Control of Wheat Root Elongation Growth

Abstract

Pritchard, J., Tomos, A. D. and Wyn Jones, R. G. 1987. Control of wheat root elongation growth. I. Effects of ions on growth rate, wall rheology and cell water relations.—J. exp. Bot. 38: 948–959. The nature of the ions in the bathing medium of hydroponically grown wheat seedlings strongly influenced root growth rate. In 0·5 mol m⁻³ CaSO₄ the growth rate was 32 mm 24 h⁻¹ (used as 100% control rate). K⁺ and SO⅔⁻ ions (10 mol m⁻³) each inhibited extension growth (to about 40% and 70% of the control value respectively). In the absence of K⁺, Cl⁻ greatly reduced the inhibition due to SO₄²⁻. Measurement of tissue plasticity and elasticity in the expanding zone with an Instron-type tensiometer indicated that both were a function of growth rate although relationship of plasticity to growth rate was the steeper and the more pronounced. Turgor pressure at the proximal end of the expanding zone was not correlated to growth, being approximately 0·65 MPa in all treatments. In mature tissue turgor pressure varied with treatment, but was also not related to growth rate. Cell membrane hydraulic conductivity (5 × 10⁻⁷ ± 1·3 (10) m s⁻¹ MPa⁻²) was not influenced by the presence of K⁺. We propose that K⁺ and SO₄^{2 −} influence root growth rates by modulating the rheological properties of the walls of the expanding cell. The physiological significance of these properties is discussed.