No abstract

Nanogram tissue samples from apical meristems of Sinapis alba were assayed for sucrose, total soluble hexosyl equivalents (-glucose and fructose plus hexoses from sucrose hydrolysis), and total soluble glucosyl equivalents (-glucose plus glucose from sucrose hydrolysis). On dry weight basis, sucrose concentration increased by more than 50% within 10 hours after the start ofeither a long photoperiod or a short photoperiod displaced by 10 hours in the 24-hour cycle ('displaced short day'). (These treatments induce flower initiation) Glucose and fructose concentrations were close to zero in vegetative meristems and remained low compared to sucrose in meristems of induced plants. Within a single meristem, the peripheral and the central zones had similar concentrations of sucrose. Our results indicate that an early physiological event in floral transition is the accumulation of sucrose in the meristem.The transition of a vegetative meristem to a floral meristem is marked by several histological, cellular, physiological, and molecular changes (2); those events that are associated with an irreversible commitment of the meristem to initiate flowers constitute the 'evocation' (12). One physiological change that is associated with floral transition in photoperiodic-and coldrequiring plants is an increase of the soluble carbohydrate concentration in the apical bud (7). In Sinapis alba, this increase has been observed regardless of the environmental inductive treatment (3,4) peripheral zone (13,17,19). At floral evocation, both the central and the peripheral zones are activated mitotically and the pithrib meristem progressively vacuolates and disappears (2,13).At least in Tradescantia paludosa, the mitotic activity of the central zone is influenced by nutritional factors (27). Thus, one possibility is that the central zone of the vegetative meristem is relatively deprived ofnutrients and that an increase ofassimilates in it is a prerequisite for floral transition (23). However, there are no data in support of this hypothesis.The highly sensitive methods for quantitative histochemical analysis described by Lowry and Passonneau (16, see also 20) are well suited to study the metabolite content of the meristem. Using these methods, we have measured the sucrose content of the peripheral zone and central zone of the apical meristem of Sinapis plants, which were induced to flower by one LD or by one displaced SD. Total soluble hexosyl equivalents and total soluble glucosyl equivalents were measured also, but with less morphological resolution. MATERIALS AND METHODSPlant Material. Sinapis alba plants were cultured as described elsewhere (3). Vegetative plants were maintained by growing them under 8-h photoperiods (SD). Sixty-five-d-old plants were induced to flower either by one 22-h LD or by one 8-h displaced SD (Fig. 1). Subsequently, the induced plants were returned to the standard SD regime. The meristem of induced plants will be referred to as 'evoked' to signify that the meristem ofthese plants is undergoing evocat...

Vegetative plants of Sinapis alba L. were induced to flower by a single long day of 20 h or by a single short day of 8 h starting at an unusual time of the 24-h cycle ("displaced short day"). The soluble sugar and starch contents of the just-expanded leaf and the apical bud were measured at various times after the start of each of these two photoinductive treatments. Associated with the induction of flowering there were temporary increases in the soluble sugar and starch contents of the leaf and of the bud. These increases were apparent 14 h after the start of the long day and 12 h after the start of the displaced short day. The starch content of the bud increased later. These results indicate that an increase of the soluble sugar content of the bud is required for its transition from the vegetative to the reproductive condition.

Results of previous investigations indicated that one of the early and essential events occurring in the apical meristem of Sinapis alba L. during the transition to flowering is the release to mitosis of the G2 nuclei; the trigger to mitosis is generated in the leaves and its movement out of the leaves begins around 16 hours after the start of the inductive treatment. The mitotic wave in the meristem culminates 10 hours later.In this paper, it is shown that a single application of a cytokinin (benzyladenine or zeatin) at concentrations ranging from 1 to 20 Ag/ml directly to the apical bud of vegetative plants, at a time corresponding to the time of movement of the mitotic trigger in induced plants, produces a mitotic wave which is very similar to that found in induced plants. It is thus proposed that the mitotic component of the floral stimulus in Sinapis is a cytokinin. As the cytokinins are completely unable to induce flowering, it appears that there is a multicomponent floral stimulus in this species.The physiology of flowering has been dominated for 40 years by the concept that there is one substance-called the floral hormone or the floral stimulus -which specifically evokes flowering and which is common to all higher plants (7,17). All attempts to isolate this stimulus have failed and its chemical nature is still completely unknown (28).Since the direct approach which consists in trying to extract, purify, and characterize the stimulus has yielded very little information so far, we thought it worthwhile to try an indirect approach, i.e. to try to gain insight into its nature from the kind of early changes that it produces at the shoot apical meristem.The plant used in this study was Sinapis alba, a long day species which can be induced to flower by a single long day. One of the first detectable changes in the apical meristem of Sinapis during the transition from the vegetative to the reproductive condition is the release to mitosis of a large population of G2 nuclei (6,12). This early mitotic wave always culminates 26 to 30 hr after the start of the inductive treatment.Further studies have shown that it was impossible to dissociate the flowering process at the meristem of Sinapis from this mitotic activation (5), and that a similar early mitotic rise occurred in the evoked meristems of all species so far examined in this respect (13,16, 25, 27). We concluded that the early mitotic wave was an essential component of floral evocation.By subjecting vegetative plants of Sinapis to a single day of 11 or 12 hr, it was however possible to induce this mitotic event in the absence of flowering (5). This possibility of fractional evocation in Sinapis suggested that the floral stimulus may consist of more than one active component and that the early mitoses are triggered by a specific component, called the mitotic component.Other supporting evidence for a multicomponent floral stimulus was supplied by defoliation experiments which showed that the patterns of movement out of the leaves of the mitotic compone...