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72 P.M. Neumann
resources from mature to young stems that facilitated the maintenance
of growth. Conversely, the fact that a phloem girdle between mature and
young stems inhibited the growth of the young stems reinforces the importance
of phloem transport tissues in interorgan communication. Although
root functioning did not appear to be essentially involved in stem growth
regulation, phloem transport clearly was. The growth regulatory function
of phloem in this case could again be considered analogous to the signaling
facilitated by the nerves of animals. However, the 3-week time scale
required for this shoot growth response to relatively rapid dehydration of
the root environment to become established was totally different from the
seconds required for most animal responses to environmental changes.
5.5
Conclusions and Future Perspectives
Some support is provided in this chapter for the drawing of analogies
between the signaling roles of plant long-distance transport systems and
animal nervous systems. However, the specific findings reviewed here do
not indicate that root apices function as essential neurobiological command
centers involved in regulating shoot growth responses to adverse
changes in the root environment. Cell-to-cell signaling via plasmodesmatal
connections and variations in long-distance transport of hormones,
essential nutrients and water via vascular tissues may conceivably provide
the regulation needed to integrate most higher-plant growth responses to
environmental changes.
References
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