J. Plant Physiol. 157. 421-427 (2000) © Urban & Fischer Verlag http://www.urbanfischer.de/journals/jpp • JOURNAL OF • PLANT PHYSIOLOGY The processes inhibited and promoted by abscisic acid in photoperiodic flowering of Pharbitis nil Takahide Maeda 1 *, Tadao Asami 2 , Shigeo Yoshida 2 , Kiyotoshi Takeno 1,3 1 Graduate School of Bioagricultural Sciences, Nagoya University, Nagoya 464-8601, Japan 2 Plant Functions Laboratory, The Institute of Physical and Chemical Research (RIKEN), 2-1 Hirosawa, Wako, Saitama 351-0198, Japan 3 Present address: Department of Biology, Faculty of Science, Niigata University, Niigata 950-2181, Japan Received April 6, 2000 . Accepted June 27, 2000 Summary A dual role of abscisic acid (ABA) in photoperiodic flowering of Pharbitis nil, cv. Violet was studied. ABA given before an inductive dark treatment lowered the flowering response without influencing the critical night length and the most effective time of a night break. ABA given after a dark treatment accelerated the transport of flowering stimulus and enhanced flower initiation response at shoot apex. Endogenous ABA level in cotyledons was determined by gas chromatography-selected ion monitoring. There was no remarkable difference between ABA levels under short-day and long-day conditions. Fluridone, an inhibitor of ABA biosynthesis, given before and during an inductive dark treatment affected flowering response only slightly. The results indicate that exogenous ABA inhibits the early phase of the flowering without influencing the time-measuring process, and promotes both transport of the flowering stimulus and flower-initiation process, although endogenous ABA plays little role in the regulation of flowering. Key words: Pharbitis nil- abscisic acid - photoperiodic flowering. Abbreviations: GAs Gibberellins. - GC-SIM gas chromatography-selected ion monitoring. - LO long-day. - NB night break. - PAR photosynthetically active radiation. - S-ABA (+ )-(S)-abscisic acid. - SO Short-day. Introduction The involvement of plant hormones in the photoperiodic flow- ering of Pharbitis nil has been suggested in a large number of reports (Vince-Prue and Gressel 1985, Takeno 1996). Gib- berellins (GAs) are not essential for flowering, although they * E-mail corresponding author: i982012d@mbox.media.nagoya-u.ac.jp promote the flower initiation (Takeno and Cleland 1990, Takeno et al. 1996). Cytokinins promote flowering (Ogawa and King 1979), and ethylene inhibits it (Suge 1972). Auxin is considered to inhibit the flowering via enhanced ethylene bio- synthesis (Wijayanti et al. 1997). Among them, the reported results on the effect of abscisic acid (ABA) were confusing. Namely, some reports showed a promotion (Bose and Harada 1970, Nakayama and Hashimoto 1973, EI-Antably and Wareing 1966) and others indicated an inhibition of flowering 0176-1617/00/157/04-421 $ 1500/0