Phenotypic flexibility is a prominent mechanism of plant adaptation to environmental variations to provide plant growth and development stability. In the paper, the distinctive quantitative criterion of phenotypic flexibility estimation is presented. It is based on analysis of isoclinal lines of response function of relationship between growth rate and limiting levels of environmental factors. The criterion represents the depth of a comfortable zone where the predefined level of efficiency is guaranteed. The major factor of plants ecological stability is plants ability to adaptation. Modeling of primary assimilate distributions between plant organs is developed in accordance with the optimality principle. It is demonstrated that teleonomic models are able to predict adaptive properties of a plant in terms of the introduced indicator. Universal quantitative criterion of ecological stability is extremely useful for solving the following practical problems: mathematical models verification of plants growth and quality assessment of new grades.