Temporal and spatial aspects of floral determination in seedling terminal buds of the qualitative short-day plant Pharbitis nil were examined using a grafting assay. Floral determination in the terminal buds of 6-day-old P. nil seedlings is rapid; by 9 hr after the end of a 14-hr inductive dark period more than 50% of the induced terminal buds grafted onto uninduced stock plants produced a full complement of flower buds. When grafted at early times after the end of the dark period the terminal buds of induced plants produced three discrete populations of plants: plants with no flowers, plants with two axillary flowers at nodes 3 and 4 and a vegetative terminal shoot apex, and plants with five to seven flowers including a terminal flower. The temporal relationship among these populations of plants produced by apices grafted at different times indicates that under our conditions, the region of the terminal bud that will form the axillary buds at nodes 3 and 4 becomes florally determined prior to floral determination of the region of the terminal bud giving rise to the nodes above node 4.
Bibliographical noteFunding Information:
The authors thank Dr. Anton Lang (Michigan State University, East Lansing, MI) and Dr. Susan Singer (Carleton College, North-field, MN) for advice on grafting techniques, and Dr. Susan Singer, Dr. Wesley Hackett (University of Minnesota), and Nandini Mendu (University of Minnesota) for critical evaluation of the manuscript. In addition, we thank Dr. Michael Simmons (University of Minnesota) for advice on statistical analysis. This work was supportedb y grants from the University of Minnesota Agricultural Experiment Station (MIN-70-030) and U.S. Public Health Service (GM 42557).JC L. was supportedi n part by a postdoctoralf ellowship from the Plant Molecular Genetics Institute of the University of Minnesota. A.D. is the recipient of an American Cancer Society Junior Faculty Research Award (JFRA No. 1’70)a nd a University of Minnesota McKnight Land-Grant Professorship Award.