References

60 T. Sachs
4.3
Mechanisms of Competition
The statement that branches of the very same plant compete calls for concrete
mechanisms. These should allow for a comparison of branches and
for the selection of the more promising alternative. Such comparisons must
occur in the absence of a central “brain” or computing center, an organ
which plants lack. Possible alternatives must take into account the fact that
plants can have numerous branches, and it is impossible for each to be
the source of a unique signal. Since competition is influenced by resource
availability, an obvious possibility is that the stronger branches act as sinks
that divert transport towards themselves (Henriksson 2001). This is supported
by evidence that the vascular channels passively transport materials
to the sinks in which they are consumed. Yet both young and mature leaves,
which have opposite sink effects, act to enhance the role of a strong branch
(Fig. 4.3a–c). A more general reason that sink effects could not suffice is
that branch competition is a long-term process, during which new vascular
channels differentiate (Sachs et al. 1993). Vascular differentiation, unlike
short-term transport, is actively oriented so that it connects the dominant
branches with the rest of the plant (Sachs 1991; Berleth and Sachs 2001).
Since hormones and essential substrates move along the vascular tissues,
their long-term transport is in fact oriented towards dominant branches.
When considered in terms of vascular differentiation it is easy to observe
the “conflict” between the vascular connections of branches that develop
on opposite sides of the same axis (Fig. 4.3d–f). There are no direct vascular
contacts between branches: all channels within the plant axis are polar,
connecting shoot and root tissues (Sachs 1991). The larger the branch
the larger its vascular supply and the more axial space this supply occupies.
The causal relations between the branch development and oriented
vascular differentiation can be readily confirmed by branch removal. The
molecular basis for processes of reoriented vascular differentiation could
be dependent on changes in the localization of the products of PIN genes
(Palme and Gälweiler 1999). The suggested role of vascular differentiation
focuses competition on the orientation and activity of cambial cells where
developing vascular systems meet (Sachs et al. 1993). This is not a proof,
however, since the evidence does not show that orientation precedes rather
than follows branch development.
The hypothesis can be taken one step further (Fig. 4.3g–i). Leaves are
known to be sources of the hormone auxin (Sachs 1991; Berleth and Sachs
2001; Ljung et al. 2001). This same auxin induces the differentiation of new
vascular tissues along the axis connecting its source with the roots (Sachs
1991; Berleth and Sachs 2001). Auxin is the only known signal whose local
source actually determines the orientation of these vascular tissues. The