A possible mechanism regulating biomass allocation, especially leaf to root ratio (L/R), optimally in response to changes in external and internal environmental factors was discussed.

Previous studies revealed that it is necessary for plant to recognize light availability in leaves, soil nitrogen availability in root, and current amount of leaves and roots for optimization of L/R in response to the changes. In other words, net assimilation rate (NAR) at leaf, specific absorption rate of nitrogen (SAR) at root, current state of L/R must be recognized.

Optimal biomass allocation rate was mathematically solved by incorporating these parameters. It was interpreted that optimal biomass allocation can be achieved by recognizing especially ratio of NAR, SAR, and net inlet flow of nitrogen per leaf area.

A dynamic system model which incorporated plant growth process and the above-mentioned regulation mechanism was developed. The model predicted that a plant can adjust biomass allocation rate optimally in response to changes in light and nitrogen availabilities, and recover from damage of leaves and root rapidly by allocating biomass to the most limiting organ preferentially.

Additionally, some assumptions used in the model were tested using Polygonum cuspidatum and role of phytohormones as signal substances was discussed.