Juvenility is the name usually given to an early phase of growth during which flowering cannot be induced by any treatment. In this way, sexual reproduction is delayed until plants reach a size sufficient to maintain the energetic demands of flowering and seed production. In many plants the transition to reproduction occurs after the juvenile phase has been completed without exposure to any particular stimulus; in others appropriate environmental treatments (such as daylength) are necessary. Sometimes the term ripe-to-flower is used for plants which have completed the juvenile phase but have not yet experienced the correct conditions for flowering. The duration of the juvenile period varies widely. In most woody plants it lasts for several years but in herbaceous plants it is rarely more than a few days or weeks. Xanthium cannot be photoperiodically induced during the first week after germination, while Perilla does not become sensitive for 15-20 days. In Glycine, the duration of the photoperiod insensitive phase varied from 11-33 days in a range of four cultivars of diverse origin and was found to be a strong determinant of the time to first flowering (Collinson et al., 1993). In extreme cases, there is no juvenile phase since flower primordia are found in the seed.

Size appears to be important in the transition to maturity and, in general, conditions that promote growth reduce the duration of the juvenile period. Two possible explanations for the effect of size have been considered. One is that a plant of sufficient size transmits one or more signals to the apex, which then undergoes a phase change from juvenile to adult. The second is that the apical meristem behaves independently and undergoes the phase transition at a particular time. There is some evidence for both views (Thomas and Vince-Prue, 1984). Once attained, the adult phase is usually highly stable and persists until sexual reproduction occurs.

The existence of a juvenile phase is obviously one of the factors associated with the lower sensitivity of younger plants to photoperiodic induction, since the daylength cycles given to juvenile plants are not effective. That this is a property of the leaves rather than of the apex is indicated by the results of experiments with Bryophyllum, where growing points from juvenile plants were able to flower when grafted to photoperiodically-induced mature plants (Zeevaart, 1962b). A similar result has been obtained in the SDP Ipomaea batatas (Takeno, 1991); here, 13-day-old seedlings flowered when grafted to Pharbitis nil in SD, but not when they were themselves exposed to inductive cycles. The hypothesis that the limitations of juvenile plants reside mainly in their inability to produce a floral stimulus is supported by the behaviour of obligatory viviperous species of grasses. Viviperous plantlets may develop a new generation of proliferated inflorescences while still attached to their parent plants but detached plantlets cannot be induced to form such inflorescences and remain vegetative (Heide, 1994). Apparently the attached plantlets are able to respond to a floral stimulus from the parent plant, but are not themselves able to produce it. Additional evidence that juvenility effects may be associated with the leaves comes from grafting experiments with Glycine, where the long-juvenile genotype appeared to be associated with one or more transmissible compounds from the leaves, which delayed flowering (Sinclair and Hinton, 1992). In contrast, seedling apices of tobacco have been shown to be less competent to respond to a given level of floral stimulus than apices from mature plants (Singer et al., 1992). Biochemical and morphological changes in the meristems have been observed to occur with increasing plant age and could confer increased sensitivity to the floral stimulus; for example, a number of polypeptides were unique to either young or aged apices in the LDP Silene coeli-rosa growing in non-inductive SD and there were also differences in morphology and mitotic index (Nougarede et al., 1989). Thus changes in both leaves and apices may contribute to the development of competence to respond to photoperiodic induction.

A few experiments have been made with woody plants in which induction is autonomous. When juvenile apices of Japanese larch were grafted to mature trees, only one (out of 56) flowered in the first year (Robinson and Wareing, 1969); when nearly-mature apices were similarly grafted, 36 of the grafts flowered. A similar situation has been reported for mango, where graft induction failed in 1-4-year-old seedlings but was successful when the receptors were 6 years old and nearly mature (Kulkarni, 1988). Thus, in these two examples, it seems that the transition from juvenile to mature in woody plants is not primarily determined by signals reaching the apex from the rest of the plant.

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