ARBUSCULAR MYCORRHIZAE OF GUTIERREZIA-SAROTHRAE AND ELEVATED CARBON-DIOXIDE - EVIDENCE FOR SHIFTS IN C ALLOCATION TO AND WITHIN THE MYCOBIONT

In a complete 2 x 2 factorial greenhouse experiment we examined the re sponses of arbuscular mycorrhizal (AM) and non-mycorrhizal fungi to Gu tierrezia sarothrae shrubs grown in elevated atmospheric carbon dioxid e (750 mu l l(-1)) and fertilized with N. AM percent infection did not change significantly with elevated CO2, but arbuscular infection incr eased 14-fold in the low-N treatment. Extraradical hyphal length incre ased on an absolute basis in elevated CO2, and also per infected root length. In the high-N treatments, increasing CO2 caused a decrease in hyphal length per infected root length, and an increase in vesicular i nfection. There was a significant positive response of AM infection in tensity to increasing CO2 for the high N treatment, and a similar tren d in the low N treatment. Infection intensity was positively correlate d with arbuscular infection and with vesicular infection. Nonmycorrhiz al fungi did not respond to any of the treatment combinations, as meas ured by percent root infection and external hyphal length. Our results indicate that C allocation to the AM fungi was increased in elevated CO2, and that the mycobiont in turn increased C allocation to external hyphae. (C) 1998 Elsevier Science Ltd. All rights reserved.