In the most complete studies of this involvement, gibberellins seem to be important(Blake, 1969;deJongandBruinsma 1974; Jeffcoat, Scott and Harris, 1969; Plack, 1958; Rao and Mohan Ram, 1979; Zieslin Biran and Halevy, 1974), but the other hormones (auxins, cytokinins, ethylene, and abscisic acid) may also be implicated especially when hormone interactions are considered (deJong and Bruinsma, 1974). The effects of applied hormones, hormone synthesis, and hormone-action inhibitors have been used to help demonstrate regulatory roles for these hormones (deJong and Bruinsma, 1974Rao and Mohan Ram, 1979). Studies showing parallel or inverse variation of endogenous hormone levels and corolla elongation add further support to the roles for the plant hormones in this growth process. Such correlations have been made for flower opening processes and auxin (Hatcher, 1945; Ilahi, 1979; Janardhan Raju and Gopal, 1977), gibberellins (Alpi et al., 1976; Einert, Staby and DeHertogh, 1972 Leshem and Ophir, 1977; Murakami, 1973 1975), cytokinins (Mayak and Halevy, 1970 Watanabe, Yokota and Takahashi, 1978), and ethylene (Beyer and Sundin, 1978Blumenfeld, 1975) in various species. All ofthese studies have been done with plants having perfect flowers, where interactions between flower parts are important. In the present work, the roles of plant hormones have been elucidated by