AN INTERPRETATION OF DOSE-RESPONSE CURVES FOR LIGHT-INDUCED CELL ELONGATION IN FERN PROTONEMATA

Protonemata of Onoclea sensibilis and Diyopteris filix-mas elongate in response to both red and far-red light. The promotion caused by far-red is larger than that caused by red light. This phenomenon differs from a typical response to phytochrome, the photoreceptor pigment immediately suggested by the activity of red and far-red light. The phenomenon has been explained by two different hypotheses, one of which holds that phytochrome is solely responsible for the response, whereas the other postulates an interaction between phytochrome and P₅₈₀, a yellow-green light absorbing pigment, to account for the response. The hypothesis that phytochrome is the sole photoreceptor leads to some specific predictions concerning the shapes of the dose-response curves for light-induced protonema elongation. These predictions were tested with both continuous and short-term irradiation. In all instances saturating far-red light caused greater elongation than did saturating red light, and no dose of red light duplicated the activity of saturating far-red light. Other experiments tested the interactions of red and far-red light and the effects of different doses of yellow-green light on protonema elongation. The results of many of the experiments were not in agreement with the hypothesis that phytochrome is the sole photoreceptor, whereas they were in agreement with the assumption that filament elongation is controlled by both phytochrome and P₅₈₀.