References

5 Essentiality of root-shoot communication 69
Table 5.1. Presence or absence (+ or –) of characteristics which may be associated with
neurobiological activity in photosynthetic organisms
Kelps Green algae Mosses Higher plants
Plasmodesmata + + + +
Vascular tissue +/– – +/– +
Hold fast + – – –
Rhizoid – +/– + –
Root – – – +
system “nerves”, remains to be determined. An obvious additional explanation
for the success of the higher plants is that differentiated root systems
and vascular tissues increase plant capacity to effectively access and/or distribute
water, mineral nutrients and organic solutes to all parts of the whole
plant structure.
5.4
Do Plant Shoot Responses to Environmental
Stresses Require Rapid Root-to-Shoot Signaling?
Experiments in which any brainlike activity in the plant root apex or chemical
and electrical signal transmissions via vascular tissues are inactivated
may reveal the degree to which neurobiological activity is essentially involved
in the functioning of the whole plant. In this section, three experiments
which examine the effects of partial or complete inactivation of the
roots on shoot growth responses to environmental change are reviewed.
In the first experiment we examined growth and physical characteristics
of the emerging first leaf of young maize seedlings with a single primary
root, shortly after imposing a defined water deficit regime [addition to hydroponic
root medium of a nonpenetrating osmolyte, poly (ethylene glycol)
6000 (PEG) at –0.5-MPa water potential)]. Water deficit rapidly inhibited
leaf growth and this inhibition was maintained for hours and days. The water
deficit treatment also induced (within minutes) associated decreases in
the extensibility characteristics of the expanding cell walls in the leaf elongation
zone (Chazen and Neumann 1994). Most importantly in the present
context, similar leaf responses were observed when the seedling roots were
killed by freeze–thaw treatment with liquid nitrogen prior to imposing water
deficit, i.e., prior to PEG addition. PEG does not penetrate the cell walls
of live or killed roots and effectively decreases water availability in each
case (Chazen et al. 1995). Supportive findings were obtained in additional
experiments on wheat seedlings (Neumann et al. 1997). An inescapable