AUTONOMIC DIURNAL CYCLES IN THE WATER RELATIONS OF NONEXUDING DETOPPED ROOT SYSTEMS

Abstract

Intake of water through the stumps of detopped root systems under a water deficit was investigated, using Helianthus annuus plants grown under the normal day-night environment of the greenhouse. Twelve -week-old plants were allowed to wilt, detopped under water, and then in a constant-environment room connected to a water supply in measuring pipettes. The rate of water intake by detopped root systems, termed negative exudation, followed an autonomic diurnal cycle with distinct maxima at midnight and minima at noon. These diurnal cycles continued with decreasing amplitude for about 6 days. This finding is in harmony with observations previously confined to roots showing positive exudation. Replacement of the atmosphere around the roots with N2 or CO2 promptly terminated the cycles. Similar cycles were observed in roots washed free of soil prior to wilting and detopping. This indicates that the cause of the diurnal cycles lies within the root and is not dependent on a periodic transfer of water between root and soil. In some cases, after the internal water deficit in the roots was satisfied, the roots exuded water from the stumps into the supply pipettes during the daytime hrs. and withdrew water at night. This suggests the existence of a diurnal cycle in the hydration of these root systems.