Integrating Hormesis and Xenohormesis in Plants Through Fitness and Response Variables

Hormesis is an adaptive response empirically characterized by a biphasic dose-response pattern, in which low or moderate levels of stress induce stimulatory responses whereas higher levels result in inhibitory or toxic effects. Although the biphasic dose-response pattern provides an empirical basis for identifying hormesis, it does not necessarily establish the adaptive significance of the observed response. Individual physiological, biochemical, or molecular variables may exhibit transient or ontogenetically dependent responses; therefore, fitness-related outcomes may provide an integrative criterion for evaluating the biological consequences and adaptive significance of hormesis. Relatively few studies have simultaneously evaluated, under the same stress factor applied across a defined dose or intensity gradient, both the hormetic performance of plants and the xenohormetic potential of bioactive compounds induced by this stress. Furthermore, this approach is insufficient to explain the complexity of the hormetic phenomenon. Therefore, there is a need to more precisely define how hormesis should be evaluated. This may be achieved by incorporating fitness-related variables, such as survival, growth, and reproductive capacity, together with response variables associated with the physiological, biochemical, and molecular mechanisms involved in stress adaptation, thus providing a framework which may be conceptually extended to xenohormesis. Direct evidence linking stress-induced metabolites to improved consumer fitness remains limited, precluding their interpretation in terms of xenohormetic potential.