Plants can respond to soil conditions in ways that can
not
readily be explained in terms of the ability of the roots
to
take up water and nutrients. Roots may sense difficult
conditions
in the soil and thence send inhibitory signals to the
shoots which harden the plants against the consequences
of
a deteriorating or restrictive environment, especially if
the
plants water supply is at risk. Generally, this behaviour
can
be interpreted as feedforward responses to the soil
becoming
too dry or too hard, or to the available soil volume
being
very small as with bonsai plants, or to roots becoming
infected with pathogens. However, soil that is too soft or
in
which the roots are forced to grow in very large pores
can
also induce large conservative responses, the significance
of
which is unclear. The inhibitory signals may affect
stomatal
conductance, cell expansion, cell division and the rate
of
leaf appearance. Their nature is still under debate, and
the
debate is becoming increasingly complex, which probably
signifies that a network of hormonal and other responses
is
involved in attuning the growth and development of a
plant
to its environment.
