Factors affecting germinable seed yield in Cypripedium calceolus var. pubescens (Willd.) Correll and Epipactis helleborine (L.) Crantz (Orchidaceae)

Abstract

One of the challenges facing orchid conservationists is to get a better understanding of seed parent fecundity. This can be especially laborious for orchids whose seed is difficult to germinate. Mature seeds of Cyripedium calceolus var. pubescens (Willd.) Correll (Orchidaceae) have been reported to require 8 weeks pre-chilling at 4d̀C and a further month at 20d̀C to induce germination, but seeds prematurely harvested at 7–8 weeks post-pollination germinate asymbiotically without chilling in less than 1 month. Mature seeds of Epipactis helleborine (L.) Crantz (Orchidaceae) have also presented a challenge but the identification of plants whose seeds are quick and easy to germinate has provided us with a tool to study patterns of production of this seed class. Preliminary field experiments suggest that timing of pollination in these two taxa is critical for the development of seed. When seeds from capsules of C. calceolus var. pubescens harvested 7 weeks after pollination were examined, the proportion of fully mature embryos increased with increasing anthesis-pollination interval over the first 10 days. Paired flowers from five genets xenogamously hand-pollinated at 2-day intervals from the second day after anthesis until the flowers wilted were significantly less likely to produce fruits as the flowers aged but the flower age at which this occurred varied between genets. The germinability of seeds from different pollen and seed parent combinations was tested on eight different media formulations and over a range of post-anthesis and post-pollination intervals. Different aged seeds germinated variably, the post-anthesis interval being the most reliable predictor of germination outcomes. Intercrossing of plants of C. calceolus var. pubescens suggests that seed production and germinability are maternally mediated traits. With E. helleborine, germinable seed can be produced both by self and by cross-pollination of selected plants but we have observed an influence of flower position in the inflorescence on selfcompatibility and on the production of the easy-to-germinate seed class. The timing of pollination in these two unrelated orchids has a considerable influence upon seed production and seed quality in plants of established fecundity. These findings suggest that flower age at pollination must be considered when interpreting similar studies in the Orchidaceae.