CARBON ECONOMY IN SOUR ORANGE IN RELATION TO MYCORRHIZAL COLONIZATION AND PHOSPHORUS STATUS
The effects of phosphorus (P) and of the mycorrhizal (M) fungus, Glomus intraradix, on the carbon (C) economy of sour orange (citrus aurantium L.) were determined during and following active M colonization. There were four treatments: mycorrhizal seedlings grown at standard-strength P (M1) and nonmycorrhizal (NM) plants grown at 1, 2 and 5 times standard-strength P (NM1, NM2 and NM5). Mycorrhizal colonization, tissue dry mass, P content, root length, leaf area, 14C partitioning and rate of c assimilation (A) were determined in five whole-plant harvests from 6 to 15 wks of age. In contrast to the effects of P nutrition on C economy in sour orange, M effects were generally subtle. Mycorrhizae increased the root biomass fraction, the root length/leaf area ratio, and the percent of 14C recovered from belowground components. Mycorrhizal plants had a higher percentage of belowground 14C in the respiration and soil fractions than did NM plants of equivalent P status. Mycorrhizal plants tended to have enhanced A at 8 wks but not at 7 or 12 wks. This temporarily enhanced A of M plants did not fully compensate for their greater belowground C expenditure, as suggested by apparently lower relative growth rates of M than NM plants of equivalent P status. Problems of interpreting the dynamic effects of mycorrhizae on C economy that are independent of p nutrition are discussed.