| Knowledge of the dynamics of VA mycorrhizal (VAM) plant systems is fundamental to the understanding of the relationships between VAM, plant and soil. Pot experiments were conducted to determine early physiological responses to VAM inoculation and identify and construct a model of VAM influence and its consequences to the physiology and ecology of VAM plants. Leucaena leucocephala seedlings, with or without the VAM fungus (Glomus aggregatum), were grown in a Wahiawa soil, Tropeptic Eutrustox, with different soil P levels (ranging from 0.005 to 0.429 mg L{dollar}sp{lcub}-1{rcub}{dollar} of P in 0.01 M CaCl{dollar}sb{lcub}2{rcub}{dollar} extract). Even with the highest P fertilization, the growth of nonmycorrhizal (NM) plants was still less than that of mycorrhizal (M) plants. Daily pinnule sampling, pot weighing methods and multiple 5-day-interval harvests revealed a series of changes in nutrient uptake, dry matter production and transpiration between M and NM plants. The order of critical events in the soil-mycorrhiza-plant system is as follows: (1) At 10 days, root P concentrations were higher in M plants than in NM plants. (2) From 10 to 15 days after inoculation, the flux of P into M roots was greater than that into NM roots. (3) After nutrient content in the shoots increased, M plants had higher growth rates than NM plants. They also allocated more assimilate to leaf growth than did NM plants, which was followed by increased transpiration. (4) Mycorrhizal plants had higher leaf area expansion rates and net assimilation rates than NM plants. (5) Greater M root growth, supporting further greater soil exploration (positive feedback). Nonmycorrhizal leucaena seedlings responses to P stress is also discussed. Despite greater initial root growth, leucaena roots alone could not acquire sufficient P to maintain growth. The outcome of P deficiency and stunted growth in NM seedlings seems to be a typical sequence of negative feedback response: (1) lowered nutrient uptake, (2) reduced carbon assimilation rate, (3) decreased rate of root proliferation, and (4) less nutrient uptake and further stunted growth. The results demonstrate how the concept of feedback helps in understanding dynamic processes in mycorrhizal systems. |
