Topic 20.3 Flowering of Juvenile Meristems Grafted to Adult Plants
By definition, juvenile plants cannot be induced to flower with an inductive photoperiod, suggesting either that the shoot apical meristems of juvenile plants are not competent to flower, or that juvenile leaves produce insufficient amounts of a floral stimulus. The LSDP Bryophyllum passes through a juvenile phase, which lasts from 5 months to a year (depending on the species), during which it cannot be induced to flower by transfer from LD to SD. However, when juvenile Bryophyllum shoots were grafted onto flowering plants, the juvenile shoots flowered promptly and profusely (Zeevaart 1985). This result indicates that the shoot apical meristems of juvenile plants are competent to flower, but fail to do so because the juvenile leaves produce insufficient amounts of the floral stimulus.
Age is another factor that can affect production of the floral stimulus. Grafting studies with the SDP Perilla have shown that the leaves at the top of a plant are stronger donors of the floral stimulus than the leaves near the base of a plant, in keeping with the juvenile character of the basal leaves. The experiment was performed as follows. Two groups of Perilla plants were sown at different times. When the first group had produced five pairs of leaves, the second group had only produced two. The fifth and the second pairs of leaves from the two sets of plants were grafted onto stocks and maintained under long days. Once the graft unions were established, the grafted leaves were induced with short days by daily enclosing them in opaque bags. Leaves from the fifth node needed only 14 short days to induce flowering while leaves from the second node required 28 short days (Zeevaart 1986). This finding shows that the more juvenile leaves from the second node produce less floral stimulus, and thus require more inductive photoperiods to induce flowering, than the fully adult leaves higher up the shoot.