Carbon Economy of Sour Orange in Relation to Mycorrhizal Colonization and Phosphorus Status

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Abstract
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 aurantium L.) were determined during and following active M colonization. There were four treatments: M seedlings grown at standard-strength (1 mM) P (M1) 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 14 C was determined from 7 to 15 weeks using a 10-min pulse followed by a 24-h chase period. For a given attribute, M1 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 14 C 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 enhance 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.Copyright 1993, 1999 Academic Press
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