The Role of Root Apices in Shoot Growth Regulation: Support for Neurobiology at the Whole Plant Level? - Communication in Plants: Neuronal Aspects of Plant Life - page 70

conclusion is that any neurobiological activity of a diffuse brain in the root
apex would have been disrupted by freeze-thaw treatment of the root and
was apparently unnecessary for the induction of the leaf growth inhibition
bywaterdeficit.Similarly,thefreeze-thawtreatmentwouldhavedisrupted
the phloem transport system in the root so that active signal transmis-
sion via phloem-related tissues acting as “nerves” was prevented. Chazen
and Neumann (1994) concluded that neither electrical nor chemical sig-
nals from the roots were essential for the rapid root-to-leaf transmission
of information concerning root water status. Instead they proposed that
passive hydraulic signaling via the xylem transport system was involved.
Rapid hydraulic signaling via pressure changes in the xylem may in this
case represent an analogue to animal nervous systems. However, root apex
“brains” are not required for such signaling.
In another set of experiments Snir and Neumann (1997, and unpublished
data) investigated the short-term effects of altered mineral nutrient supply
on leaf growth in maize seedlings with a single primary root about 8 cm
in length, a caryopsis and an emerging first true leaf. The interactive effect
of excising a 1.5 cm section from the root apex was investigated with the
aim of determining whether upward transport of electrical or chemical
signals (e.g., plant hormones such as cytokinins) from the root tip might
be involved in regulating leaf growth. Leaf growth was measured for 15 h
following partial root excision and under different nutrient supply regimes.
Root excision caused a reduction in subsequent leaf growth in seedlings
continuously supplied with mineral nutrient solution (Table 5.2). These
findings suggest that for seedlings supplied with mineral nutrient solution,
root tip excision may have prevented the generation and transport to the
shootofgrowthessentialhormone(s)orelectricalsignalsfromtheroot
tip. Alternatively, tip excision simply reduced root length and surface area,
thereby limiting rates of uptake and transport to the leaf growing zone of
growth-essential mineral nutrients.
When seedlings were grown on a minimal physiological solution (1 mM
CaCl 2 without additional mineral nutrients) leaf growth was slightly re-
duced but root tip excision had no additional growth inhibitory effect. The
nutrients required for the ongoing growth of leaves in the low-nutrient
treatment were presumably transported from reserves in the caryopsis.
The fact that excision of the root apex from low-nutrient seedlings had
no inhibitory effect on leaf growth indicates that hormonal or electrical
signals from any diffuse brain in the root tip were not essential for the
maintenance of leaf growth under these conditions.
In a third experiment, relationships between water deficit and growth
were studied in succulent, photosynthetic stem cuttings of Hylocereus un-
datus ((Haworth) Britton and Rose), a vine cactus from central America
(Nerd and Neumann 2004). This species is grown as a fruit crop in Israel