SYMBIOTIC EXCHANGE OF CARBON AND PHOSPHORUS BETWEEN CUCUMBER AND 3 ARBUSCULAR MYCORRHIZAL FUNGI

Cucumber (Cucumis sativus L.) was grown in an irradiated soil:sand mix ture in PVC tubes containing a root compartment, a hyphal compartment and a hyphal P-32-labelling compartment. The soil was inoculated with either Scutellospora calospora (Nicol. & Gerd.) Walker & Sanders, Glom us sp. or Glomus caledonium (Nicol. & Gerd.) Trappe & Gerdemann or lef t uninoculated. At 23 d from sowing 20 muCi of an aqueous solution of carrier-free P-32 was pipetted into the hyphal labelling compartment. Just 24 h later the plants were labelled with (CO2)-C-14 for 16 h. At harvest the plants were analysed for mycorrhizal colonization and dist ribution of C-14 and P-32. Hyphae of G. caledonium proved to have the most rapid P-32 uptake and transfer system of the three fungi studied. The host was provided with an order of magnitude more P-32 than suppl ied by either Glomus sp. or S. calospora. The S. calospora-colonized p lants contained the least P-32 counts of the three mycorrhizal treatme nts. 75% of the total P-32 in the plant was present in the shoots of t he plants colonized by G. caledonium; the corresponding figures for th e shoots of S. calospora and Glomus sp. were 45 and 25%, respectively. The proportion of assimilated C-14 allocated to below-ground events w as 35, 32 and 25 with S. calospora, Glomus sp. and G. caledonium, resp ectively. The specific incorporation of C-14 into roots colonized by G lomus 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 r oot respiration (Bq mg-1 d. wt) was similar in the mycorrhizal treatme nts. The C-14 flow into external hyphae was approximately 0.8% of the total assimilated C-14 for all three fungi. Based on these results, my corrhizal efficiency was calculated as hyphal C-14 use per unit P-32 t ransported, below-ground C-14 use per unit P-32 transported, P-32 tran sported per metre of hyphae and hyphal C-14 use per metre of hyphae.