An Interpretation of Dose‐response Curves for Light‐induced Cell Elongation in Fern Protonemata

Abstract

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.

The Photobiology of Fern Gametophytes. Ii. The Photocontrol of Filamentous Growth and Its Implications for the Photocontrol of the Transition to Two‐dimensional Growth

“…The general shapes of the quantum dose-A filament length curves for 560, 660, and 720 nm irradiation (Fig. 4,6,8) bear close resemblance to the shapes of published curves (Greany and Miller, 1976). The only important difference is that those authors observed that high doses of 660-nm light elicit a stable optimal response, whereas Fig.…”

“…Intensity measurements were performed periodically in the course of experimentation to confirm that the output of the light source had remained constant. It has been reported that the photoresponses of filamentous gametophytes exhibit dose-response reciprocity within the range of the quantum doses of the wavelengths employed in this study (Miller and Miller, 1967a;Greany and Miller, 1976).…”

Modulation of Gene Activity During Photomorphogenesis of Gametophytes of the Fern, Onoclea sensibilis

“…These results do not implicate phytochrome as the primary photoreceptor in red light-induced filament elongation in this species. Despite the fact that blue light is generally implicated in planar growth, form change in the gametophytes of O. sensibilis is also induced by high intensity red light (Greany and Miller, 1976), while short periods of blue light induce filamentous growth (Miller and Miller, 1964). Although there is no evidence for a "light intensity factor" in the present experiments, one cannot completely eliminate the possibility of a photosynthetic rather than a photomorphogenetic effect oflight in this system.…”

The Control of the Orientation of Cell Divisions in Fern Gametophytes

“…However, this obligate dependence on the blue light receptor does not seem characteristic of all gametophytes. Red light of moderate to high intensity will cause the morphological transition in filaments of Alsophila australis (Mohr and Barth, 1962), Pteris vittata and Osmunda japonica (Kato, 1968), Lygodium japonicum (Raghavan, 1973), and Onoclea sensibilis (Greany and Miller, 1976). These red-lightmediated transitions in growth form argue against Ohlenroth and concept that blue light acts as a specific trigger that induces/derepresses the gene constellation for two-dimensional growth.…”