Nectar secretion dynamic links pollinator behavior to consequences for plant reproductive success in the ornithophilous mistletoe Psittacanthus robustus

Abstract

The mistletoe Psittacanthus robustus was studied as a model to link flower phenology and nectar secretion strategy to pollinator behaviour and the reproductive consequences for the plant. The bright-coloured flowers presented diurnal anthesis, opened asynchronously throughout the rainy season and produced copious dilute nectar as the main reward for pollinators. Most nectar was secreted just after flower opening, with little sugar replenishment after experimental removals. During the second day of anthesis in bagged flowers, the flowers quickly reabsorbed the offered nectar. Low values of nectar standing crop recorded in open flowers can be linked with high visitation rates by bird pollinators. Eight hummingbirds and two passerines were observed as potential pollinators. The most frequent flower visitors were the hummingbirds Eupetomena macroura and Colibri serrirostris, which actively defended flowering mistletoes. The spatial separation between anthers, stigma and nectar chamber promotes pollen deposition on flapping wings of hovering hummingbirds that usually probe many flowers per visit. Seed set did not differ between hand-, self- and cross-pollinated flowers, but these treatments set significantly more seeds than flowers naturally exposed to flower visitors. We suggest that the limitation observed in the reproductive success of this plant is not related to pollinator scarcity, but probably to the extreme frequency of visitation by territorial hummingbirds. We conclude that the costs and benefits of plant reproduction depend on the interaction strength between flowers and pollinators, and the assessment of nectar secretion dynamics, pollinator behaviour and plant breeding system allows clarification of the complexity of such associations.

Reproductive biology and nectar production of the Mexican endemic Psittacanthus auriculatus (Loranthaceae), a hummingbird‐pollinated mistletoe

“…; P. schiedeanus , Ramírez & Ornelas ; P. robustus , Guerra et al . ). Flowers of P. auriculatus are long (38 mm), bright red‐pink in colour, petals form a floral tube which remains tubular with age, filaments and style are erected favouring pollination by hummingbirds while removing nectar from the top (Castellanos et al .…”

“…), Tristerix corymbosus (Aizen ) and three Psittacanthus species (Azpeitia & Lara ; Ramírez & Ornelas ; Guerra et al . ). The observed reproductive system in P. auriculatus flowers allows self‐ and cross‐pollination; however, protandry, male‐biased nectar production and variation in nectar offered in P. auriculatus flowers are traits that promote cross‐pollination and reduce geitonogamy (see also Fisogni et al .…”

“…90–120 μl per flower at the beginning of anthesis (Guerra et al . ). The floral tube (petals partially forming a nectar chamber) of P. auriculatus flowers (19.6 mm) is larger than in P. schiedeanus (8 mm; Ramírez & Ornelas ) but smaller than those of flowers of P. robustus (20–30 mm; Guerra et al .…”

“…In contrast, Guerra et al . () found that flowers of P. robustus naturally exposed to flower visitors set significantly fewer seeds than flowers exposed to hand‐, self‐ and cross‐pollination. They suggested that the limitation observed in the reproductive success of P. robustus was not related to pollinator scarcity, but the high frequency of visitation by territorial hummingbirds favoured geitonogamy.…”

“…(Guerra et al . ). In these studies, the breeding system and nectar production patterns are described to assess the interaction between the mistletoe and its floral visitors.…”