FLOWER-BUD FORMATION IN EXPLANTS OF PHOTOPERIODIC AND DAY-NEUTRAL NICOTIANA BIOTYPES AND ITS BEARING ON THE REGULATION OF FLOWER FORMATION

The capacity to form flower buds in thin-layer explants was studied in flowering plants of several species, cultivars, and lines of Nicotian a differing in their response to photoperiod. This capacity was found in all biotypes examined and could extend into sepals and corolla. It varied greatly, depending on genotype, source tissue and its developme ntal stage, and composition of the culture medium, particularly the le vels of glucose, auxin, and cytokinin. It was greatest in the two day- neutral plants examined, Samsun tobacco and Nicotiana rustica, where i t extended from the inflorescence region down the vegetative stem, in a basipetally decreasing gradient; it was least in the two qualitative photoperiodic plants studied, the long-day plant Nicotiana silvestris and the short-day plant Maryland Mammoth tobacco, the quantitative lo ng-day plant Nicotiana alata and the quantitative short-day plant Nico tiana otophora line 38-G-81, where it was limited to the pedicels (and , in some cases, the sepals). Regardless of the photoperiodic response of the source plants, the response was the same in explants cultured under long and short days. The finding that capacity to form flower bu ds in explants is present in all Nicotiana biotypes studied supports t he idea that it is regulated by the same mechanism(s), regardless of t he plant's photoperiodic character. However, the source plants were al l in the flowering stage, and no flower-bud formation can be obtained in explants from strictly vegetative Nicotiana plants. Hence, flower f ormation in the explants is not identical with de novo flower formatio n in a hitherto vegetative plant: it is rather the expression of a flo ral state already established in the plant, although it can vary widel y in extent and spatial distribution. Culture conditions that permit f lower-bud formation in an explant are conditions that maintain the flo ral state and encourage its expression; conditions under which no flow er buds are formed reduce this state and/or prevent its expression.