Root-shoot growth interactions of sorghmn (Sorghwn Bicolor L. Moench) in response to mechanical impedance

Abstract

Soil mechanical impedance has a widespread influence on plant organ expansion, penetration, and growth. Studies on root-shoot interactions 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 gap 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 (UI), 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 a PR of 0. 025 MPa. Matric suction was 5.kPa in both media. Results were that: (i) Impedance to 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 the 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 of the factors responsible for 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.

Keywords: Root-shoot interactions, mechanical impedance, crop establishment, sorghum

Tanzania J. Agric. Sc. (2000) Vol.3 No.1, 11-20