Photophysiology of turion germination in Spirodela polyrhiza (L.) Schleiden IX. Interaction of light and gibberellic acid in the removal of dormancy

Abstract

The effect of gibberellic acid (GA 3), exogenously applied in the range 10 −8–10 −4 M, on the germination of turions of Spirodela polyrhiza grown in continuous white light (fluorescence lamps) or in darkness (etiolated turions) was investigated. The germination was studied in darkness, after a short red light irradiation, or in continuous white light. Non-dormant (after-ripened) as well as dormant (freshly harvested) turions were tested. In non-dormant turions GA 3, applied at concentrations greater than 10 −6 M, inhibits germination independently of the light conditions. Dormant turions do not germinate in complete darkness or after a single red light pulse and this inhibition cannot be overcome by GA 3 application. However, in continuous white light, in continuous red light or with short daily applied red light irradiation (5–120 min) GA 3 acts synergistically in the light-stimulated germination of dormant turions. As shown by far-red light reversibility this light effect is mediated by the photoreceptor phytochrome. However, the possibility that other photoreceptor(s) could be involved cannot be excluded. No specific light-sensitive phase was detected by red light step-down experiments. Synergism between the far-red light absorbing form of phytochrome (P fr) action and GA 3 is only obtained if P fr is allowed to act for several days. In the absence of GA 3 the P fr presentation time required to induce a certain germination response in dormant turions is significantly shorter than that required for the synergistic interaction in the presence of GA 3.