Insect Pollination and Helioptropism in Oritrophium limnophilum (Compositae) of the Andean Paramo

Oritrophium limnophilum (Sch.Bip.) Cuatr. (Compositae), a rosette plant species of the paramo (alpine) region of the Venezuelan Andes, has strongly helioptropic flower stalks. Flower heads facing the sun are warmer than heads held away from the sun, and, except at high sun angles, attract more insect pollinators. Because the paramo is frequently cloudy, and because Oritrophium pollinators are generally inactive in cloudy weather, optimization of pollination rates during sunny periods may increase fitness. Oritrophium limnophilum (Sch.Bip.) Cuatr. (Compositae) is a common herbaceous rosette species occuring on seepage slopes and bog edges of the paramo (alpine) region cf the Venezuelan Andes. Each basal rosette produces one to five unbranched flower stalks from 5 to 15 cm high. Each stalk has a single flower head 1.4 to 2.5 cm in diameter. The ray (pistillate) flowers are white, and disc (staminate) flowers are yellow. Ten marked flower heads lasted 31 to 69 days before wilting. Disc flowers in the marked heads began to open several days after the ray flowers; the last disc flowers remained open up to three weeks after ray flowers had wilted. This situation suggests that there may be selection for outcrossing. In clear weather, the stalks show strong helioptropic movement. By mid-morning, the stalks are often inclined approximately 450 from the vertical with flower heads facing east; by mid-afternoon the same stalks may be inclined 450 from the vertical, facing west. In cloudy weather, the stalks remain more or less vertical all day. The flower heads are approximately paraboloid in shape. When they face the sun, the temperatures both of flower tissue, and of any insects sitting on the head surface, are raised above ambient air temperatures (see table 1), (cf. Heinrich and Raven 1972, Hocking and Sharplin 1965, Kevan 1972). It is hypothesized that, in addition to raising tissue temperature and thus speeding flower and seed development, helioptropism, by creating a more favorable microenvironment for insects, may increase the probability of visits by pollinators. Flower heads facing directly into the sun may also be more conspicuous and thus more attractive to pollinators than are shaded heads. In addition, higher flower-tissue temperatures may increase the rate of nectar production, further increasing attractiveness. This hypothesis was tested in Oritrophium limnophilum at Mucubaji, Estado Merida, Venezuela (8048'N), at an elevation of 3550 meters. Observations were made during a three-day period of unusually clear weather. Two groups of 16 flowering plants of the test species were located on a bog edge. Each group occupied an area of approximately 0.5 m2. In the first group, the flower stalks of 8 plants were bent so that the heads were inclined to the east at a 450 angle. The stalks were kept in this position throughout the study period by fastening them to small metal stakes. Near each experimental plant, a control plant was located with a flower stalk of similar age and size. The stalks of control plants were allowed to move freely. The second group of 16 plants was treated similarly, except that the stalks of the 8 experimental plants were bent so that the heads were facing west, rather than east, at a 450 angle. Experimental flower heads were bent at 450 angles so, that, at low sun angles, the difference in shading between control and experimental heads would be optimized, while retaining similar orientation relative to the ground. A between-treatment difference in flower-head angle relative to the ground might, independently of shading, cause differences in number of insect visits, due to differences in visibility of inflorescences. All observations in the morning were made on the group of plants with experimental stalks pointing west, so that 8 shaded (experimental) flower heads could be compared with 8 control heads facing the sun. For the same reason, observations in the afternoon were made on the group with experimental stalks pointing east. During observation periods, the time and duration of each insect visit were recorded, along with the identification number of the plant visited. 1 Present Address: Smithsonian Tropical Research Institute, Box 2072, Balboa, Canal Zone, and Department of Biology, University of Pennsylvania, Philadelphia, Pennsylvania 19174, U.S.A. 284 BIOTROPICA 7(4): 284-286 1975 This content downloaded from 157.55.39.176 on Sat, 09 Apr 2016 06:34:31 UTC All use subject to http://about.jstor.org/terms Air and tissue temperatures were measured on five different days with a potentiometer and blackened thermocouple. Use of a blackened, unshaded thermocouple to measure air temperatures provided an estimate of the effects of flower-head orientation on heat loads of insect visitors (cf. Hocking and Sharplin 1965). Results are presented in tables 1 and 2.