Title: Interspecific variation in resting bud morphologies and seasonal heteroblastic leaf sequences among temperate, woody plants
Abstract: To observe the continuous organogenesis from a shoot apical meristem that is characteristic of plants, one needs only look inside the resting bud of a temperate, woody plant. The contained sequence of embryonic leaves at different stages of development discloses the bud’s dynamic nature. At the same time, as a dormant structure, a resting bud is demonstrative of the selective pressure on plants in temperate environments only to grow when environmental conditions are favorable. As such, resting buds are frequently regarded as static units that require cataphylls, or bud scales, to protect the packaged preformed embryonic foliage leaves during winter dormancy.
This winter-protective view is challenged by my inventory of the morphological diversity in temperate, woody angiosperm taxa of naked buds that overwinter without cataphyll protection. Naked buds are not, as long presumed, markedly rare in temperate, woody floras. They occur in at least 87 genera in 42 families throughout the angiosperm phylogeny in various morphologically distinct manifestations. The geographic distribution of species with naked buds in temperate areas was found to be associated with summer precipitation, but not with winter climatic variables. These results prompt questions about the direction of evolutionary character transitions of resting bud types in families with temperate, woody representatives, and about the relationship between resting bud structure and summer growth.
Both these topics were investigated in the terminal resting buds of Juglandoideae, the temperate subfamily of Juglandaceae, which includes species that develop cataphylls; do not develop cataphylls; or develop caducous cataphylls that are lost before winter. The complete seasonal heteroblastic sequences of leaf forms in six species of Juglandoideae were characterized and studied comparatively. In a common garden setting over a seven-month growing season, the dimensions of 2249 individual metamers were tracked from first exposure to abscission along the shoots of saplings and mature trees. The origin and developmental timing of metamers within the resting bud were investigated using micro-CT scanning. The occurrence of cataphylls in a heteroblastic sequence is associated with a single cohort of foliage leaves that flush and abscise synchronously. This growing pattern is highly determinate, with next year’s terminal buds initiated even before leaf out in spring. In contrast, in sequences without cataphylls, slightly shorter-lived foliage leaves appear and abscise in a staggered fashion. An ancestral state reconstruction of bud types in the family suggests that its last common ancestor had naked buds and that four independent transitions occurred to groups of taxa with cataphylls and determinate growth.
Chlorophyll fluorescence data recorded for 1581 of the metamers in the survey reveal that the effective light-capture area of species with determinate growth in most cases reaches a peak early in the growing season and slowly declines thereafter. An indeterminate growth pattern, on the other hand, ultimately leads to a steady increase in effective light-capture area several months into the growing season with a peak in late summer. Therefore, the observed determinate and indeterminate modes of sequential leaf development in Juglandoideae, and their associated terminal resting bud types, may be reflective of distinct ecological strategies during the growing season.
While the protective role of cataphylls during the dormant season has been the subject of much speculation, the developmental consequences of inserting cataphylls into a dynamic seasonal sequence of leaf forms potentially reach far beyond winter in Juglandaceae. In a wider context, bringing a leaf economic spectrum mindset to seasonal heteroblasty could increase our understanding of the ecological functioning of shoot development in temperate, deciduous, woody species and their seasonal patterns of carbon sequestration.
Committee: Ned Friedman (Advisor), Missy Holbrook, Andy Knoll, Elena Kramer (Chair)