The determinate and indeterminate dwarf shoots of Pinus longaeva (bristlecone pine)

Abstract

The dwarf shoot of the genus Pinus is normally regarded as a determinate short shoot, but the fate of the dwarf shoot apex after needle maturity and the origin of proliferated dwarf shoots remain controversial. To resolve these issues, 1- to 28-year-old dwarf shoots of P. longaeva were microscopically examined and experimentally manipulated. Eighty-eight percent of the lateral long shoots and all trunks had "proliferated dwarf shoots," or dwarf shoots that had become long shoots. On trunks 44.3% of the dwarf shoots had terminal buds, versus 6.6% on lateral long shoots. Most of these buds were microscopic, "interfoliar buds," derived from the original dwarf shoot apex. With advancing age of dwarf shoots there was no statistically significant change in the percentage of dwarf shoots with buds. The fate of the dwarf shoot apex depends upon the position of the dwarf shoot within its age-class and the type of long shoot. On lateral long shoots, only distal dwarf shoots have interfoliar buds and only these may spontaneously proliferate or be experimentally induced to proliferate into long shoots. Proximal dwarf shoots have a suberized apex. On trunks, both distal and proximal dwarf shoots often have buds and can thus proliferate. Dwarf shoot proliferations therefore arise predictably from interfoliar buds and not adventitiously.