The relative contribution of hyphae and roots to phosphorus uptake by arbuscular mycorrhizal plants, measured by dual labelling with 32P and 33P

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Abstract
SUMMARY: The aim of this investigation was to measure the relative contribution of hyphae and roots to the total uptake of phosphorus by mycorrhizal plants. Cucumber plants were grown in three‐compartment systems where 32P was applied to a lateral root‐free compartment (HC) and 33P applied to an identical lateral compartment with both roots and hyphae (RHC). The cucumber seeds were sown into the main root compartment (RC) which was inoculated with one of the following three arbuscular mycorrhizal fungi: Scutellospora calospora (Nicol. & Gerd.) Walker & Sanders [isolate WUM 12(2)]; Glomus sp. [isolate WUM 10(1)]; Glomus caledonium (Nicol. & Gerd.) Trappe & Gerdemann (isolate RIS 42) or left uninoculated to represent a control treatment. The plants were harvested at 17 and 27 d. The hyphal uptake of 32P from the HC increased as follows –S. calospora ≥ Glomus sp. ≥G. caledonium. The uptake of 32P from the HC was equivalent to 7, 21 and 109 % of the uptake of 33P from the RHC in plants colonized by S. calospora, Glomus sp. and G. caledonium, respectively. This indicates that the relative contribution of the roots in total P uptake varied greatly between the three mycorrhizal treatments. Although 33P uptake was variable within treatments, the P uptake systems of roots colonized by G. caledonium appeared to be inactive when compared to hyphal 32P uptake. This may have been due to feedback mechanisms being activated because of the high hyphal P uptake. The roots colonized by S. calospora had higher rates of root‐P uptake compared with the control roots, suggesting that the root‐P uptake systems have been stimulated by the presence of the fungus. This would ensure that the P supply was sufficient to maintain photosynthesis in this symbiosis where the fungus gives only little P in return for its use of plant carbon substrate from the host.