Elevation in the Sucrose Content of the Shoot Apical Meristem of Sinapis alba at Floral Evocation

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...

Cytokinin Fluxes during Floral Induction in the Long Day Plant Sinapis alba L.

“…The increase in dry weight occurring in response to daylength extension is probably due to an increased sucrose content as suggested by TLC analysis of exudates for carbohydrates. Increased levels in starch and/or soluble sugars were also recorded in leaves and apical buds of induced plants from the 10th h of the LD (6,7,18). The rise in cytokinin export from the leaves fits well with the hypothesis that cytokinins represent the mitotic trigger which moves out of the leaves 16 h after start of the LD (2).…”

The florigen mystery

“…Abscisic acid (ABA) is also involved in flowering 20 . In studies with Rudbeckia bicolor 21 , Sinapis alba 17,22 and Perilla nankinensis 23 it was found that sugars (sucrose) are involved in flowering. There are other cases in which the carbohydrates play a role in flowering, namely in Fuchsia hybridia 24 , and the LD-grass Lollium temulentum.…”

“…Dit is reeds beskryf dat in die geval van Sinapis alba, 17,22 sukrose 'n rol speel by blomvorming. S. alba en R. bicolor is albei LD-plante, maar dit is ook by 'n KD-plant, naamlik Perilla nankinensis, 23 bevind dat suikers 'n rol by blomvorming speel.…”

Interaction Between Flowering Initiation and Photosynthesis

“…It is known that many plant developmental, physiological and metabolic processes are regulated by soluble sugars such as glucose and sucrose and by other signaling molecules (Gibson, 2004). A lot of scientists investigated the sucrose distribution in apex and in other plant tissues (Bodson and Outlaw, 1985;Lejeune et al, 1993;King and Ben-Tal, 2001). It is presumed that the supply of sucrose to apical meristemic tissues is important for flower induction.…”