The aim of this work was to determine the direct inhibitory effects of 60Co γ-rays on plantlets and to control them in a future program of experimental mutagenesis. Gerbera jamesonii plantlets were irradiated in vitro using two sources of 60Co γ-rays. The following variables were tested: dose rate, total dose, irradiation of the growth medium, time interval between the exposure to γ-rays and the subdivision and transfer to fresh medium, and dose fractionation. These variables were evaluated using regeneration rate of new shoots, callus production, and plantlet growth rate (fresh weight). The optimal dose ranged around 50 Gy at a low dose rate (8.4 Gy∙h−1) and 30 Gy at high dose rate (600 Gy∙h−1). These conditions assured minimal development of callus and 50% of the normal multiplication rate. The γ-irradiation may be done during a 1-week interval before or after transfer on fresh medium, without causing any additional effects. Interrupted doses (3 × 10 Gy at 48-h intervals) caused less important inhibitory effects than a single dose (30 Gy), but the intensity of the reaction varied according to clone and the measured parameter. The clone "Duplex" reacted favourably; this type of treatment ensured better callus and plantlet growth and a higher rate of regeneration. In the case of "Mardi Gras," which is less sensitive to fractionated doses, only the weight of the plantlets was significantly less reduced.

SUMMARY The inflorescential axis of Gerbera jamesonii is a determined one, not fully differentiated when growers cut the flowers. A region located 10 cm below the capitulum is very sensitive to “stem break”. Histology of this axis has been studied. Three different levels have been analysed: A, under the capitulum; B, 10 cm below the capitulum; C, half. way on the axis. Among the principal characteristics observed are a very important medullary parenchyma, a morphological endodermis, a half girdle of sclerenchyma under the capitulum and a full girdle of sclerenchyma in the mature zone. The distribution of the different tissues allows us to link the bending to the deficiency in supportive elements, typical of certain cultivars and of certain developmental stages.

Résumé. --L'inflorescence uniflore de Streptocarpus rexii représente un thyrse eymoïde vestigial ; cette constatation sc base sur la comparaison de l'inflorescence typique uniflore avec les formes pluriflores, obtenues expéri-mentalement (traitements à la !dbbérelline A 3 et à la 6-bcnzylaminopurine) de même qu'avec les inflorescences de genres voisins. Les gibbérellines A 3 ct A 7 inhibent le développement floral, particulièrement celui des pièces femelles ; la 6-bcnzylaminopurine entraîne la production de fleurs fortement pléiomères et exerce un effet féminisant très marqué : production de plusieurs verticilles femelles pléiomères ct formation de carpelles à partir du cycle staminal.Zusammenfassung. -Die cinblütige « Infloreszcnz » von S. r. cntspricht cinem extrem reduzierten Cymoid. Diesc Feststellung basicrt auf Vergleichen der Normalform mit experimcntcll erhaltcnen mehrblütigcn lnflorcszcnzen (Behandlung mit GA 3 oder mit 6-BAP) sowie mit Blütenstanden verwandtcr Gattungen. Die Gibberclline A 3 und A 7 hemmen die Blütenentwicklung, besondcrs die der weiblichen Teile. ii-BAP fürdcrt die Bildung stark plciomcrer Blüten und übt zugleich eine betont feminisierende 'Virkung aus : Bildung von etagierten plciomcren Gynoeceen und Transformierung von Staub-zu Fruchtblattern.

SOMMAIRETrois types de boutures de Streptocarpus wendlandii Sprenger (Gesneriacee) ont ete realisees it partir du phyllomorphe cotyledonaire de plantes temoins et pretraitees (gibberelline A3 et/ou cytokinine 6-benzylaminopurine). L'initiation de racines et, par la suite, de bourgeons, se fait it partir de l'extremite proximale des nervures sectionnees, Le prelevement des explantats it partir de la region proximale du limbe, it partir de plantes en periode preflorale ou pretraitees avec AG3 et l'absence de reste de la nervure mediane sur l'explantat favorisent la regeneration. SUMMARYThree types of cuttings of Streptocarpus wendlandii Sprenger (Gesneriaceae) have been realised from the phyllomorph of untreated and pretreated lamina with gibberellin A3 and/or 6-benzyladenin. The products of regeneration obtained varied according to factors as age and size of the phyllomorph and the explant, propagation during preflowering and flowering periods, vascularisation of the explant, treatments with growth substances.

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