What does it cost a plant to produce floral nectar?

To understand the adaptive nature of floral nectar production it is necessary to determine for individual plants the associated costs and benefits in terms of growth and/or reproduction^1-3. Nectar production may use up to 37% of a plant's available energy^4,5 but might not affect growth or reproduction. I report here that removal of nectar from flowers of Christmas bells (Blandfordia nobilis) increased the plant's net nectar production but reduced its ability to produce seeds. To our knowledge this is the first demonstration that nectar production entails a cost to a plant in terms of growth and/or reproduction and that both the gains and costs associated with nectar production may be estimated in the same 'currency' (seeds). As a plant's nectar production increases, there should therefore be a trade-off between pollinator-mediated increases in numbers of fertilized seeds^1-3 and decreases in seed number due to the costs of producing the nectar.