Flowers, phenology and pollination of the endemic New Zealand greenhood orchid Pterostylis brumalis

Abstract

ABSTRACTGreenhood orchids are most diverse in New Zealand and Australia, with at least some species pollinated by sexual deception of male fungus gnats. Preliminary investigation of endemic New Zealand Pterostylis brumalis via field phenology, hand-pollination and insect trapping revealed P. brumalis is self-compatible but relies on insects for cross pollination. Natural fruit-set was low, averaging 2.6% across two populations. High fruit-set in hand-pollinated flowers (66.7%) indicates severe pollinator limitation. In comparison, only one collected insect, a female Mycetophila vulgaris, bore orchid pollinia. Potential pollinator attractants (colour and scent) were analysed. UV reflectance may enhance pollinator detection of the flower against the background, but visual modelling generated conflicting results: a categorical fly vision model indicated excellent camouflage against herbivory, but a generic tetrachromat vision model suggested flowers are easily distinguishable against the background. Both models have flaws that create issues with drawing conclusions, and identification of the pollinator is required to understand their visual system further. Scanning electron microscopy (SEM) revealed putative scent-releasing osmophores (lateral sepals) and uniform papillate trichomes (labellum) that may mechanically direct pollinators into the floral trap. The trap is a pressure sensitive labellum that temporarily traps the visitor inside the flower.