Cortical Intracellular Electrical Potential in Roots of Unstressed and Stressed Sunflower Seedlings. II. Radial Profiles and Oscillations

Abstract

Various models have been proposed to explain how plants can extract water from dry soil against a gradient in water potential. According to one model, it is hypothesised that water uptake from dry soil is associated with a radial gradient in the intracellular electrical potential (E_c) of root cortical cells such that the potential in the outer cells is relatively hyperpolarised. As a partial test of this model, a microelectrode was used to measure radial profiles of E_c in intact roots of sunflower (Helianthus annuus) seedlings. The seedlings were subjected to either a control (unstressed) or one of two stress treatments. Water stress was rapidly imposed and relieved by lowering and raising the level of nutrient solution in a tank which contained all the seedling roots except the lateral root in which E_c was measured. There was a gradient in E_c in roots of unstressed plants with the potential of the outer cortical cells being relatively depolarised. The imposition of stress shifted the gradient in accordance with the hypothesis. Increasing the magnitude of the osmotic potential of the solution perfusing the measured portion of a lateral root of a stressed seedling resulted in a discontinuity in the profile between the second and third cortical layers. The gradients in the profile of E_c indicate there is a limitation in intercellular electrical coupling and the appearance of a discontinuity may indicate a decrease in coupling. Three types of occasional, spontaneous oscillations in E_c are characterised. One type of oscillation may be additional evidence that the electrical coupling between the cortical cell layers is variable and depends on the transport and status of water in the roots.