Seasonal Variation in the Nectar Sugar Concentration and Nectar Quantity in the Western Prairie Fringed Orchid, Platanthera praeclara (Orchidaceae)

Abstract

The endangered western prairie fringed orchid, Platanthera praeclara, is found in remnant tall grass prairie in the central plains of North America. Platanthera praeclara is pollinated by several species of hawkmoths (Sphingidae) in Canada. There is minimal overlap between the flight period of hawkmoth pollinators and the flowering period of the Canadian orchid population, with orchids normally flowering at the end of the hawkmoth flight period. This study was designed to determine if orchids produce more nectar and higher concentrations of sugars in the nectar at the beginning of the flowering period coinciding with maximal pollinator flight times. As pollinators are nocturnally active, we also determined if orchids produce more nectar or nectar with increased concentrations of sugars during the night versus the daytime. Nectar sugar concentration and the amount of nectar per flower were highest at the beginning of the flowering period, and significantly decreased as the season progressed. Nectar volume increased in the evening and during the night but sugar concentration remained relatively unchanged. We examined nectar volume and sugar concentration by flower position, as the majority of the flowers encountered by pollinators in the first few days of the bloom period were lower-positioned flowers due to the sequential flowering nature of the orchid. Within the inflorescence, lower-positioned flowers had higher sugar concentrations in comparison to higher-positioned flowers at the start of the flowering period, but sugar concentrations converged as the season progressed. There was no difference in the amount of nectar present in low- or top-positioned flowers. Orchids may show increased nectar sugar concentrations and amounts at the beginning of the flowering period to capitalize on the presence of hawkmoth pollinators.