The initiation, growth and morphogenesis of wheat tissue cultures.

Investigations were made into the behaviour of wheat tissues in vitro, with regard to the initiation of callus and its subsequent growth and morphogenesis, and with particular emphasis upon defining the factors leading to plant regeneration. Despite quantitative genotypic differences, consistent callus initiation was possible from all parts of the embryo (mature and immature), the young seedling and mature plant, with the excepation of the scutellum, the expanded leaf blade and certain segments of internode (endosperm cultures were not tested). Auxin was essential for callus initiation and growth; the concentration of 2,4-D required for optimum yield of callus depended on the source of explant. Primary cultures had slightly higher auxin requirements than older callus. Typically the primary callus grew rapidly, but increase in fresh weight declined during subsequent passages. Use of several different media had little effect on growth. The time for the culture to double its biomass ("doubling-time") was affected by passage length, hormone concentration and age of the culture. Addition of cytokinins was found to elicit little response other than a slight increase in rhizogenesis. Rhizogenesis was readily stimulated by decreasing the auxin level, but shoot formation was sporadic. A low proporation of shoot-bearing cultures was observed in many separate experiments, under cultures was observed in many separate experiments, under different conditions of hormone treatment and using callus derived from explants of many sources. Shoot formation was more common during the second passage, and occurred either as a single clump from within the callus, or (more rarely) as superficial leafy outgrowths. The former type of shoot formation is suggested to be due to expression of auxin-inhibited primordial, originating from the explants, continuing development once auxin is depleted. Histological examinations revealed the source of callus initiation from different types of explants, and showed the internal structure of organogenetic cultures. Wheat cultures grew well in liquid medium, but as aggregates greater than 1.0 mm diameter; aggregation was little affected by the level of auxin in the medium. In liquid medium, rhizogenesis and greening were the only types of differentiation observed. A project of limited soope inovolving study of the behaviour of uninucleate pollen in cultured wheat anthers demonstrated the low incidence of further mitosis and the rapid loss of pollen viability.