PS 65-116
The reproductive consequences of deception in the orchid Cypripedium candidum
Attraction of pollinators in deceit-pollinated species often relies on producing a conspicuous floral display. Cypripedium candidum is a non-rewarding orchid that provides no nectar reward to floral visitors, but produces a variable number of single-flowered stems, depending on age and available resources. Higher numbers of flowering stems may increase visibility to pollinators, but in-turn may increase selfing. The lack of a nectar reward may discourage pollinators from foraging on multiple flowers on a single plant, while encouraging pollinia dispersal. Understanding the factors limiting pollination and how deceit pollination influences reproductive success will provide important insight into the evolution of deception in flowering plants.
In order to address the role of deception in C. candidum, a nectar addition experiment was conducted. Six 100m transects were established across a population at the study site. At 20m intervals along each transect a randomized block design was employed that randomly assigned the four closest plants to the transect point to receive one of four histochemical dyes. Two individuals selected to receive nectar received 2μl of 25% sucrose solution in the labellum of the flower while the other two individuals received no additional nectar. Plants were scored for pollen receipt and removal every two days throughout the flowering period. The density of surrounding nectar producing plants was assessed by measuring and identifying the distance to the three nearest nectiferous plants. Number of fruits produced, fruit mass and fruit abortion were scored at the end of the experiment.
Results/Conclusions
Receipt of self and out-cross pollinia were measured using Mann-Whitney U tests blocked by quadrat, due to the lack of normality within the samples. The addition of nectar significantly increased (p<0.0001) self-pollination by nearly 3x (mean of 0.26 vs. 0.78 for non-nectar and nectar plants respectively). Plants not receiving nectar had significantly greater (p<0.0001) numbers of non-self pollinia deposited on the stigmatic surface (mean of 0.80 vs. 0.22 for non-nectar and nectar plants respectively). The color of dye used in the experiment had no significance on any variables measured. There was no significant difference (p=0.0645) in fruit set amongst the nectar treatments, although there was a positive trend for the non-nectar plants to set more fruit. The presence of nectar had no significant effect on the number of pollinia removed. This study demonstrates the role deception plays in increasing the rate of outcrossing and provides insight into why this physiologically expensive floral design may have evolved.
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