Elevated atmospheric CO2 concentration alters the effect of phosphate supply on growth of Japanese red pine (Pinus densiflora) seedlings

We demonstrated that the inorganic phosphate (P(i)) requirement for growth of Japanese red pine (Pinus densiflora Sieb. & Zucc.) seedlings is increased by elevated CO(2) concentration (CO(2)]) and that responses of the ectomycorrhizal fungus Pisolithus tinctorius (Pers.) Coker & Couch to P(i) supply are also altered. To investigate the growth response of non-mycorrhizal seedlings to P(i) supply in elevated CO(2)], non-mycorrhizal seedlings were grown for 73 days in ambient or elevated CO(2)] (350 or 700 micromol mol(-1)) with nutrient solutions containing one of seven phosphate concentrations (0, 0.02, 0.04, 0.06, 0.08, 0.10 and 0.20 mM). In ambient CO(2)], the growth response to P(i) was saturated at about 0.1 mM P(i), whereas in elevated CO(2)], the growth response to P(i) supply did not saturate, even at the highest P(i) supply (0.2 mM), indicating that the P(i) requirement is higher in elevated CO(2)] than in ambient CO(2)]. The increased requirement was due mainly to an altered shoot growth response to P(i) supply. The enhanced P(i) requirement in elevated CO(2)] was not associated with a change in photosynthetic response to P(i) or a change in leaf phosphorus (P) status. We investigated the effect of P(i) supply (0.04, 0.08 and 0.20 mM) on the ectomycorrhizal fungus P. tinctorius in mycorrhizal seedlings grown in ambient or elevated CO(2)]. Root ergosterol concentration (an indicator of fungal biomass) decreased with increasing P(i) supply in ambient CO(2)], but the decrease was far less in elevated CO(2)]. In ambient CO(2)] the ratio of extramatrical mycelium to root biomass decreased with increasing P(i) supply but did not change in elevated CO(2)]. We conclude that, because elevated CO(2)] increased the P(i) requirement for shoot growth, the significance of the ectomycorrhizal association was also increased in elevated CO(2)].