EFFECTS OF PHOSPHORUS AVAILABILITY AND VESICULAR-ARBUSCULAR MYCORRHIZAS ON THE CARBON BUDGET OF COMMON BEAN (PHASEOLUS-VULGARIS)

Low phosphorus availability is often a primary constraint to plant pro ductivity in native soils. Here we test the hypothesis that root carbo n costs are a primary limitation to plant growth in low P soils by ass essing the effect of P availability and mycorrhizal infection on whole plant C budgets in common bean (Phaseolus vulgaris L.). Plants were g rown in solid-phase-buffered silica sand providing a constant supply o f low (1 mu M) or moderate (10 mu M) P. Carbon budgets were determined weekly during the vegetative growth phase. Mycorrhizal infection in l ow-P plants increased the root specific P absorption rate, but a concu rrent increase in root respiration consumed the increased net C gain r esulting from greater P uptake. The energy content of mycorrhizal and non-mycorrhizal roots was similar. We propose that the increase in roo t respiration in mycorrhizal roots was mainly due to increased mainten ance and growth respiration of the fungal tissue. Plants grown with lo w P availability expended a significantly larger fraction of their tot al daily C budget on below-ground respiration at days 21, 28 and 35 af ter planting (29-40 %) compared with plants grown with moderate P supp ly (18-25 %). Relatively greater belowground respiration in low P plan ts was mainly a result of their increased root:shoot ratio, although s pecific assimilation rate was reduced significantly at days 21 and 28 after planting. Specific root respiration was reduced over time by low P availability, by up to 40 %. This reduction in specific root respir ation was due to a reduction in ion uptake respiration and growth resp iration, whereas maintenance respiration was increased in low-P plants . Our results support the hypothesis that root C costs are a primary l imitation to plant growth in low-P soils.