Leaf Initiation and Shoot Apical Diameter in Relation to Phase Transition in Cineraria

Abstract

SummaryTransition from the juvenile to the competent phase of vegetative development in cineraria cvs Cindy Blue and Cindy Dark Red was coincident with a two-fold increase in leaf initiation rate and the attainment of a critical shoot apical diameter of 0.15-0.18 mm. The base (Tb), optimum (T0) and maximum (Tm) temperatures for leaf initiation in plants grown at constant temperatures in growth rooms were derived to be 3.6, 19.8 and 38.1°C respectively in cv. Cindy Blue, and Tb = 3.6, T0 = 19.7 and Tm = 37.8°C in cv. Cindy Dark Red. The thermal time required for the initiation of each leaf in the juvenile phase was 87°Cd in cv. Cindy Blue and 78°Cd in cv. Cindy Dark Red, calculated from the cardinal temperatures. Juvenile development in cv. Cindy Blue was completed after 6–7 leaves had been initiated, which occurred 565 ± 43°Cd after radicle emergence and in cv. Cindy Dark Red juvenility ended after initiation of 7–8 leaves at 585 ± 39°Cd from radicle emergence. Thermal times for leaf initiation and for the completion of juvenility derived from growth room data were validated in plants grown under variable environmental conditions in glasshouses. Shoot apical diameter (˂0.15-0.18 mm) was linearly related to leaf number in glasshouse-grown plants during juvenile and transition phases regardless of the time of year but after phase transition, this relationship became environmentally dependent.