How Can Plants Choose the Most Promising Organs? - Communication in Plants: Neuronal Aspects of Plant Life - page 54

branched systems, similar to those seen in the upper parts of the very same
trees. Reiterated developmental processes are thus only a framework that
is subject to long-term modification.
The purpose here is to consider the mechanisms that determine which
branches are retained. This question calls for careful, repeated experi-
ments in defined conditions, for which trees are hardly suitable. The work
described here is therefore based on a model system, using seedlings of
an annual plant. The results suggest that branches compete, and possible
mechanisms of this internal competition are briefly considered. The discus-
sion proposes that competition and selection which follows rather than pre-
cedes initial development is common at all levels of biological organization.
4.2
An Experimental Model Demonstrates Branch Competition
4.2.1
The Experimental System
The development of annual plants has been used extensively for studies
of the inhibitory influence of a growing shoot on bud growth (Snow 1937;
Sachs 1991; Cline 1994). The choice of a successful branch from among
a number that have started growing does not, however, usually occur in
annual plants. A possible evolutionary reason is that branch shedding is not
adaptive in plants with a short life span (Novoplansky et al. 1994). A valuable
model system has nonetheless been developed in annuals (Snow 1931;
Sachs 1966; Novoplansky et al. 1989). It uses the unstable state of a plant
regenerating after severe damage. The first or seminal shoot was removed
from pea seedlings (Fig. 4.1a). This treatment distorted the balance between
the shoot and root systems of the growing plant, and, as in cut trees, shoot
number was increased (Sachs et al. 1993; Sachs 2004). Two buds grew
into more or less equal shoots or branches (Fig. 4.1b,c). Gradually these
two shoots became increasingly unequal (Fig. 4.1d). The smaller shoot
stopped growing and in some varieties it eventually died, returning the
plant to a single axis state (Fig. 4.1e; Sachs 1966). The deterioration of
the smaller shoot was prevented whenever the larger shoot was removed
(Fig.4.1f),demonstratingthatinteractionsbetweentheshootsareamajor
determinant of their individual fate.
“Two-shoot” pea seedling are an experimental system in which it is pos-
sible to ask how the plant chooses the shoot that continues developing. The
following simple experiments demonstrate that there is no key parame-
ter. Instead, the plants appear to be integrating information from varied
sources.