Carbon economy of sour orange in relation to mycorrhizal colonization and phosphorus status

The effects of plant phosphorus (P) status and the mycorrhizal (M) fungus, Glomus intraradices Schenck & Smith, on the carbon (C) economy of sour orange (Citrus aurontium L.) were determined during and following active M colonization. There were four treatments: M seedlings grown at standard-strength (1 mM) P (Ml) and nonmycorrhizal (NM) plants grown at one, two and five times standard-strength P (NM1, NM2 and NM5). Mycorrhizal colonization, tissue dry mass, P content, root length and leaf area were determined in five harvests from 6 to 15 weeks of age. Rate of C assimilation (A) was determined at 7, 8 and 12 weeks by gas exchange. Partitioning of ₁₄C was determined from 7 to 15 weeks using a 10-min pulse followed by a 24-h chase period. For a given attribute. Ml plants were compared to the curve defining the NM response as a function of tissue P concentration. In contrast to the large effects of P nutrition on C economy of sour orange, M effects were generally subtle. Mycorrhizae increased the root biomass fraction, the root length/leaf area ratio and the percentage of 14C recovered from below-ground components. A higher percentage of below-ground ₁₄C was in the respiration and soil fractions in M than NM plants of equivalent P status. Mycorrhizal plants tended to enhance A only for a brief period. Mycorrhizal plants had lower relative growth rates than NM plants of equivalent P status, suggesting that the temporarily enhanced A of M plants did not fully compensate for their greater below-ground carbon expenditure. Problems of interpreting the dynamic effects of mycorrhizae on C economy that are independent of P nutrition are discussed.