Phosphorus supply and arbuscular mycorrhizas increase growth and net gas exchange responses of two Citrus spp. grown at elevated [CO2]

We hypothesized that greater photosynthate supply at elevated [CO2] could c ompensate for increased below-ground C demands of arbuscular mycorrhizas. T herefore, we investigated plant growth, mineral nutrition, starch, and net gas exchange responses of two Citrus spp. to phosphorus (P) nutrition and m ycorrhizas at elevated atmospheric [CO2]. Half of the seedlings of sour ora nge (C. aurantium L.) and `Ridge Pineapple' sweet orange (C. sinensis L. Os beck) were inoculated with the arbuscular mycorrhizal (AM) fungus, Glomus i ntraradices Schenck and Smith and half were non-mycorrhizal (NM). Plants we re grown at ambient or 2X ambient [CO2] in unshaded greenhouses for 11 week s and fertilized daily with nutrient solution either without added P or wit h 2 mM P in a low-P soil. High P supply reduced AM colonization whereas ele vated [CO2] counteracted the depressive effect of P on intraradical coloniz ation and vesicle development. Seedlings grown at either elevated [CO2], hi gh P or with G. intraradices had greater growth, net assimilation of CO2 (A (CO2)) in leaves, leaf water-use efficiency, leaf dry wt/area, leaf starch and carbon/nitrogen (C/N) ratio. Root/whole plant dry wt ratio was decrease d by elevated [CO2], P, and AM colonization. Mycorrhizal seedlings had high er leaf-P status but lower leaf N and K concentrations than nonmycorrhizal seedlings which was due to growth dilution effects. Starch in fibrous roots was increased by elevated [CO2] but reduced by G. intraradices, especially at low-P supply. In fibrous roots, elevated [CO2] had no effect on C/N, bu t AM colonization decreased C/N in both Citrus spp. grown at low-P supply. Overall, there were no species differences in growth or A(CO2). Mycorrhizas did not increase plant growth at ambient [CO2]. At elevated [CO2], however , mycorrhizas stimulated growth at both P levels in sour orange, the more m ycorrhiza-dependent species, but only at low-P in sweet orange, the less de pendent species. At low-P and elevated [CO2], colonization by the AM fungus increased A(CO2) in both species but more so in sour orange than in sweet orange. Leaf P and root N concentrations were increased more and root starc h level was decreased less by AM in sour orange than in sweet orange. Thus, the additional [CO2] availability to mycorrhizal plants increased CO2 assi milation, growth and nutrient uptake over that of NM plants especially in s our orange under P limitation.