Symbiotic exchange of carbon and phosphorus between cucumber and three arbuscular mycorrhizal fungi

Abstract

SUMMARY Cucumber ( Cucumis sativus L.) was grown in an irradiated soil:sand mixture in PVC tubes containing a root compartment, a hyphal compartment and a hyphal 32 P‐labelling compartment. The soil was inoculated with either Scutellospora calospora (Nicol. & Gerd.) Walker & Sanders, Glomus sp. or Glomus caledonium (Nicol. & Gerd.) Trappe & Gerdemann or left uninoculated. At 23 d from sowing 20 μCi of an aqueous solution of carrier‐free 32 P was pipetted into the hyphal labelling compartment. Just 24 h later the plants were labelled with 14 CO 2 for 16 h. At harvest the plants were analysed for mycorrhizal colonization and distribution of 14 C and 32 P. Hyphae of G. caledonium proved to have the most rapid 32 P uptake and transfer system of the three fungi studied. The host was provided with an order of magnitude more 32 P than supplied by either Glomus sp. or S. calospora. The S. calospora ‐colonized plants contained the least 32 P counts of the three mycorrhizal treatments. 75% of the total 32 P in the plant was present in the shoots of the plants colonized by G. caledonium ; the corresponding figures for the shoots of S. calospora and Glomus sp. were 45 and 25%, respectively. The proportion of assimilated 14 C allocated to below‐ground events was 35, 32 and 25 with S. calospora, Glomus sp. and G. caledonium , respectively. The specific incorporation of 14 C into roots colonized by Glomus sp. was 1.8 and 3.1 times as high as into roots colonized by G. caledonium and S. calospora , respectively. In contrast, the specific root respiration (Bq mg −1 d. wt) was similar in the mycorrhizal treatments. The 14 C flow into external hyphae was approximately 0.8% of the total assimilated 14 C for all three fungi. Based on these results, mycorrhizal efficiency was calculated as hyphal 14 C use per unit 32 P transported, below‐ground 14 C use per unit 32 P transported, 32 P transported per metre of hyphae and hyphal 14 C use per metre of hyphae.