LEAF INITIATION RATES AND VOLUME GROWTH RATES IN THE SHOOT APEX OF CHRYSANTHEMUM

The leaf initiation rate and apical volume growth rate of lateral shoots of Chrysanthemum inorifolium 'Improved Albatross III' were determined for plants growing under controlled environment conditions. After removal of the terminal bud, the top two lateral buds produced leaves at a rate of 1.4 to 1.6 leaves/day for the first 9 or 10 days. At about the 10th day there appeared to be an abrupt shift to a lower initiation rate of 0.7 to 0.8 leaves/day. Defoliation by periodic removal of leaves larger than 1 cm caused a statistically significant increase in initiation rate, but not to as high a rate as that of the first 10 days of bud growth. Volume growth rates of the apical region were determined from transverse sections of five apices, according to the method of Richards (1951). The apical region was found to have a radial expansion rate of 0.1623/plastochron and a vertical expansion rate of 0.0494/plastochron. The volume growth rate was calculated to be 0.3740/plastochron and the volume doubling time to be 1.85 plastochrons or 56 to 63 hr. IN THE STUDY of shoot apical growth several parameters of growth can be determined directly, and from these other parameters can be derived. However, relatively few such quantitative studies of growth in the shoot apex have been carried out. The rate of leaf primordia initiation and the rate at wvhich leaf primordia grow away from the center of the apical dome can be determined directly (Richards, 1948, 1951). Other parameters which could be determined directly include ontogenetic or seasonal changes in cell volume and apical volume. From such information estimates can be made of growth rates, cell division rates, and ontogenetic changes in growth rates and division rates. This paper presents some quantitative information about the growth of the vegetative chrysanthemum apex. The overall objective of this series of studies was to determine the rate(s) of cell division in the vegetative shoot apex and to relate these rates to the rates of leaf initiation and apical volume growth. The specific objective of the work presented in this paper was the determination of leaf initiation rate, volume growth rate, and the degree of constancy of growth rate. MATERIALS AND METHODS-Culture of plant mater-ial-Cuttings of Chrysanthemum morzfolium Bailey 'Improved Albatross III' were provided by Cal-Florida Chrysanthemum Propagators, 1 Received for publication 5 June 1968. Supported in part by a National Science Foundation Cooperative Graduate Fellowship granted to the senior author at the University of California, Davis, and in part by NSF Grant GB-6591 to the junior author; submitted as part of a doctoral dissertation. 2 Present address: U.S.D.A., Forest Service, Forest Fire Laboratory, Riverside, California 92507. Fremont, Calif. The rooted cuttings were selected for uniformitv of size, vigor, and root mass. They were potted singly in 4-inch clay pots in a potting mixture consisting of 50 % sand, 25 % peat, and 25 % fir bark (Baker, 1957). The pots were then placed in the greenhouse for 2-4 weeks and were watered daily with one-half-strength Hoagland's solution (Hoagland and Arnon, 1950). The plants were again selected for uniformitv and the terminal part of the stem was removed (pinched), leaving eight to nine leaves on the plant. After final selection the plants were placed in a homemade controlled environment chamber. The light intensity in the chamber was 13001500 ft-c at plant height. The light source was provided by 24 T-8 cool-white fluorescent and 14 60-w incandescent bulbs placed 18 inches above the plants. Daylength was kept at 20 hr of continuous light and 4 hr of darkness in order to provide a maximum growth rate and to maintain the plants in a vegetative state. Later the light intensity was increased, and the plants used in later experiments were grown under 1800-2000 ft-c. The rate of leaf initiation at the higher intensity was compared with that under the lower light-intensity regime. The temperature was held constant at 20 ?4 1.5 C. The plants were watered daily with one-half-strength Hoagland's solution. Leaf initiation rate-The top two lateral buds of the pinched plants quickly became dominant over the lower buds. Because of the obvious differences between top buds and lower buds, only the top two buds were used. Samples of 10-16 buds were collected from the population at intervals of 2 or 3 days from the beginning of the growth period (in all observations and experi-