Inorganic phosphorus nutrition in green-leaved terrestrial orchid seedlings

Abstract

Background and aims: Many terrestrial orchids have an obligate dependence on their mycorrhizal associations for nutrient acquisition, particularly during germination and early seedling growth. Though important in plant growth and development, phosphorus (P) nutrition studies in mixotrophic orchids have been limited to only a few orchid species and their fungal symbionts. For the first time, we demonstrate the role of a range of fungi in the acquisition and transport of inorganic P to four phylogenetically distinct green-leaved terrestrial orchid species (Diuris magnifica, Disa bracteata, Pterostylis sanguinea and Microtis media subsp. media) that naturally grow in P-impoverished soils.

Methods: Mycorrhizal P uptake and transfer to orchids was determined and visualized using agar microcosms with a diffusion barrier between P source (33P orthophosphate) and orchid seedlings, allowing extramatrical hyphae to reach the source.

Key results: Extramatrical hyphae of the studied orchid species were effective in capturing and transporting inorganic P into the plant. Following 7 d of exposure, between 0.5 % (D. bracteata) and 47 % (D. magnifica) of the P supplied was transported to the plants (at rates between 0.001 and 0.097 fmol h-1). This experimental approach was capable of distinguishing species based on their P-foraging efficiency, and highlighted the role that fungi play in P nutrition during early seedling development.

Conclusions: Our study shows that orchids occurring naturally on P-impoverished soils can obtain significant amounts of inorganic P from their mycorrhizal partners, and significantly more uptake of P supplied than previously shown in other green-leaved orchids. These results provide support for differences in mycorrhiza-mediated P acquisition between orchid species and fungal symbionts in green-leaved orchids at the seedling stage. The plant-fungus combinations of this study also provide evidence for plant-mediated niche differentiation occurring, with ecological implications in P-limited systems.

Keywords: 33P; Ceratobasidium; Tulasnella; in vitro; mineral nutrition; mycorrhizal networks; niche partitioning; orchid; phosphorus.

Fig. 1.

Digital autoradiographs of microcosms containing green-leaf orchid seedlings (left), 7 d after labelling the mycelial compartment (right) with ³³P. Orchid species clockwise from top left are Pterostylis sanguinea, Diuris magnifica, Disa bracteata and Microtis media subsp. media. Inset image at top left shows microcosm with plant in RHC (on left) and hyphal network extending across the diffusion barrier to the HC (on right); the plug was removed for isotope source after the photograph was taken, and the species shown is D. magnifica. Position of orchid seedling is denoted on the left (arrow) and position of isotope plug is denoted on the right of each autoradiograph image (circle). The colour scale represents the number of counts of radioactivity detected in 1 h per 0.25-mm² pixel.