LEAF INITIATION AND SHOOT APICAL DIAMETER IN RELATION TO PHASE-TRANSITION IN CINERARIA

Transition from the juvenile to the competent phase of vegetative deve lopment 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 (T-b), opt imum (T-o) and maximum (T-m) temperatures for leaf initiation in plant s grown at constant temperatures in growth rooms were derived to be 3. 6, 19.8 and 38.1 degrees C respectively in cv. Cindy Blue, and T-b = 3 .6, T-o = 19.7 and T-m = 37.8 degrees C in cv. Cindy Dark Red. The the rmal time required for the initiation of each leaf in the juvenile pha se was 87 degrees Cd in cv. Cindy Blue and 78 degrees Cd in cv. Cindy Dark Red, calculated from the cardinal temperatures. Juvenile developm ent in cv. Cindy Blue was completed after 6-7 leaves had been initiate d, which occurred 565 +/- 43 degrees Cd after radicle emergence and in cv. Cindy Dark Red juvenility ended after initiation of 7-8 leaves at 585 +/- 39 degrees Cd from radicle emergence. Thermal times for leaf initiation and for the completion of juvenility derived from growth ro om data were validated in plants grown under variable environmental co nditions in glasshouses. Shoot apical diameter (< 0.15-0.18 mm) was li nearly related to leaf number in glasshouse-grown plants during juveni le and transition phases regardless of the time of year but after phas e transition, this relationship became environmentally dependent.