Composition of Cypripedium calceolus (Orchidaceae) seeds analyzed by attenuated total reflectance IR spectroscopy: in search of understanding longevity in the ground

Abstract

Premise of the study: Orchid seeds are minute and covered with a thin coat, yet they often have a long life after dispersal. They are notorious for low and irregular germination, in nature as well as in vitro. Since orchids are often rare species of conservational and commercial interest, reproduction by seeds is an important concern. The purpose of this study was to learn more about the resilience of these highly specialized seeds and stimulatory processes toward germination. •

Methods: We studied testa and embryos of Cypripedium calceolus to identify natural components in intact seeds and the impact of 7 yr in soil in its natural habitat. We also analyzed the effects of Ca(OCl)2, used technically to enhance germination for cultivation in vitro. For the first time with this kind of plant material, we used attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy, an ideal method for minute sample sizes and surface selectivity. Thus, we could link treatments with changes in seed surface chemistry. •

Key results: A lignin-like polymer is an essential testa component that undergoes degradation by soil or hypochlorite processes. In both cases, we found a build-up of CaCO3 on the testa, which could interact with lignin to enhance germination. Very minor changes occurred in embryo reserve nutrient content after a long sojourn underground, which supports their continued viability. •

Conclusions: We suggest that degradation of lignin and enrichment of the testa surface with CaCO3 are important stimulants of germination both in the habitat and during laboratory sowing.

Keywords: ATR-FTIR; IR; attenuated total reflectance–Fourier transform infrared spectroscopy; calcium; internal reflection; lignin; orchid; seed bank; seed chemistry; seed germination.