Growth depression of mycorrhizal Citrus seedlings grown at high phosphorus supply is mitigated by elevated CO2

Gas exchange and growth responses of pot-grown sour orange (Citrus aurantium) and sweet orange (C. sinensis) were studied at high soil-P where growth depression is predicted. Seedlings were either inoculated (AM) with the arbuscular mycorrhizal fungus Glomus intraradices or not inoculated (NAA), and grown at elevated (eCO(2)) or ambient CO2 (aCO(2)) for 11 wk. At aCO(2), growth of AM sour orange was depressed (18%) compared with NM seedlings, but at eCO(2), AM sour orange plants were 15% larger than NM plants. Growth depression coincided with increased rhizosphere respiration, reduced root starch concentration, and lower relative growth rates. Net photosynthesis (Pn) of both genotypes was enhanced by eCO(2). For sour orange, the stimulation was greater in AM than in NM seedlings and this may have compensated for the carbon (C) expenditure on mycorrhizas. Gas exchange and growth of sweet orange were unresponsive to colonization by G. intraradices. Differential responses to treatments suggest that C expenditure on mycorrhizas is more tightly, regulated in citrus genotypes of low mycorrhizal dependency (MD) such as sweet orange than in high MD genotypes (e.g. sour orange).