MAXIMISATION OF REPRODUCTIVE SUCCESS BY EUROPEAN ORCHIDACEAE UNDER CONDITIONS OF INFREQUENT POLLINATION

摘要

The pollination biology of a group of European orchids (Dactylorhiza, Ophrys, Orchis, Plantanthera, Goodyera, and Serapias species) are investigated, and their anthecological characteristics considered in relation to natural levels of reproductive success. Pollen:ovule (P:O) ratios of the European orchids surveyed range from 10:1 (Goodyera repens) to 24:1 (Platanthera chlorantha). Average pollen-load : ovule ratios are consistently lower than P:O ratios. Naturally occurring pollen lends range from 1 massula to > 1 pollinium. Even the smallest pollen load is sufficient to stimulate embryogenesis in experimentally pollinated Dactylorhiza purpurella flowers, although more seeds are set with larger loads. Pollen tubes grow rapidly through the stylar canal and into the top of the ovary within 2 or 3 days of pollination, and grow down either side of the 3 parietal ridges in the ovary. Fertilisation occurs throughout the length of the ovary but its distribution is non-random, especially when pollen loads are limiting, with more seeds being set at the sty lar end. All species of Dactylorhiza, Ophrys, and Orchis studied are highly self-compatible, In the absence of pollination, Ophrys and Orchis flowers remain open and fertile for at least 3 weeks. Pollinated flowers remain receptive to further pollinations for at least 8 days. Some fruits can even be obtained on selfing 20-day-old unpollinated Orchis morio flowers. Excised pollinia retain germinability for a long time, up to 51 days in Dacrylorhiza purpurella. The arrival of pollen on the stigma hastens floral senescence, but post-pollination changes are relatively slow when compared with those reported for tropical orchid species. It is concluded that characteristics of the pollination biology of European orchids act to maximise reproductive success by (1) prolonging the opportunity for effective pollen deposition both pre- and post-pollination, (2) increasing the likelihood of widespread dispersal, (3) reducing pollen wastage, and (4) increasing seed quality by promoting some pollen competition. As most European orchids are xenogamous and require pollen to arrive on the stigma before seed can be set, reproductive maximisation is of particular adaptive advantage because many of them art infrequently visited by insects so that the probability of successful pollination can be very low. [References: 47]