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more modest changes ( Goebel, 1900 ). Heteroblasty has since acquired a
broader meaning, and is now often used to describe any morphological var-
iation along the length of a shoot, independent of the nature or degree of this
change ( Zotz et al., 2011 ). An additional terminological complication is that
the terms “juvenile” and “adult” were initially employed to describe differ-
ent stages of vegetative development ( Goebel, 1889; Hildebrand, 1875 ), but
are now more often used to refer to vegetative versus reproductive
(flowering) shoots. Indeed, some authors have criticized the use of these
terms to describe changes in vegetative morphology ( Jones, 1999, 2001;
Zotz et al., 2011 ), arguing that they should be used exclusively to describe
a change in reproductive competence. As described below, reproductive
competence is controlled by multiple pathways, which interact with the
pathway controlling vegetative maturation at various points. Because veg-
etative and reproductive maturation are equally important aspects of shoot
development and are regulated in concert, it is not obvious why the devel-
opmental state of the shoot should only be defined by one of these processes.
To avoid confusion it has been suggested that vegetative and reproductive
maturation be described using terms specific to each process, for example,
juvenile and adult phases of vegetative development and juvenile and
adult phases of reproductive development ( Poethig, 1990 ). In this chapter
the terms “juvenile” and “adult” will refer specifically to phases of vegetative
development,
that
is, developmental changes
that occur prior
to
floral induction.
3. HETEROBLASTY AND VEGETATIVE PHASE CHANGE
Variation in the morphology or physiology of a shoot can occur for
many reasons. Traits that change in a coordinated fashion at a predictable
time prior to flowering and which are not readily modified by environmen-
tal conditions, are the basis for the division of shoot development into
juvenile and adult vegetative phases. The process responsible for this type
of variation has been termed “ontogenetic maturation” ( Wareing, 1959 )
or “phase change” ( Brink, 1962; Poethig, 1990 ). A second type of variation
is evident in traits that change gradually over the life of the shoot, and
which can be modified by rerooting or by grafting shoots onto a more
vigorous root stock ( Day et al., 2002; Greenwood, Day, & Schatz, 2010;
Mencuccini, Martinez-Vilalta, Hamid, Korakaki, & Vanderklein, 2007 ).
This type of heteroblasty has been termed “physiological aging”
( Wareing, 1959 ), and is thought to be a consequence of an increase in
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