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 compensate for increased below-ground C demands of arbuscular mycorrhizas. Therefore, we investigated plant growth, mineral nutrition, starch, and net gas exchange responses of two Citrus spp. to phosphorus (P) nutrition and mycorrhizas at elevated atmospheric [CO2]. Half of the seedlings of sour orange (C. aurantium L.) and ‘Ridge Pineapple’ sweet orange (C. sinensis L. Osbeck) were inoculated with the arbuscular mycorrhizal (AM) fungus, Glomus intraradices Schenck and Smith and half were non-mycorrhizal (NM). Plants were grown at ambient or 2X ambient [CO2] in unshaded greenhouses for 11 weeks and fertilized daily with nutrient solution either without added P or with 2 mM P in a low-P soil. High P supply reduced AM colonization whereas elevated [CO2] counteracted the depressive effect of P on intraradical colonization and vesicle development. Seedlings grown at either elevated [CO2], high P or with G. intraradices had greater growth, net assimilation of CO2 (ACO2) 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 decreased by elevated [CO2], P, and AM colonization. Mycorrhizal seedlings had higher 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, but AM colonization decreased C/N in both Citrus spp. grown at low-P supply. Overall, there were no species differences in growth or ACO2. 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 mycorrhiza-dependent species, but only at low-P in sweet orange, the less dependent species. At low-P and elevated [CO2], colonization by the AM fungus increased ACO2 in both species but more so in sour orange than in sweet orange. Leaf P and root N concentrations were increased more and root starch level was decreased less by AM in sour orange than in sweet orange. Thus, the additional [CO2] availability to mycorrhizal plants increased CO2 assimilation, growth and nutrient uptake over that of NM plants especially in sour orange under P limitation.