Experimental fertilization increases amino acid content in floral nectar, fruit set and degree of selfing in the orchid Gymnadenia conopsea

Abstract

Floral traits have evolved to maximize reproductive success by attracting pollinators and facilitating pollination. Highly attractive floral traits may, however, also increase the degree of self-pollination, which could become detrimental for plant fitness through inbreeding depression. Floral nectar is a trait that is known to strongly mediate pollinator attraction and plant reproductive success, but the particular role of the nectar amino acid (AA) composition is poorly understood. Therefore, we experimentally manipulated the nectar AA composition and abundance of the Lepidoptera-pollinated orchid Gymnadenia conopsea through soil fertilization, and we quantified AA content and AA composition through high performance anion exchange chromatography with pulsed amperometric detection. Mixed models were then used to evaluate differences in pollinia removal, fruit set, seed set and degree of selfing between fertilized and control individuals. Selfing rates were estimated using microsatellite markers. We found that fertilized individuals had a significantly higher nectar AA content and an altered AA composition, whereas plant height, number of flowers, nectar volume and sugar concentration remained unchanged. Fertilized individuals also had significantly more pollinia removed and a higher fruit set, whereas control plants that did not receive the fertilization treatment had significantly fewer selfed seeds, and more viable seeds. Although we cannot exclude a role of changes in floral scent following the fertilization treatment, our results strongly suggest a relation among nectar AA composition, fruiting success and selfing rates. Our results also indicate potential consequences of nutrient pollution for plant reproductive success, through the induced changes in nectar AA composition.

Nutrient enrichment is associated with altered nectar and pollen chemical composition in Succisa pratensis Moench and increased larval mortality of its pollinator Bombus terrestris L.

“…For instance, Hoover et al . [13] showed that bumblebees were more attracted to a nectar solution mimicking nectar of plants under nitrogen fertilization, despite a twenty percent reduction in survival rate of individual workers compared to workers fed by control nectar solutions. Therefore, elucidating the precise effect of amino acid and sugar composition of pollen and nectar on pollinator health and fitness merits further research.…”

“…Nevertheless, our results are in line with the results of Hoover et al . [13], who found a higher mortality of bumblebee workers feeding on a controlled synthetic amino acid and sugar solution mimicking that of fertilized plants, free from the possible confounding effects of secondary metabolites.…”

“…Food quality of floral resources is primarily contingent upon the sugar and amino acid composition of nectar and pollen, which is known to greatly vary within plant species [12, 13]. The quality of nectar and pollen can be influenced by altered environmental conditions.…”

“…However, while loss of plant diversity results in reduced food quantity, which is being unraveled by a growing number of studies as discussed above, little is known regarding the effects of reduced food quality following nutrient pollution. Nevertheless, there is scant evidence that food quality may deteriorate under nutrient pollution and that, in response, pollinators actively change their foraging behavior [13, 23]. For instance, nectar composition of individuals of the orchid Gymnadenia conopsea that were growing on more fertile soils contained lower nectar amino acid diversity through an increase in relative abundance of glycine and serine [24].…”

“…For instance, nectar composition of individuals of the orchid Gymnadenia conopsea that were growing on more fertile soils contained lower nectar amino acid diversity through an increase in relative abundance of glycine and serine [24]. This was further experimentally corroborated by a fertilization experiment, showing similar changes in nectar amino acid content and higher self-pollination in fertilized plants [13]. …”