Abstract:

Recent research suggests that the timing of flowering and leafing out are genetically controlled. It remains unknown, however, whether it is genetic or environmental factors that control the flowering-leafing phenorhythm, and whether this character can be linked to the systematic evolution of a group. In this study, the flowering-leafing phenorhythm type of 42 Rhododendron species belonging to 4 subgenera, 4 sections and 13 subsections were determined conserved ex situ at two sites with different elevations in Dujiangyan, Sichuan Province, China. The phenorhythm of the Rhododendron species was divided into five types: (1) flowering before leafing (F→L); (2) flowering ahead and covering part time of leafing (F+L); (3) leafing covers flowering (L∈F); (4) leafing ahead and covering part time of flowering (L+F); and (5) leafing before flowering (L→F). We tested the following hypotheses: flowering-leafing phenorhythm type is a heritable trait for Rhododendrons, and phenorythm is related to evolutionary processes resulting in an evolutionary sequence among the five types. The results showed that Rhododendron species at the same site or at different sites displayed only one flowering-leafing phenorhythm type that did not change throuth time, thereby supporting the hypothesis that phenorhythm is heritable. Floristic components, proportion of Rhododendron species at different elevations, and the different taxa showed that the phenorhythm type is related to the phylogenetic evolution supporting the second hypothesis. The primitive species are present at the mid- elevation range and the advanced species are present at a lower and higher range. The evolution of these phenorhythm types agree with life form and leaf trait evolution. Environmental stress, growing season length and light resource availability could be selective pressures driving phenorhythm type evolution. Overall, the results indicate that the timing sequences of phenological events are more stable than the specific timing of any single phenological event in response to climate change.

Key words: flowering-leafing phenorhythm, phenology, phylogenetic evolution, Rhododendron