Floral determination in internode tissues of day-neutral tobacco first occurs many nodes below the apex.

The temporal and spatial patterns of floral determination in cultured internode segments of Nicotiana tabacum L. cv. Wisconsin 38 were investigated. Segments from internodes, when cultured on hormone-free medium, either produced some de novo floral shoots or produced only vegetative shoots. Segments that produced some floral shoots were considered to contain florally determined tissues. The first internode tissues to exhibit floral determination were located 24 nodes (about 15 cm) below the terminal meristem on plants that had 21 leaves of length >/=3 cm (i.e., plants of age 21). Since the terminal meristem was morphologically vegetative at this age, floral determination in internode tissues did not result from a signal emanating from floral structures. As the plant continued growing, internode tissues progressively higher on the main axis gained the capacity to produce de novo floral shoots in culture. These data indicate that the inductive signal which elicited the state of floral determination in internode tissues was active in the upper half of the plant beginning at about age 21.