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

Plants censused on 5 july had from 1-8 open flowers (3.68 � 1.86). There was no difference in the number of open flowers between isolated plants and those in dense patches (ANOVA: F^sub [1,116]^ = 2.29; P = 0.133). Of 435 flowers, 89 (20.5%) had been visited at least once. Field had no effect on the proportion of flowers visited (Kruskal-Wallis [chi]^sup 2^ = 4.18; d.f. = 2; P = 0.120) and a slight but nonsignificant effect on the number of pollinaria removed (Kruskal-Wallis [chi]^sup 2^ = 5.25; d.f. = 2; P = 0.075). As in the first census, flowers of isolated plants were significantly more likely to have been visited and had more missing pollinaria than plants from dense monocultures (Table la,b).

Like the naturally growing plants, flowers on potted plants placed in dense stands had more missing pollinaria than those placed among four neighbors in a small patch or those isolated from other plants (Table Ib). The proportion of flowers visited did not differ significantly among the three treatments (Table Ia). The field into which we placed the potted plants did not have a significant effect on either of these response variables (Kruskal-Wallis tests, P > 0.6 for both variables).

EFFECT OF FLOWER VISITATION ON FRUIT SET. Of the 156 marked flowers on the 16 potted plants, 39 (25.0%) produced a mature fruit. For the 15 plants that had more than one flower, there was no significant parental effect on the probability that a flower would produce a mature fruit (Logistic model: Likelihood ratio [chi]^sup 2^ for effect of parent = 21.87; df = 14; P = 0.080). Visitation, on the other hand, had a significant effect on fruit production; of the flowers that had been visited, 38.2% produced mature fruits, whereas only 17.8% of flowers that were not visited produced fruits (Logistic model; Likelihood ratio [chi]^sup 2^ for effect of visitation = 4.87; df = 1; P = 0.027).

EFFECT OF VISITATION ON SEED PRODUCTION, WEIGHT, GERMINATION AND EMBRYONY. For the nine potted plants that produced more than one fruit, parent plant had a significant or marginally significant effect on seed production, mean seed weight, proportion of seeds germinated, and mean number of embryos emerging from each seed (Table 2). Whether or not a fruit was the product of a visited flower was a significant predictor of only the average embryony of seeds in that fruit (Table 2). Fruits from visited flowers contained seeds that produced 1.72 � 0.054 (SE) seedlings; those from nonvisited flowers produced 1.54 � 0.068 seedlings.

HAND POLLINATION. The emasculation treatment was not 100% effective; however it resulted in a significant reduction in the percentage of flowers forming fruits (Table 3; Likelihood ratio [chi]^sup 2^ = 20.8, P = 0.0001). Percent fruit production in the other three treatments-self-pollen, cross-pollen and control-did not differ significantly (Table 3; Likelihood ratio [chi]^sup 2^ = 3.18, P = 0.20). The three fruits that were formed by flowers in the emasculation treatment were single follicles. In the other three treatments 79.9% of the fruits had two follicles; there was no difference in the proportion of two-follicle fruits among these three treatments (Likelihood ratio [chi]^sup 2^ = 2.30, P = 0.316).

The number of seeds per fruit did not differ significantly among treatments (ANOVA: F^sub [3,36]^ = 2.60, P = 0.070); however, since only three fruits were produced in the emasculation treatment, the power to detect differences between this treatment and the other three is low. Comparison of the other three treatments-cross-pollen, self-pollen and control-revealed no significant difference among treatments (ANOVA: F^sub [2,34]^ = 0.615, P = 0.546). No differences in seed weight were observed among the cross-pollen, self-pollen and control treatments (ANOVA: F^sub [2, 34]^ = 0.354, P = 0.705). Neither the arcsin-transformed germination rate nor the mean number of embryos emerging from seeds differed significantly among these three treatments (ANOVAs germination rate: F^sub [2, 34]^ = 0.244, P = 0.785; embryony: F^sub [2, 34]^ = 1.82, P = 0.161).