ENDOGENOUS RHYTHM IN OPENING AND ODOR OF FLOWERS OF CESTRUM NOCTURNUM

OVERLAND, LILLIAN. (Missouri Botanical Garden, St. Louis.) Endogenous rhythm in opening and odor of flowers of Cestrum nocturnum. Amer. Jour. Bot. 47(5): 378-382. flius. 1960.The opening and closing of flowers of Cestrum nocturnum have been found to occur in a cyclic manner both in constant light and constant darkness at constant temperature. The strong odor, which in nature occurs at night and was previously believed to be a direct result of darkness, has been shown to exist as an endogenous rhythm, occurring under constant conditions of temperature and light, as well as in darkness. Although in nature the odor cycle is so synchronized by the environment as to have a period of roughly 24 hr. , in constant conditions temperature affects the cycle by lengthening it at low temperature and reducing it at high temperature. The cycle may be disrupted by shifting between extreme temperatures, but it is not destroyed. The controlling mechanism for odor production seems to be located in the tips of the corolla and the cycle exists independently in the excised tips. CESTRUM NOCTURNUM (Night Blooming Jessamine) of the family Solanaceae is commonly known for its abundant, small, yellow flowers which, in nature, open at night and emit a powerful fragrance. This plant, which is a semi-tropical shrub native to islands in the Caribbean, is pollinated by a night-flying insect, obviously attracted to it by its fragrance. Plants grown in insect-free greenhouses, although exhibiting the same cycles of flowering and odor, never form seed due to the absence of pollinating insects. The occurrence of the odor so prominently at night led to the general belief that darkness is an essential factor in its formation, preceded possibly by the production of some necessary intermediates during the light period. The present investigations on the behavior of Cestrum flowers under controlled environmental conditions were undertaken in order to determine the possibility of the association of flower opening and odor production with endogenous rhythms. As described by Biinning (1956), the term endogenous rhythm refers to a biological process which alters periodically in a pendulumlike manner although external conditions remain constant. This phenomenon has long been known to exist in plants, but it is only relatively recently that it has been shown to control a larg,e number of basic physiological processes in both plants and animals. Excellent reviews have been published on this subject by Biinning (1956, 1957) and Harker (1958). MATERIALS AND METHODS. -The material used in these experiments was obtained from cuttings from the same stalk grown in controlled-tempera1 Received for publication August 17, 1959. The author would like to express sincere thanks to Dr. F. W. Went for having made this work possible, as well as for his advice and encouragement. Thanks are also due to Dr. I. I. Chaudhri for his many valuable suggestions, and to Mr. G. P. Keyes for the graphs. ture greenhouses at 230C. -dav and 170C. -night temperatures. Either the entire plant bearing flowers or cut flowers in cups of water were placed in constant-temperature rooms and moved from one condition to another for photoperiod or temperature changes. In the constant-temperature rooms, the average light intensity was 500 ft. -c. The preliminary experiments were crude; the odor was followed merely through the sense of smell, the odor, when fully developed, being unmistakable. The objective confirmation on many occasions by several observers served to validify the basic results. Further definite confirmation of the cyclic behavior was provided by time-lapse photography of the opening and closing of flowers under constant conditions. For this purpose a Bollard movie camera with either Cine Kodak Fine Grain Positive film for light, or High Speed Infrared plus infrared lights for darkness, was used. The flowers were placed at a distance of about 21,/? 1 ft. from the lens, and an exposure of 8 sec. was t ken automatically every 10 min. by means of a time clock. An electrical time clock, included in the photograph, accurately recorded the time of each exposure. Projection of the film on a screen revealed a distinct opening and closing of the flowers in a rhythmic manner. The angle of opening was then measured, with 1800 taken as the fully open position. OPENING AND CLOSING OF FLOWERS IN CONSTANT CONDITIONS. -Constanlt temnperature and light. -A plant bearing young flowers, as well as separate cut flowers were photographed at 170C. in constant light at 10-min. intervals over a 5-day period. The behavior of the intact flowers as well as of cut ones was essentially the same, showing a distinct cyclical opening and closing. The results, as shown in fig. 1, indicate the existence of a 27-28-hr. cycle which corresponds very well with the odor cycle as followed by the senses.