Geographical Transition of Sylleptic/Proleptic Branching in Three Cinnamomum Species with Different Bud Types

Abstract

Three native Japanese Cinnamomum(Lauraceae) species exhibit either sylleptic and proleptic, or proleptic branching depending on species-specific types of bud-structure (hypsophyllary and scaled-buds) and geographical distribution. Major branching shifted from sylleptic at the axils of cataphylls and hypsophylls in the southernmost C. doederleinii(latitudinal distribution range 24–28° N) to both sylleptic at cataphyllary- and proleptic at hypsophyllary-axils in mid-latitudinal C. daphnoides(27–33° N), and to proleptic at the axils of foliage-leaves in the northernmost species C. japonicum(25–37° N). In all three Cinnamomum species, the sylleptic and proleptic branches expand around the boundary between two successive yearly shoot-modules. The predominant, fertile axils having branches shifted from cataphylls (n year) and hypsophylls (n−1 year) in C. doederleinii and C. daphnoides, to foliage-leaves (n−1) in C. japonicum. The buds of proleptic branches at the axils of hypsophylls and foliage-leaves inevitably experience winter dormancy, hence the shoots are equipped with protective cataphylls instead of a hypopodium. Shoot length and leaf number gradually increased from the most distal cataphyllary sylleptic (n) shoots towards the most basal proleptic shoots at the axils of foliage-leaves (n−1). Proleptic shoots at the axils of foliage-leaves perform similarly to main shoots in all three Cinnamomum species studied. The relative number of leaves borne on proleptic shoots gradually increased from C. doederleinii, to C. daphnoides, and C. japonicum. Under the seasonal, short growing periods of temperate latitudes, the increased shoot performance and bud protection of proleptic shoots are made possible through the earlier onset of bud formation (prolepsis) at the axils of the previous year's hypsophylls and/or foliage-leaves in trees with rhythmic growth.