Fungal symbionts may modulate nitrate inhibitory effect on orchid seed germination

Many orchid species are threatened, while some disappear from their natural habitats without obvious reasons. Eutrophication has been suggested as a possible factor and nitrate, which is able to suppress non-symbiotic orchid seed germination even at very low concentrations, and could pose a serious threat for natural orchid populations. Early ontogenesis of all orchids entirely depends on orchid mycorrhizal symbiosis, and at this initial mycoheterotrophic stage, many terrestrial green orchids associate with polyphyletic fungal symbionts (i.e., mycobionts), collectively called "rhizoctonias." We asked whether these fungi might also have some non-nutritional roles, i.e., whether they might confer resistance to eutrophication. To test this hypothesis, we co-cultivated seeds of the terrestrial orchid Dactylorhiza majalis with five rhizoctonias (two Tulasnella, two Ceratobasidium and one Serendipita isolate) at various ecologically meaningful nitrate concentrations (0 to 100 mg/L). With the exception of one Tulasnella isolate, all mycobionts supported the growth of protocorms and formed orchid mycorrhiza, i.e., intracellular hyphal pelotons, in the protocorms. Nitrate suppressed asymbiotic, as well as symbiotic, seed germination in all but one fungal treatment; the seeds co-cultivated with one of the Ceratobasidium isolates were indeed insensitive to nitrate. We conclude that nitrates also negatively affect symbiotic orchid germination, depending on the available compatible mycobionts. Thus, eutrophication with nitrate may decrease the number of orchid mycobionts capable of supporting seed germination.