Arbuscular mycorrhizal (AM) root respiration can impose a respiratory sink on host reserves under low P conditions, but it is not known how AM roots respond to short-term supply of sufficient P. Therefore, the effect of P stress alleviation on the respiration of AM roots was investigated in 5-week-old tomato plants. Plants were inoculated with Glomus mosseae in sand culture and grown hydroponically in a low P (2 microM) nutrient medium for 3 weeks. P stress was alleviated by the supply of 2 mM P for 72 h. With P stress alleviation, the improved root P status coincided with a decline in AM fungal activity and a reduction in root CO2 and O2 fluxes of the AM plants. During P stress alleviation, the AM roots had lower concentrations of organic acids, derived from root-zone CO2 assimilation, in their root exudates. These results show that short-term alleviation of low P conditions in AM roots rapidly affects AM fungal symbiont activity, AM root respiration, and root-zone CO2-derived organic acid metabolism.