Peculiarities of calcium and iron effects on some wild terrestrial orchids in vitro compared to in vivo

There are 32 species from the family Orchidaceae in Latvia, and 26 are rare and endangered, and their preservation in an in vitro bank is vital. The death of in vitro grown wild terrestrial orchids is mainly caused by the release of phenolic compounds from root tissues. As mineral nutrients form a significant component of culture media, a hypothesis was advanced that increased doses of both Ca and Fe could prevent phenol oxidation and improve micropropagation. Liparis loeselii (L.) Rich. and Gymnadenia conopsea (L.) R. Br. calcicole plants, and Dactylorhiza russowii (Klinge) Holub, a non-calcicole plant, were used as model species. Modification of the culture media with increased Ca gluconate monohydrate concentrations significantly improved the quality of L. loeselii and D. russowii plants, especially in the presence of elevated levels of ferric citrate. There was no benefit of Ca for G. conopsea, except in combination with the highest level of ferric citrate tested. The results revealed species-specific stimulatory or inhibitory impacts of changes in the pH of culture media on orchid plantlet quality. These findings demonstrate the crucial role of iron to prevent necrosis. The results indicated that for species with relatively higher adaptation potential to growth in habitats with different pH levels (L. loeselii and D. russowii), the shoot quality in vitro was better if the Ca to Fe ratio in the culture medium was 2:1. Despite the fact that G. conopsea is a calcicole species, the optimal Ca to Fe ratio in the culture medium was 1:1.