Utilization of exogenous saccharides by protocorms of two terrestrial orchids

Orchid protocorms are completely mycoheterotrophic structures. Although saccharides are proposed as the main energy and carbon (C) sources provided by fungi, there is only limited knowledge on their effects. For the first time, utilization of a wide range of saccharides by in vitro axenic protocorms of two terrestrial orchids from two subfamilies, Ophrys iricolor subsp. lojaconoi and Oeceoclades, was tested. Protocorm size and, in the first of these also rhizoid length and soluble saccharide contents, were analysed. The endogenous saccharide spectra reflected the supplied saccharides and their metabolism. In both species, sucrose supported protocorm growth best. Surprisingly, fructose inhibited O. iricolor subsp. lojaconoi protocorm growth while O. decaryana ones grew well on it. Interestingly, mannitol abundant in mycorrhizal fungi was not utilized while sorbitol not found in fungi was usable. Galactose was toxic at pre-germination stage. Protocorm rhizoid length correlated with protocorm size but revealed several signalling effects of some saccharides. In conclusion, the orchid's ability to utilize various saccharides reflects more likely species life strategy rather than phylogenetic relations or saccharide abundance in mycorrhizal fungi.