Effects of pollination limitation and seed predation on female reproductive success of a deceptive orchid

Abstract

For many species of conservation significance, multiple factors limit reproduction. This research examines the contributions of plant height, number of flowers, number of stems, pollen limitation and seed predation to female reproductive success in the deceit-pollinated orchid, Cypripedium candidum. The deceptive pollination strategy employed by many orchids often results in high levels of pollen limitation. While increased floral display size may attract pollinators, C. candidum's multiple, synchronously flowering stems could promote selfing and also increase attack by weevil seed predators. To understand the joint impacts of mutualists and antagonists, we examined pollen limitation, seed predation and the effects of pollen source over two flowering seasons (2009 and 2011) in Ohio. In 2009, 36 pairs of plants size-matched by flower number, receiving either supplemental hand or open pollination, were scored for fruit maturation, mass of seeds and seed predation. Pollen supplementation increased proportion of flowers maturing into fruit, with 87 % fruit set when hand pollinated compared with 46 % for naturally pollinated flowers. Inflorescence height had a strong effect, as taller inflorescences had higher initial fruit set, while shorter stems had higher predation. Seed predation was seen in 73 % of all fruits. A parallel 2011 experiment that included a self-pollination treatment and excluded seed predators found initial and final fruit set were higher in the self and outcross pollination treatments than in the open-pollinated treatment. However, seed mass was higher in both open pollinated and outcross pollination treatments compared with hand self-pollinated. We found greater female reproductive success for taller flowering stems that simultaneously benefited from increased pollination and reduced seed predation. These studies suggest that this species is under strong reinforcing selection to increase allocation to flowering stem height. Our results may help explain the factors limiting seed production in other Cypripedium and further emphasize the importance of management in orchid conservation.

Keywords: Conservation; orchid; plant reproduction; plant–insect interactions; pollen limitation; pollination ecology; reproductive trade-offs; seed predation; supplemental pollination..

Figure 1.

Effect of hand and open pollination treatments on the mean per cent of initial and final fruit set. Plants receiving supplemental pollen produced higher fruit set for both the initial time period (Hand mean = 0.87, SE = 0.037; Open mean = 0.46, SE = 0.06; t = −4.11, n = 36, P < 0.001) and final time period (Hand mean = 0.87, SE = 0.033; Open mean = 0.46, SE = 0.057, t = −4.30, n = 36, P < 0.001).

Figure 2.

Relationship between per cent fruit set (number of fruits/number of flowers) of open-pollinated plants and the mean height of flowering stems. Linear regression, % fruit set = −0.34 + 0.037 × avg. height flowering stems, F_1,35 = 7.0, P = 0.012, R² = 0.17.

Figure 3.

Relationship between the percentage of matured capsules preyed upon (number of capsules preyed upon/number of capsules produced) and the mean height of flowering stems of all plants in the study. Linear regression, % capsules preyed upon = 1.57–0.033 × avg. height flowering stems, F_1,61 = 10.12, P = 0.023, R² = 0.144.

Figure 4.

Effect of pollination treatments on initial and final fruit set in the 2011 study. N = 30. Mean fruit set for the initial (Self = 0.633, SE = 0.084; Outcross = 0.433, SE = 0.084; Open = 0.166, SE = 0.084) and final fruit maturation times (Self = 0.466, SE = 0.08; Outcross = 0.40, SE = 0.08; Open = 0.1, SE = 0.08) were significantly different as indicated by Tukey–Kramer HSD. Treatments separated by letter indicate significant difference. ANOVA: initial fruit set: F_2,89 = 7.73, P < 0.001; final fruit set: F_2,89 = 5.73, P = 0.004.

Figure 5.

Effect of pollination treatment on seed mass (g) per fruit. The outcross (mean = 0.027, SE = 0.001) and open-pollinated plants (mean = 0.027, SE = 0.002) produced significantly higher seed mass than the self-pollinated plants (mean = 0.017, SE = 0.001) as indicated by Tukey–Kramer HSD, N = 29. Treatments separated by a different letter are significantly different. ANOVA: F_2,28 = 18.72, P < 0.001, R² = 0.59.