Vegetative phase change and photosynthesis in Eucalyptus occidentalis: architectural simplification prolongs juvenile traits

To understand the effect of shoot architecture on vegetative and reproductive phase changes, seedlings of Eucalyptus occidentalis (Myrtaceae) were grown as free-branching or as single-stem plants, the latter treatment resulting from the continual removal of axillary vegetative buds. In E. occidentalis, vegetative phase change was characterized by increasing leaf length/width ratios. In contrast to the behaviour of other woody species subjected to architectural manipulation of this kind, vegetative phase change was faster in branched plants than in single-stem plants, which continued to exhibit juvenile leaf morphology throughout the duration of this study. However, the first appearance of flowers occurred approximately synchronously in both treatments after 9 months, demonstrating that vegetative phase change and floral transition are developmentally uncoupled in this species. Leaf morphological changes that characterized phase change lagged behind changes in leaf anatomy and gas exchange. In single-stem plants with juvenile leaves, leaf intercellular airspace (20.9%) was almost double that in branched plants with adult foliage (11.2%). Photosynthetic gas exchange analyses indicated that the juvenile leaves of single-stem plants had greater Rubisco and electron transport capacities than those of free-branching plants. Higher leaf N concentrations were recorded in single-stem plants than in branched plants. These observations support the hypothesis that the complexity of shoot architecture impacts the rate of vegetative phase change, but does not affect reproductive phase change in this species.