A dry soil is generally a hard soil. Thus, the effects of water stress
and mechanical impedance on plant growth are
difficult to separate. To achieve this we have developed a growth cell that
allows manipulation of the strength of
growth media (i.e. mechanical impedance) without altering the availability of
water or nutrients. We monitored
leaf elongation rates of barley and wheat seedlings before and after the
mechanical impedance to root growth was
increased. Results show that a large and rapid reduction (within 10 min) of
leaf elongation rates occurred after
impedance to the roots was increased. The average reductions for barley and
wheat, with associated standard
errors, were 22·6% (4·84) and 36·2% (5·48),
respectively. The data are consistent with the hypothesis that
mechanical impedance of roots might have a direct negative effect on leaf growth
even where nutrients and water
are in plentiful supply to the plant. The implications of the rate of the
response are examined with respect to the
underlying mechanisms controlling root–shoot signalling.