Landscape scale variation in nectar amino acid and sugar composition in a Lepidoptera pollinated orchid species and its relation with fruit set

Abstract

Summary1. Understanding landscape scale variation in reproductive and pollination success is a major aim of plant population biology. A potential determinant of reproductive success that has received surprisingly little attention so far is variation in nectar chemical composition across plant individuals and populations. Using the Lepidoptera pollinated fragrant orchid (Gymnadenia conopsea) as a model species, we aimed at providing a first account of landscape scale variation in nectar amino acid and sugar composition, its environmental drivers, and how it may affect plant fruit set. 2. Nectar was sampled from 986 flowers from 296 individuals across eleven discrete populations of the G. conopsea. The proportions of sugars and amino acids were determined using high-performance anion-exchange chromatography. Nectar composition was related to soil characteristics of the populations, and to fruit set, using linear mixed models.3. Approximately, 20% of the variance in nectar traits was situated between populations, whereas the highest proportion of variance (c. 45%) was found among flowers within individuals. Soil carbon and nitrogen content affected both nectar concentration and composition. Furthermore, fruit set of G. conopsea individuals was found to be significantly related to nectar amino acid and sugar composition. 4. Synthesis. These results show that landscape scale variation in nectar amino acid and sugar composition should be taken into account in future studies of plant reproductive success. Furthermore, there is also high within-plant variation in both nectar amino acid and sugar composition. This variation may reduce geitonogamous pollination, but it may also limit rapid pollinator-mediated selection on nectar composition.

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

“…A variety of long-tongued Lepidoptera species pollinate G. conopsea including diurnal and nocturnal moths and butterflies (Gustafsson 2000;Huber et al 2005;Sletvold et al 2012a). Fruit set of G. conopsea in the study area is pollen and not resource limited as supplemental hand pollinations significantly increased fruit set to nearly 100 % (Meekers et al 2012;Gijbels et al 2014).…”

“…During mid-summer G. conopsea produces one dense spike of 10-80 pale pink to intense violet flowers (Meekers et al 2012). Flowers have a long spur half filled with sucrose-dominant nectar, rich in AAs (Gijbels et al 2014) and produce scent composed of 45 volatiles, including the physiologically active benzyl acetate, eugenol and benzyl benzoate (Huber et al 2005). Each flower contains two sectile pollinia, which adhere to pollinators singly or in pairs.…”

“…Nectar AA and sugar composition of all samples were analysed with high performance anion exchange chromatography with pulsed amperometric detection analysis on an ICS3000 chromatography system (Thermo Scientific, Dionex, Sunnyvale, CA) (Gijbels et al 2014). When nectar samples arrived at the lab they were diluted with 10 μl NaN 3 /water (0.02 % w/v) and frozen (−20 °C) until further analysis.…”

“…Samples of different populations and treatments were analysed in a random order. Nectar AA and sugar concentrations of individual plants were determined as the average over all sampled flowers from that individual, as previous publication showed high variation between flowers (Gijbels et al 2014). …”

“…To conclude, AAs can be expected to elicit specific pollinator behavioural responses, and mediate the dilemma of attracting sufficient pollinators while avoiding too intensive pollinator visits (González-Teuber and Heil 2009;Nepi 2014). Two recent observational studies of the Lepidopterapollinated orchid Gymnadenia conopsea have revealed the importance of nectar AAs in mediating pollen transfer, showing that nectar AA composition affected fruit set (Gijbels et al 2014(Gijbels et al , 2015. However, the precise effects that nectar AA concentration and composition can have on pollen flow patterns and plant reproductive success are still largely unknown.…”