0073-5655/81/0825-0830 $01.50/0 IN VITROVol. 17, No. 9, September1981 9 1981 Tissue Culture Association, Inc. GIBBERELLIC ACID REGULATION OF ADVENTITIOUS SHOOT FORMATION FROM TUBER DISCS OF POTATO ROBERT L. JARRET, PAUL M. HASEGAWA,' AND RAY A. BRESSAN Department of Horticulture, Purdue University, ff/est Lafayette, Indiana 47907 (Received November 7, 1980; accepted May 1, 1981) SUMMARY The formation of adventitious shoots from potato tuber discs explanted onto a modified Murashige and Skoog (MS) medium containing N6-benzylaminopurine (BAP) (3.0 mg/l), and a-naphthaleneacetic acid (NAA) (0.01 mg/l), was affected by gibberellic acid (GA). The presence of GA in the explant medium was required for shoot formation and 3 x 10-'o M GA appeared optimum. However, microscopic examination of the tissue pro- tuberances on the surface of the tuber discs from which shoots arose revealed that GA in- hibited the formation of shoot meristems. Tuber discs cultured for 6 wk on MS medium containing BAP and NAA without GA did not initiate adventitious shoots that could be determined visually, but microscopic examination of the tissue protuberances revealed the presence of numerous shoot meristems. Subsequent transfer of these tuber discs to medium with GA but without BAP or NAA resulted in the formation of shoots from 100% of the re- cultured discs. Thus it appears that although GA inhibits shoot meristem initiation from potato tuber discs, it is required for shoot development once meristems are initiated. Key words: Solanum tuberosum L.; tissue culture. INTRODUCTION Gibberellic acid (GA) has been reported to in- hibit shoot formation from tissue cultures of tobacco (1-4) by inhibiting the initiation of meri- stematic nodulelike structures that eventually gave rise to shoot primordia (3). From these (1-3) and other reports ~5,6) it appears that GA sup- presses the initiation of organs but stimulates the development of organs that have been already ini- tiated (7). However, Kartha et al. (8) have re- ported that shoot initiation could be enhanced if callus derived from shoot meristems of cassava (Manihot ultissima Pohl.) was cultured on medium containing GA in addition to N 6- benzylaminopurine (B AP). Research from this laboratory (9) indicated that GA was essential for the formation of adventitious shoots from cultured tuber discs of potato, al- though the exact role of GA in the shoot formation process was not determined. The need for a more thorough understanding of the action of GA and other hormones in the organogenetic process and 'To whom correspondence should be addressed. subsequent organ development is fundamental to our ability to regulate morphogenesis in vitro (10). In this study we examined the effects of GA on the shoot formation process (separated into initiation and development phases) from tuber discs of potato (Solanum tuberosum L. cv. Superior). MATERIALS AND METHODS Preparation and Culture of Tuber Disc Explants Culture of freshly excised discs (Stage I). Tubers of "Superior" used in this study were har- vested at Rensselaer, Indiana, on October 4, 1979, and stored at 4 ~ C. Tuber tissue segments were surface disinfested, and tuber disc explants were excised and cultured as previously described (11). The freshly excised tuber discs were placed onto nutrient media containing the following basal constituents: Murashige and Skoog (MS) salts (12), folic acid (0.5 mg/l), d-biotin (0.05 mg/l), pyridoxine" HCI(0.5 mg/l), nicotinic acid (0.5 mg/l),thiamine" HCI (0.1 mg/l), glycine (2.0 mg/l), i-inositol (100 mg/1), casein hydro- lysate (1 g/l), sucrose (25g/1), Bacto agar 825