Leaf Movement Regulated by Light Condition in Oxalis triangularis

Abstract

Oxalis triangularis, called the love plant, exhibits circadian rhythms of leaf movement by moving its leaves to upward (open) in the light and downward (close) in the darkness, which is well known as a nyctinastic movement in leguminous plant. In this study, the effects of various light conditions on leaf movement of O. triangularis were evaluated in accordance with the changes of light condition as following: transfer 1) from constant darkness to various light intensities, 2) from various dark durations to constant light, or 3) from various light durations to constant darkness. Period exposed to light or darkness as well as light intensity affected response rate to open or close in O. triangularis. A further increase in light intensity between 7.5 and 80 μmol･mּ-2･s-1 resulted in rising in opening rate and degree with a similar response. Dark-light transition gradually decreased opening rate with increased dark duration in response to the constant light intensity, and 24 hour-darkness brought about 8 minute-delay in the half maximal opening degree as compared to response in dark adapted leaves for 2 hour. Light-dark transition mostly made the leaflets close completely within 100 minutes with nearly the same tendency except for 1 hour-light exposure. With one hour-light exposure, leaf closing rate was significantly decreased, taking leaves 150 minutes to reach the complete closing phase. As a result of the simulated lines by fitting, half maximal closing degrees were obtained as 48 and 85 minutes after light exposure for 24 and 1 hour-dark treatment, respectively. Despite darkness without light, periodic leaf opening and closing were observed within a period of approximately 26 h, showing gradual delay in period and decrease in opening degree. Also, another experiment was carried out to find an external light perception region which was directly related to leaf opening in O. triangularis. The result show that light-receptive part is not the leaflet moving upward or downward but pulvinus located between petiole and leaflet.