Journal Number 100
August 2006

EDITORIAL

Carlos Lehnebach's Papers On The Pollination Of NZ Orchids
By Ian St George

The Chilean postgraduate student Carlos Lehnebach wrote his Masters thesis on the Pollination ecology of New Zealand orchids, and from it has co-authored, with his colleagues at Massey University, two important papers, one on epiphytes and one on terrestrials.

His methods were similar in both studies: he studied live plants in the wild, and among other things tested self-pollination by bagging flowers; tested for apomixis by removing pollinia, then bagging; tested for self-compatibility by bagging until the stigma was receptive, hand-pollinating with pollinia from the same inflorescence, then re-bagging; cross-pollinated by hand; and observed natural pollination. He stained flowers looking for osmophores (scent glands).
He looked for pollinators in the field. In the epiphytes he assessed nectar production.  

EPIPHYTES

Earina autumnalis and Earina mucronata are self-compatible, whereas Earina aestivalis and Winika cunninghamii appear to be partially self-incompatible. All four orchids are incapable of autonomous selfing and are therefore completely dependent on pollinators to set fruits.

Floral visitors observed in Earina belong to Diptera, Coleoptera and Hymenoptera and to Diptera and Hymenoptera in W. cunninghamii. Thus, unlike many epiphytic orchids in the tropics, the orchid-pollinator relationship in these orchids is unspecialised and flowers are visited by a wide range of insects. Putative pollinators are flies of the families Bibionidae, Calliphoridae, Syrphidae and Tachinidae.

All four orchids display anthecological adaptations to a myophilous pollination system such as simple flowers, well-exposed reproductive structures, easily accessed nectar and high pollen : ovule ratios.

Some snippets of general interest

None of these species set seed through apomixis or autonomous self pollination; therefore they depend entirely on pollinating agents for their reproduction.

Osmophores (scent glands) were difficult to identify by staining in E. autumnalis (despite its strong fragrance); top and base of column in E. mucronata and E. aestivalis, as well as tip of lip under nectary in E. mucronata. Column wings, around stigma and lateral lobes and ridges of labellum in Winika.

Nectaries: E. autumnalis has two short crescent shaped ridges near the base of the labellum; these leading down to a small, brightly coloured pit, the nectary. E. mucronata and E. aestivalis have, near the base of the labellum, two inconspicuous ridges leading down to a small pit where the nectary is located. In Winika, the three lobed labellum is attached to the column by a column foot, where the nectary is located.

The energy content of the nectar produced by these orchids per flower is substantial, and similar to that of NZ plants that attract honeyeater birds.

Nectar seems to be the only reward offered; the pollinia are strongly packed and waxy, and very difficult to break apart mechanically so are unlikely to be harvested by flower visitors.

Many insects visit the flowers and that may reflect the rich reward they offer, but it also makes them more vulnerable to visits by nectar thieves - exotic or generalist nectar feeding insects - causing pollen loss.

These orchids may be significant nectar sources for the local insect community, especially E. autumnalis. This orchid was the only rewarding species flowering from autumn to early winter near the population studied.

E. autumnalis was visited by seven insect species, mainly dipterans, but only the cosmopolitan syrphid fly Eristalis tenax was captured twice carrying the four pollinia attached to the mouthparts, and is regarded as a probable pollinator.

Thus pollination of these NZ epiphytic orchids is neither highly specialised nor species specific. All four appear to be primarily pollinated by generalist Dipterans. They have simple, scented, white yellowish flowers, with easy access to the nectary, well exposed reproductive structures and high pollen/ovule ratios - all considered as floral adaptations to this pollination syndrome. The most outstanding feature of the reproductive biology of NZ flora is the large proportion of plants pollinated by dipterans - more than on any other landmass of continental origin.  

TERRESTRIALS

The researchers studied the pollination ecology of four terrestrial orchids: Gastrodia
cunninghamii (or was it G. "long column"?), Thelymitra longifolia, Pterostylis alobula, and
P. patens. Reproduction of these orchids relies on contrasting reproductive strategies.

Thelymitra longifolia is predominantly self pollinated, whereas both Pterostylis species are cross pollinated and have an absolute dependence on pollinators. T. longifolia, P. alobula, and P. patens are self compatible. Results for G. cunninghamii were unclear. Insect visits are uncommon in these species and were observed only in G. cunninghamii and P. alobula.

Aphids were usually found inside the flowers of G. cunninghamii, but the role they may have as pollinators is undetermined. In P. alobula, male fungus gnats of Zygomyia (Mycetophilidae: Diptera) were considered pollinators. The two Pterostylis species are more likely to be adversely affected by disruption of the plant pollinator mutualism because of the specialist nature of the plant pollinator interaction.  

Snippets

All 4 spp. are scentless to humans, but all stained in a way that suggested scent glands are present - especially on Pterostylis, around the column, lip and sepal tips.

T. longifolia, P alobula and P. patens are self compatible. Autogamous self pollination occurred only in G. cunninghamii and T. longifolia. Natural fruit set was low in the Pterostylis, especially in P. alobula.

Male fungus gnats of the genus Zygomyia were found alive and dead in P. alobula.

T. longifolia has a mixed pollination strategy: it is autogamous, but cross pollination can occur. Indeed, flowers may remain open after deposition of pollen on the stigma, in case a pollinator (a burrowing bee, Leioproctus fulvescens) should visit. Self pollination is a fallback position in case bad weather or competition from other flowers makes pollinators scarce.

P. alobula probably attracts its male fungus gnat insect pollinator by sexual deceit, with the emission of a species-specific sexual attractant; pseudocopulation or sapromyophily seem unlikely. The insect enters, triggers the labellum, is imprisoned, and then crawls down attracted by light entering via the clear "windows" of the flower's base, then up the tunnel formed by the labellum and column wings, to detach pollen and emerge to pollinate another flower. This may be the first species-specific insect pollinator system recorded for New Zealand.

The reproductive success of P. alobula is limited by (1) winter flowering, when insects may be scarce. It adapts by long floral persistence (a month for individual flowers), a long flowering season (4 months), and a high count of pollen grains; (2) herbivory by the native leaf-roller caterpillar; (3) deposition of Pinus nigra pollen on the stigma, reducing the surface area available for P. alobula pollen; (4) habitat disturbance.

References

1. Lehnebach CA, Robertson AW. Pollination ecology of four epiphytic orchids of New Zealand.
    Annals of Botany 2004; 93: 773-781.
2. Lehnebach CA, Robertson AW, Hedderley D. Pollination studies of four New Zealand terrestrial orchids
    and the implication for their conservation. NZ J Bot 2005; 43: 467-477.