Intracellular Mechanism of Growth Inhibition by Radiant Energy

Abstract

The intracellular mechanism by which visible radiation inhibits stem growth in intact plants has been investigated in Pisum sativum, variety Alaska. Various physical properties of the stem tissue were measured 3 hours after the onset of red radiation. This is within 1 hour after growth inhibition begins, allowing measurement of the direct effects of radiation inhibition. Osmotic pressure of the growing tissue was measured by a plasmo-lytic method and osmotic pressure of expressed sap was measured thermoelectrically. No difference was found between dark-grown and irradiated tissue. No difference was found in water permeability of dark-grown and irradiated tissue cells. No difference was found in the diffusion-pressure-deficit of dark-grown and irradiated tissue. Cell wall plasticity was the only parameter correlated with growth inhibition. It decreased markedly at the time of growth inhibition. Plasticity was measured as the irreversible bending of turgid tissue and as the irreversible extension of frozen-killed tissue. It is concluded that visible radiation inhibits stem elongation by limiting stem cell plasticity. When glbberellic acid is applied to the plants prior to irradiation, plasticity of irradiated plants remains identical to that of dark-grown plants. Application of gibberellic acid to dark-grown plants has no effect on their plasticity. It is concluded that irradiation reduces the effective level of endogenous gibberelin, and a high level of gibberellin is required for maximum cell wall plasticization.