The giant extra-floral nectaries of carnivorous Heliamphora folliculata: Architecture and ultrastructure

Extra-floral nectaries commonly occur in carnivorous plants, forming pitfall traps to attract nectar-feeding insects. Although they are not connected with pollination, extra-floral nectaries promote the reproductive functions of carnivorous plants by increasing the supply of animal-sourced nutrients and thereby increasing the plant's vigor. Our main purpose here was to study the functional ultrastructure of the giant nectaries in Heliamphora, focusing on nectar production and secretion. We wanted to determine whether specialization of the shape and structure of Heliamphora nectar spoons has an influence on nectary structure. Heliamphora folliculata, with its unique nectar storage chamber, may also have specialized giant nectaries differing from other species in the genus. In Heliamphora folliculata the largest nectaries occur in a nectar storage chamber. Regardless of their size, the nectaries have similar ultrastructure. Key features of their cells are ER-sheathed leucoplasts and vacuoles with large osmiophilic phenolic inclusions. The former is characteristic for cells producing monoterpenes; indeed, the giant nectaries produce volatile compounds and may have a function similar to osmophores. Nectary cells are isolated from ordinary parenchyma cells by cutinized walls lacking plasmodesmata (endodermis). Symplastic transport is possible only between nectary cells and special parenchyma cells that have wall thickenings. Between them are many plasmodesmata; thus the nectary is a symplastic and apoplastic field. These specialized parenchyma cells are similar to the flange cells described in parasitic plants. Why has a special spoon with a nectar chamber evolved in Heliamphora folliculata? One answer given is that it protects nectar against being washed away by frequent rainfalls so that the plant produces less nectar and saves energy. Also, when nectar is not easily accessible the insects have to spend more time near the trap entrance to look for it, and they are more likely to be trapped. Regardless of the shape and structure of Heliamphora nectar spoons (pitcher appendages), giant nectaries apparently have the same architecture throughout the genus. So far as is known, pollinator-prey conflict does not exist in Heliamphora; nectaries in this genus are formed only for nectar-feeding prey.