Reproductive biology of Vincetoxicum rossicum (Kleo.) Barb. (Asclepiadaceae), an invasive alien in Ontario1

Journal of the Torrey Botanical Society, Jan-Mar 2004 by St Denis, Melissa, Cappuccino, Naomi

HAND POLLINATION. Potted V. rossicum were grown in the greenhouse for six months from seed collected at several local sites. When these plants had begun to bloom, four flowers at a single node on each of seventeen plants were randomly assigned to the following treatments: 1) emasculation-removal of all five pollinaria, 2) self-pollen-emasculation and insertion of five pollinaria from another flower of the same plant, 3) cross-pollen-emasculation and insertion of five pollinaria from another plant, and 4) unmanipulated control. Pollinaria were removed by inserting an entomological pin (no. 000) beneath the corpusculum and pulling upward and outward. In the self- and cross-pollination treatments, a pollinarium was placed in each of the five nectar-filled cavities that lie directly beneath the corpuscula. Fruits forming from each of the flowers were noted. all other flower buds were removed from the plants as they were formed so that resources would be allocated to the four experimental flowers/fruits. When the fruits were ready to dehisce, the seeds were counted, weighed and planted to test for germinability as described in the previous experiment.

STATISTICAL ANALYSKS. all analyses were performed using JMP version 3.2.5 (SAS Institute Inc. 1995). To assess the effect of isolation on visits by pollinators in the two censuses of naturally growing plants, as well as in the observations on potted plants, two response variables were calculated for each plant: the proportion of flowers that were visited (missing at least one pollinarium) and the mean number of pollinaria removed per flower. Because these variables were not normally distributed, separate Wilcoxon/Kruskal-Wallis rank-sums tests were used to test the effects of field and treatment.

Likelihood ratio khgr;^sup 2^ tests (JMP's logistic regression platform) were performed to test the effects of parent plant and visitation (visited/not visited) on whether or not the flowers in the potted-plant experiment produced fruits. For the nine plants that produced two or more fruits, we used 2-way ANOVA to examine the effect of parent plant and flower visitation (yes/no) on the following response variables: number of seeds produced per fruit, mean seed weight per fruit, proportion of seeds germinating per fruit and mean number of seedlings produced by the seeds of a fruit. The proportion of seeds germinating was arcsin transformed prior to analysis.

Likelihood-ratio tests were used to determine whether fruit production (yes/no) differed for the four treatments in the hand-pollination experiment. ANOVAs were then used test for an effect of pollination treatment on seed weight, arcsintransformed proportion germinating and mean embryony.

Results. EFFECT OF ISOLATION ON VISITS BY POTENTIAL POLLINATORS. On 15 june, the censused plants had from 2-13 open flowers (7.3 � 0.25, mean � SE). Isolated plants tended to have more open flowers (7.6 � 0.3) than plants in dense patches (6.7 � 0.44); however, this difference was not statistically significant (ANOVA: F^sub [1,88]^ = 3.45; P = 0.06). Of 661 flowers, 158 (23.9%) were missing at least one pollinarium. The proportion of flowers that were visited, as well as the mean number of pollinaria removed was higher for isolated plants than for plants in dense stands (Table Ia, b). Field was not a significant predictor of either proportion of flowers visited (Kruskal-Wallis [chi]^sup 2^ = 1.46; d.f. = 2; P = 0.480) or mean number of pollinaria missing (Kruskal-Wallis [chi]^sup 2^ = 1.03; d.f. = 2; P = 0.597).