Environmental and Experiraental Botany, Vol. 18, pp. 225 to 228 0098 8472/78/1201 0225 $02.00/0 © Pergamon Press Ltd. 1978. Printed in Great Britain ENHANCED BUD FORMATION IN GAMMA-IRRADIATED TISSUES OF NICOTIANA TABACUM L. CV WISCONSIN-38 K. G. HELL, W. HANDRO and G. B. KERBAUY Plant Tissue Culture Laboratory, Department of Botany, University of Sao Paulo, C.P. 11461, 05421 Sao Paulo, Brasil (Received October 28 1977; revision received March 7 1978) HELL K. G., HANDRO W. and KERBAUY G. B. Enhanced bud formation in gamma-irradiated tissues 0fNicotiana tabacum L. cv Wisconsin-38. ENVIRONMENTAL AND EXPERIMENTALBOTANY 18, 225-- 228, 1978.--Pith and callus tissues ofNicotiana tabacum cv W-38, explanted from three different regions of the stem, were irradiated with 2 Kr of gamma-radiation. The observed enhance- ment of bud regeneration in explants and callus was related to the origin of the explanted tissue and to growth regulators added to the media prior to and after irradiation. INTRODUCTION STIMULATORY AND INHIBITORY effects of ionizing ra- diation on growth and differentiation of plant tissues cultured in vitro have been reported3 l' 9, 11, 13) These results have been related mainly to radiation effects on endogenous IAA levels in plant tissues, c6'12~ Recently, new data were added showing that gamma-radiation effects could also be related to cytokinins ~s) and to other organic substances usually present in the culture media. {4's'11) In this work, tobacco tissue cultures were used to investigate morpho- genetic effects induced by gamma-radiation with respect to: (a) the position of the explan- ted tissue in relation to the shoot apex, (b) the growth stage of the explanted tissues, (c) the growth substances added to the media and (d) the culture conditions prior to irradiation. MATERIAL AND METHODS The explants were taken from young plants of ACicotiana tabacum L. cv Wisconsin-38. All plants were at the vegetative growth-stage. Stem segments from ca. 100cm tall plants were cut at 5cm (A), 20cm (B) and 40cm (C) respectively from the shoot apex. These seg- ments were surface sterilized by a 10-rain im- mersion in a 53~ solution of Calcium hypo- chlorite and then rinsed with sterile, distilled water. Pith tissue cylinders of 5mm dia were removed with the aid of a corkborer. From these pith cylinders, 1 mm thick discs were cut. The tissues were cultured in 25 x 150mm tubes with 20ml of MURASHIGE and SKOOO'S revised medium (RM-65) ls) containing indoleacetic acid (IAA), 2,4-dichlorophenoxyacetic acid (2,4-D), kinetin (K) and coconut milk from green nuts (CM) at concentrations as indicated under Results. The cultures were incubated with a photoperiod of 16hr light (about 40001x) at temperatures of 29°C (day) and 24°C (night) or kept in the dark at a constant temperature of 27°C. The material was gamma- irradiated with 2 Kr from a 13VCs source at a rate of 1.6Kr/hr. The pith tissues were ir- radiated prior to the explantation. Callus tissues were subcultured 3 times in each of the dif- ferent growth media, before irradiation. The tissues for transfers and for the irradiation ex- periments were 4 week-old cultures. Callus ti- ssues were irradiated on wet filter paper discs placed in Petri dishes under aseptic conditions and then transferred to the new culture media. The results were measured 40 days after the transfer to the medium under investigation. Twelve tubes were used for each treatment, and 225