Plant hormesis and Shelford’s tolerance law curve

Abstract

Shelford's law of tolerance is illustrated by a bell-shaped curve depicting the relationship between environmental factor/factors’ intensity and its favorability for species or populations. It is a fundamental basis of ecology when considering the regularities of environment impacts on living systems, and applies in plant biology, agriculture and forestry to manage resistance to environmental limiting factors and to enhance productivity. In recent years, the concept of hormesis has been increasingly used to study the dose–response relationships in living organisms of different complexities, including plants. This requires the need for an analysis of the relationships between the hormetic dose–response model and the classical understanding of plant reactions to environments in terms of Shelford's law of tolerance. This paper analyses various dimensions of the relationships between the hormetic model and Shelford’s tolerance law curve under the influence of natural environmental factors on plants, which are limiting for plants both in deficiency and excess. The analysis has shown that Shelford’s curve and hormetic model do not contradict but instead complement each other. The hormetic response of plants is localized in the stress zone of the Shelford’s curve when adaptive mechanisms are disabled within the ecological optimum. At the same time, in a species range, the ecological optimum is the most favorable combination of all or at least the most important environmental factors, each of which usually deviates slightly from its optimal value. Adaptive mechanisms cannot be completely disabled in the optimum, and hormesis covers optimum and stress zones. Hormesis can modify the plant tolerance range to environmental factors by preconditioning and makes limits of plant tolerance to environmental factors flexible to a certain extent. In turn, as a result of tolerance range evolution, quantitative characteristics of hormesis (width and magnitude of hormetic zone) as well as the range of stimulating doses, may significantly differ in various plant species and even populations and intra-population groups, including plants at different development stages. Using hormetic preconditioning for managing plant resistance to environmental limiting factors provides an important perspective for increasing the productivity of woody plants in forestry.