Temperature sensitivity of in vitro germination and seedling development of Dactylorhiza majalis (Orchidaceae) with and without a mycorrhizal fungus

Abstract

Abstract. Symbiotic germination and development in vitro of Dactylorhiza majalis seeds with a strain of Rhizoctonia is very temperature dependent. Above an optimum at 23–25 °C there is a marked decline in germination percentage. Seeds that did germinate at higher temperatures had only little or no development of mycorrhiza, and developed few or no rhizoids compared with seedlings raised at optimal or lower temperatures. Six‐week‐old seedlings grown for additional 4 weeks on a range of temperatures had an optimal length increase at 23–24.5 °C mean temperature. At superoptimal temperatures (26 °C), the seedlings contained smaller starch reserves than those at lower temperatures and increased about as much in length as seedlings grown at 13 °C but much less than those grown at optimum. Temperature also influenced the differentiation of the leafy shoot, seedlings growing to a larger size before shoot initiation in the temperature range of optimal growth. Because of the small span between optimal and too‐high temperatures, a careful assessment of temperature optimum will be necessary in any orchid/fungus relationship before judging the success of symbiosis. At optimal temperature, symbiotic germination gave a germination percentage about twice that using a good asymbiotic method. The increase in seedling length was about 45% per week in symbiotic culture compared with less than 30% in the asymbiotic culture.