Mycorrhiza formation and elevated CO2 both increase the capacity for sucrose synthesis in source leaves of spruce and aspen

The effects of mycorrhiza formation in combination with elevated CO2 concen trations on carbon metabolism of Norway spruce (Picea abies) seedlings and aspen (Populus tremula x Populus tremuloides) plantlets were analysed. Plan ts were inoculated for 6 wk with the ectomycorrhizal fungi Amanita muscaria and Paxillus involutus (aspen only) in an axenic Petri-dish culture at 350 and 700 mu l l(-1) CO2 partial pressure. After mycorrhiza formation, a sti mulation of net assimilation rate was accompanied by decreased activities o f sucrose synthase, an increased activation state of sucrose-phosphate synt hase, decreased fructose-2,6-bisphosphate and starch, and slightly elevated glucose-6-phosphate contents in source leaves of both host species, indepe ndent of CO2 concentration. Exposure to elevated CO2 generally resulted in higher net assimilation rates, increased starch as well as decreased fructo se-2,6-bisphosphate (aspen only) content in source leaves of both mycorrhiz al and nonmycorrhizal plants. Our data indicate only slightly improved carb on utilization by mycorrhizal plants at elevated CO2. They demonstrate howe ver, that both factors which modulate the sink-source properties of plants increase the capacity for sucrose synthesis in source leaves mainly by allo steric enzyme regulation.