Timing of the floral induction in spinach

A general outline of the cellular and metabolic events linked to floral induction in spinach, a LDP, will be presented. A special emphasis will be put on the chronology of these events in the meristem, in the leaves and in the petioles. The vegetative and the floral states were first characterized by time series studies including electron microscopy, cytochemical and biochemical techniques. The development of the induced state was then kinetically followed. Various inhibitors and promoters of the flowering process have also been used. According to this experimental protocol, it was shown that the first sign of floral evocation in the meristem is an activation of the pentose phosphate pathway (PPP) which occurs three hours after the critical photoperiod has been reached. The first ultrastructural events (budding and fragmentation of the plastids) are seen only six hours later. In the leaves, newly synthesized proteins, which are specific for the induced state, are detectable nine hours after the critical photoperiod. Earlier physiological markers have been identified. Among them, the R-FR photocontrol of a peroxidase activity is switched to its specific floral response after three hours, while changes in energy metabolism (adenine and pyridine nucleotides ratios) already occur after one hour. In the petioles, the glucose level is immediately and steadily increased and reaches its maximum value when the PPP is activated in the meristem. Other results have shown that the signal transmitted from the leaves to the meristem can be slowed down by a LiCl pretreatment, suggesting that membrane permeability changes could be implied in floral induction. It can, therefore, be proposed that most of the earliest events of the transition to the floral state are rapid biophysical and biochemical modifications of the structures supporting vegetative growth. Changes in hormonal balances, though still not studied, are also considered as part of these early events which finally result in gene activation, the ultimate event of flower induction