Influence of elevated CO2 and ultraviolet-B radiation levels on floral nectar production: a nectary-morphological perspective

Abstract

Investigations of the effects of two global events – elevated CO2 levels and enhanced ultraviolet-B (UV-B) radiation – on floral nectar production are reviewed from twelve dicotyledonous families. Furthermore, to allow comparisons between nectary morphology and nectar production in treated plants of these fifteen species, new data on floral nectary structure are provided for Malcolmia maritima (L.) R. Br. (Brassicaceae) and Scabiosa columbaria L. (Dipsacaceae). All but the last taxon possessed mesenchymatic floral nectaries with surface stomata. Few clear relationships existed between nectary morphology and various physiological responses to CO2 or UV-B enrichment, indicating that species responded notwithstanding nectary structure itself. Overall, nectar-solute concentration was least affected by elevated CO2 or UV-B radiation; consequently, changes in nectar volume were responsible for differences in nectar-sugar production per flower. Three species of Fabaceae experienced no change in floral nectar production upon exposure to elevated CO2. To date, no study of enhanced UV-B radiation reported a consistent reduction in floral nectar production; three species of Brassicaceae responded differently, but various levels of ozone depletion were simulated. Experimentation with more taxa – including those possessing nectary types such as septal (gynopleural) nectaries (e.g. many monocotyledons) or aggregations of glandular trichomes – and expanding such physiological studies to species possessing extrafloral nectaries, are recommended.