Biology Reference
In-Depth Information
1. INTRODUCTION
During its postembryonic development, a plant becomes established as
a young seedling, increases in size and complexity, undergoes sexual repro-
duction, and eventually senesces and dies. This process involves gradual
quantitative changes, as well as more rapid qualitative changes that occur
at particular times in shoot development. The most obvious and best under-
stood of these transitions is the switch from vegetative to reproductive
development, which is accompanied by the production of novel reproduc-
tive structures, such as flowers or cones (
Amasino, 2010; Andres &
Coupland, 2012; Huijser & Schmid, 2011; Wilkie, Sedgley, & Olesen,
2008
). This transition is preceded by a change in the competence of the
shoot to respond to stimuli that induce reproductive development, and
by changes in a variety of other traits, including leaf and stem morphology,
growth rate and orientation, branching patterns, and disease or herbivore
resistance. Variation in these latter traits has been described in many different
species, starting with the observations of Goethe and Knight in the eigh-
teenth century (
Goethe, 1790; Knight, 1795
). But—in contrast to the
vegetative-to-reproductive transition—the molecular mechanism of these
vegetative changes is still largely unknown. Here, the author presents some
of the major questions about this process that remain to be answered, focus-
ing on the phenomenon of vegetative phase change in herbaceous plants.
The extensive literature on this topic in vines and woody plants has been
discussed in several excellent reviews (
Day, Greenwood, & Diaz-Sala,
2002; Doorenbos, 1965; Greenwood, 1995; Hackett, 1985; Lee &
Richards, 1991; Schaffalitzky de Muckadell, 1954; Zotz, Wilhelm, &
Becker, 2011
), and readers should consult these for other perspectives on
this problem.
2. TERMINOLOGY
Hildebrand (1875)
and
Goebel (1889)
were the first to recognize that
shoot development can be divided into juvenile and adult stages on the basis
of species-specific vegetative traits such as leaf shape, the orientation of
branch growth, and the capacity for sexual reproduction. Goebel noted that
the degree of variation in these traits varied considerably between species,
and coined the term “heteroblasty” to describe species that undergo major
morphological changes, and “homoblasty” to refer to plants that display
