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

Abstract

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 32P‐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 32P was pipetted into the hyphal labelling compartment. Just 24 h later the plants were labelled with 14CO2 for 16 h. At harvest the plants were analysed for mycorrhizal colonization and distribution of 14C and 32P. Hyphae of G. caledonium proved to have the most rapid 32P uptake and transfer system of the three fungi studied. The host was provided with an order of magnitude more 32P than supplied by either Glomus sp. or S. calospora. The S. calospora‐colonized plants contained the least 32P counts of the three mycorrhizal treatments. 75% of the total 32P 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 14C allocated to below‐ground events was 35, 32 and 25 with S. calospora, Glomus sp. and G. caledonium, respectively. The specific incorporation of 14C 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 14C flow into external hyphae was approximately 0.8% of the total assimilated 14C for all three fungi. Based on these results, mycorrhizal efficiency was calculated as hyphal 14C use per unit 32P transported, below‐ground 14C use per unit 32P transported, 32P transported per metre of hyphae and hyphal 14C use per metre of hyphae. Copyright © 1993, Wiley Blackwell. All rights reserved