Abstract:
Soil mechanical impedance has a widespread influence on plant organ expansion, penetration, and
growth. Studies on root-shoot intera'ctions in relation to mechanical impedance have only investigated
the effect on shoots of mechanical impedance imposed on roots. The aim of the reported study was therefore
to fill the identified iap in knowledge, including an investigation into all root-shoot interations in response
to mechanical impedance. Individual pregerminated sorghum (Sorghum bicolor L. Moench)
seeds cultivar ICSV-I12 were grown for 8 days in a growth room in seven replicate cylinders per treatment.
Treatments were: (a) impeded root and impeded shoot (II), (b) impeded root and unimpeded shoot
(lU), (c) unimpeded root and impeded shoot (VI), and (d) the control, in which both the root and shoot
systems were unimpeded (UU). The impeding growth medium was a mixture of sand and vermiculite
packed to give a penetration resistance (PR) of 1.18 MPa below the seed and 0.32 MPa above the seed.
Control cylinders were completely packed with expanded vermiculite to a bulk density of 0.2 Mg m-3 giving
aPR ofO. 025 MPa. Matnc suction was 5.kPa in both media. Results were that: (i) Impedance tg the
shoot significantly (P < 0.05) delayed emergence, more so when the root was, also impeded. (ii) Shoots
emerging through a mechanically impeding layer, had significantly greater extension rates after emergence
than unimpeded ones. (iii) Mesocotyls became significantly thicker only when (he root systems
were impeded. (iv) Impeding the shoot system, significantly increased root extension rate. (v) The
length of the first internode, the number of leaves and the spacing of lateral roots were not changed by
any of the treatments. Root-shoot signalling is suggested as one ofthefactors responsiblefor these interactions.
Generally, our findings indicate that mechanical impedance which may be caused by sUrface
crusting and hardsetting soils and shallow tilth achieved with a hand hoe results in poor crop establishment
and probably total. crop failure.
