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 thecarbon (C) economy of sour orange (Citrus aurantium L.) weredetermined during and following active M colonization. Therewere four treatments: M seedlings grown at standard-strength(1 mM) P (M1) and nonmycorrhizal (NM) plants grown at one, twoand five times standard-strength P (NM1, NM2 and NM5). Mycorrhizalcolonization, tissue dry mass, P content, root length and leafarea were determined in five harvests from 6 to 15 weeks ofage. Rate of C assimilation (A) was determined at 7, 8 and 12weeks by gas exchange. Partitioning of ¹⁴ C was determined from7 to 15 weeks using a 10-min pulse followed by a 24-h chaseperiod. For a given attribute, M1 plants were compared to thecurve defining the NM response as a function of tissue P concentration.In contrast to the large effects of P nutrition on C economyof sour orange, M effects were generally subtle. Mycorrhizaeincreased the root biomass fraction, the root length/leaf arearatio and the percentage of ¹⁴C recovered from below-groundcomponents. A higher percentage of below-ground ¹⁴ C was inthe respiration and soil fractions in M than NM plants of equivalentP status. Mycorrhizal plants tended to enhance A only for abrief period. Mycorrhizal plants had lower relative growth ratesthan NM plants of equivalent P status, suggesting that the temporarilyenhance A of M plants did not fully compensate for their greaterbelow-ground carbon expenditure. Problems of interpreting thedynamic effects of mycorrhizae on C economy that are independentof P nutrition are discussed.Copyright 1993, 1999 Academic Press Citrus aurantium L., sour orange, carbon economy, ¹⁴carbon, CO₂ assimilation, vesicular-arbuscular mycorrhizae, phosphorus fertilization, phosphorus nutrition