How Can Plants Choose the Most Promising Organs? - Communication in Plants: Neuronal Aspects of Plant Life - page 59

Fig. 4.3. a - c Theinfluenceofsimulatedherbivoryonshootrelations.The numbers are
theratiosofthefreshweightsoftheinitiallysmallerovertheinitiallylargershootafter
adding the expected weight of removed leaves; they should be compared with the control
of 0.37 (Fig. 4.1d). a Removing a mature leaf at an early stage from the somewhat larger
shootresultedinthetwoshootsbeingmoreorlessequal. b Removing a young, expanding
leaf from the larger shoot had a larger effect than removing a mature, photosynthetic leaf.
c An expanding leaf was removed from the larger shoot and a mature leaf from the smaller
shoot. The two shoots grew to be essentially equal, dramatically changing the outcome seen
in the controls. d - f The evidence of vascular differentiation. The lines are only indications
of overall structure, not a record of all the numerous vascular channels. d The cambial
space that had originally been occupied by vascular channels ( broken lines )thatledtothe
removed seminal shoot (Fig. 1a) was taken over by new connections of the new growing
shoots with the roots. There were no direct contacts between the new shoots. e Where the
two shoots were unequal their vascular contacts and the space they occupied were correlated
with their size and developmental rates. f When a weak shoot died the space occupied by its
vascular channels, but not the channels themselves, was taken over by the growing shoot.
g - i . An hypothesis: the choice between shoots is mediated by hormonal relations with the
roots.Theshootsarethesourceofsignalsthatinformtheplantabouttheirstate.The
response to these signals that they receive from the roots limits their development. g In
the cut plant essential supplies from the root are present in excess, and the shoots do not
compete. h As the shoots develop, their young and, to a lesser extent, their mature leaves
increase their signals and their requirements. A large proportion of the supplies from the
roots are diverted towards the larger shoot and they enhance its continued development.
i A positive feedback between development and continued development ensues and an
increasing proportion of the resources are diverted towards only one of the two shoots.
(Based on Sachs and Novoplansky 1997)
removal of a photosynthetically active leaf (Fig. 4.3b). A plant forced to
“choose” between two damaged branches discriminated against the one
that had lost a young rather than a mature leaf (Fig. 4.3c).
4.2.5
Hypothesis: Branches Compete
The results of the experiments with pea seedlings suggest that branches of
thesameplantcompeteforresources,theirgeneticidentitynotwithstand-
ing (Sachs et al. 1993; Sachs 2004). This competition is temporarily lifted
when the shoot/root balance is disrupted by the removal of the seminal
shoot and is intensified when the plants are in stress conditions. Com-
petition means that the plants not only respond to local environmental
conditions, such as low light and local leaf damage, but they also actually
“choose” for continued development the most promising of the available
alternatives. An important criterion for this choice is rapid development,
which predicts future rather than immediate photosynthetic performance.