The callus model of plant flowering

Abstract

The callus model of flowering consists of cultures of stem calluses of three tobacco varieties, of photoperiodically neutral Trapesond, short-day Mammoth ( Nicotiana tabacum ) and long-day Sylvestris ( Nicotiana sylvestris ). Stem calluses of the Trapesond variety taken from flowering plants produce generative buds with flowers; those from young vegetating plants form vegetative buds. Stem calluses from both flowering and young vegetating plants of the Mammoth and Sylvestris varieties form only vegetative buds. The capacity for generative or vegetative morphogenesis is retained not only on prolonged cultivation of the primary calluses but also in passage culture. The differences in the ability of calluses to form generative or vegetative buds consists in the fact that in the photoperiodically neutral Trapesond variety all components of the hormone flowering complex are formed irrespective of day length in all parts of the plants, including the stems from which they are then transferred to the stem calluses (autonomous regulation); however, in the photoperiodically responsive Mammoth and Sylvestris varieties only part of the components of the hormone complex are formed autonomously whereas the others are produced as a result of biosynthesis which occurs in leaves under an optimal day length (photoperiodic regulation). At present the most promising seems to be a stem callus culture from neutral Wisconsin 38 and Trapesond plants. The ability of these calluses to form generative and vegetative buds is variable and changes on alteration of cultivation conditions. Formation of generative buds on stem calluses of these varieties occurs when the glucose concentration in the medium is sufficiently high and becomes more pronounced with increase of the content of the kinetin and some purine and pyrimidine bases. Vegetative buds are formed when the glucose content is low and increase of auxin content accelerates their formation. The callus model of flowering can be applied for comparative investigations of changes in the hormone complex and in protein-nucleic acid metabolism which occur at the transition from vegetative growth to flowering.