Apical reactions to gibberellic acid application according to the genotype inSilene armeria

Two selected strains ofSilene armeria L. were used: S_1-2 (GA-line, not induced to flowering by GA₃ in SD of 8 h) and S_2-1 (GA+ line which reacts to GA₃ with flowering in non-inductive photoperiod). Moreover S_1-2 and S_2-1 differ in their critical daylength 14.5 and 8.0 h, respectively. Changes in the mitotic index and DNA content of cells in the various zones of the apical meristem during GA₃ treatment were described. At the start of the experiment, the functioning of the apex was characterized by a predominance of G₁ phase in the two strains. Therefore S_2-1 differed by a higher mitotic activity specially in the central zone. In S_1-2, GA₃ acted mainly on the rib-meristem cells, which resulted in stem elongation. Lack of response in the cells of the central zone explains why GA₃ fails to induce flowering in this strain. In S_2-1, GA₃ acted mainly on the central zone where a gradual increase in mitotic acitivity and in the percentages of nuclei at the (S + G₂) level was recorded. This stimulation brought the meristems into the prefloral stage. The differences between the two strains are discussed according to the status of control meristems. The reactions induced by GA₃ were compared with the pattern of changes during induction, in LD. This work was supported by grants from the “Centre National de la Recherche Scientifique” (UA 572).     

Mitotic Activities and Levels of Nuclear DNA in the Apical Meristem of Silene armeria (strain SI.2) Following Application of Gibberellin A3

Strain S1.2 of Silene armeria was grown under an 8h-photoperiodand treated with GA₃ every day for 20 days. This growth substancecaused stem elongation, but no flowering in this long-day plant.Changes in the mitotic index and DNA content of cells in thevarious zones of the apical meristem before, during and afterGA₃ treatment were described. Mitotic activity and increasein the proportion of nuclei at the 4C level (S+G₂ phase of thecell cycle) were strongly stimulated in the rib-meristem, andto a lesser extent in the lateral zone, but not in the axialzone. This stimulation of apical activity reached a peak aftertwo GA₃ treatments and then declined gradually, so that after20 days the activity in GA₃-treated meristems was lower thanthat in untreated controls; at this point most cells were inthe G₁ phase. When the GA₃ treatment was discontinued, there was a gradualincrease in the mitotic activity which ultimately reached thesame level as that in controls. Stem elongation ceased and leavesformed aerial rosettes. It is concluded that in vegetative plants of strain S1.2 ofSilene armeria GA₃ acts mainly on the rib-meristem cells whichresults in stem elongation. Lack of response in the axial cellsexplains why GA₃ fails to induce flowering in this strain ofSilene armeria. (Received June 18, 1983; Accepted August 3, 1983)     

Histoautoradiographic analysis of the thermoinductive processes in the shoot apex of Arabidopsis thaliana L. Heynh, vernalized at different stages of development

The cytochemical reactions of various zones of the shoot apexof an annual winter plant (Arabidopsis thaliana), vernalizedat two different ages (26 and 85 days), were analyzed usinghistoautoradiography after incorporation of tritiated thymidine.The following sequence of events was observed during a coldtreatment of 40 days: subapical activation (in DNA synthesis)of the rib meristem (this took place regardless of the age ofthe plants; dedifferentiation of axial cells (the beginningof which was earlier in the older plants). After cold treatment, the plants were resubmitted to warm temperaturesunder long day conditions. A re-establishment of the zonationin the meristem was seen during the first week for plants inthe "intermediate stage". The activation of axial cells increasedduring the next two weeks and a few days later both groups ofplants entered the prefloral stage followed by the formationof their first floral buds. These apical reactions were comparedto those of a French variety of A. thaliana, a quantitativelong-day plant, submitted to different photoperiods and to gibberellicacid (GA₃). (Received January 17, 1977; )     

Photoperiod and gibberellic acid – control of the cell cycle in the meristem of Silene armeria and its effects on flowering

Plants of Silene armeria L., strain S_2.1, a quantitative long-day (LD) species which is known to react to GA₃ by flowering after attaining, the'intermediate stage', were induced by two LD or by two GA₃ applications. Changes in the mitotic index and DNA content (microdensitometric estimation) of cells in the axial zone, lateral zone and rib meristem of the shoot apex were observed during the first 48 h of each treatment. Similar mitotic activation occurred in response to LD or GA₃ after a 6-8 h lag period. This was preceded by a decrease in the proportion of nuclei with a 2C DNA content, indicating that in this species the control point for the shortening of the cell cycle was essentially in G₁. A second mitotic peak was observed 16 h later in photoinduced meristems, resulting in more pronounced cellular synchronization. These further events were not seen in GA₃-treated plants where only the meristematic activity was slightly, but reproducibly higher than in the control. Thus, two successive synchronizations of cell division are a typical feature of LD induction. The data are discussed with regard to the competence of shoot apical cells to be reactivated. The essential changes for the transition to flowering depend on these differential patterns of cell reactivation.     

Effectiveness of gibberellins and 6-benzyladenine on flowering of Arabidopsis thaliana

The different phases of development of the apical meristem in two ecotypes of Arabidopsis thaliana (L.) Heynh. [one French (a quantitative long-day plant), the other one Swedish (an absolutely cond-requiring plant)] were established, under conditions either inductive or not inductive of flowering. Then the effects of gibberellins (GA₃, GA_4/7) and cytokinin (6-BA) on these ecotypes were studied at different stages of their ontogenic development. The rate of plant growth was estimated by measuring the plastochron duration. The activities of the different apical zones were determined with cytochemical methods and by histoautoradiography after incorporation of 3H-thymidine into DNA. When treated during the vegetative stage, i.e. at a young age (French ecotype) or in absence of cold treatment (Swedish ecotype) flowering was not induced. The tested chemicals had similar effects: at first a stimulation of the phylogenetic activity of the cells of the lateral zone and of the rib meristem and, after extended treatments, a differentiation of the axial cells of the meristem and, as a result, an inhibition of flowering. On the other hand, the different hormonal applications on meristems which have reached the intermediate stage, were effective in inducing the reproductive state. The activity of the central cells already in the process of dedifferentiation was increased and, by this, flowering was promoted.