To examine flower opening and closing of a Portulaca hybrid, flower buds were placed in darkness for 12 h (2030-0830 h) at 20 mC and then exposed to various light-temperature conditions. Flower buds exposed to light at 25, 30 or 35 mC opened within 1 h, and wilted 10-14 h later. Flower buds exposed to light at 20 mC started to open after 4 h but opened slowly and not completely. Flower buds subjected to 25, 30 or 35 mC in darkness also opened rapidly, but did not reach full opening. Flowers opened at 30 mC in light, and partially closed and opened repeatedly in response to cycles of a 2-h exposure to 20 mC and a 2-h exposure to 30 mC at any time between 1000 to 1600 h. Similar phenomena were observed when the flowers opened at 30 mC in light and then were subjected to darkness and light alternately at 30 mC, although the effect of light was less obvious than that of alternating temperature. Flower opening and closing were not affected by relative humidity. These results indicate that a rise in temperature is required for rapid flower opening in the buds kept at 20 mC, and that light intensifies the effect of high temperature. Exposure to light without a temperature change delayed and slowed flower opening which was never complete. The involvement of an endogenous rhythm in flower opening by Portulaca is indicated.
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Kazuo Ichimura (1998) studied this question.