Nitrogen (N) plays a key role in the plant life cycle. It is the main plant mineral nutrient needed for chlorophyll production and other plant cell components (proteins, nucleic acids, amino acids). Crop yield is affected by plant N status. Thus, the optimization of nitrogen fertilization has become the object of intense research due to its environmental and economic impact. This article focuses on reviewing current methods and techniques used to determine plant N status. Kjeldahl digestion and Dumas combustion have been used as reference methods for N determination in plants, but they are destructive and time consuming. By using spectroradiometers, reflectometers, imagery from satellite sensors and digital cameras, optical properties have been measured to estimate N in plants, such as crop canopy reflectance, leaf transmittance, chlorophyll and polyphenol fluorescence. High correlation has been found between optical parameters and plant N status, and those techniques are not destructive. However, some drawbacks include chlorophyll saturation, atmospheric and soil interference, and the high cost of instruments. Electrical properties of plant tissue have been used to estimate quality in fruits, and water content in plants, as well as nutrient deficiency, which suggests that they have potential for use in plant N determination.

Nitrogen plays a key role in crop yields. Hence, farmers may apply excessive N fertilizers to crop fields, inducing environmental pollution. Crop N monitoring methods have been developed to improve N fertilizer management, most of them based on leaf or canopy optical-property measurements. However, sensitivity to environmental interference remains an important drawback. Electrical impedance has been applied to determine the physiological and nutritional status of plant tissue, but no studies related to plant-N contents are reported. The objective of this article is to analyze how the electrical impedance response of plants is affected by their N status. Four sets of lettuce (Lactuca sativa L.) with a different N-source concentrations per set were used. Total nitrogen and electrical impedance spectra (in a 1 to 100 kHz frequency range) were measured five times per set, three times every other day. Minimum phase angles of impedance spectra were detected and analyzed, together with the frequency value in which they occurred, and their magnitude at that frequency. High and positive correlation was observed between plant N content and frequency values at minimum phase angle with no significant variations detected between days of measurement. These results suggest that electrical impedance can be sensitive to plant N status.

Throughout evolution, living beings have had to face and resist adverse conditions that tested their adaptive capacity. As a result, they have developed processes such as hormesis to ensure their survival and their ability to thrive in challenging environments. Currently, this process is recognized as a key mechanism that complements Darwin and Wallace’s theory of evolution, making it necessary to explore its relationship with other processes linked to natural selection such as adaptation, adaptability, plasticity, variation, and variability, among the main ones. Subsequent research within the framework of Neo-Darwinism and Modern Synthesis better explains hormesis and the understanding of the complexity of biological responses. In this framework, there is a great need to put hormesis in context based on the laws of variation and inheritance and establish a consistent, updated, and expanded definition that allows the integration of hormesis with evolutionary processes. In addition, the biological mechanisms through which hormesis may be related to the evolutionary process are discussed.

Hydrogen peroxide (H₂O₂) application in several plant species has been widely studied as a plant biostimulant; however, the use of acoustic emissions related to hydric stress (MHAF) in biostimulating plants has not been widely studied, including the response of plants to the interaction of different stress factors. The aim of the present work was to evaluate the stress response in some morphological, biochemical, and molecular variables of the single or combined application of H₂O₂ and MHAF in C. annuum L. plants. Acoustic emission frequencies were obtained in a previous study where the frequencies came from C. annuum plants submitted to medium hydric stress (MHAF). Our results showed that the combination of the two stressors evaluated has a possible synergistic effect on variables such as SOD activity and relative gene expressions of ros1, met1, and MAPkinases (mkk5, mpk4-1, mpk6-2), as well as an antagonistic effect for flavonoid content, DPPH, and ABTS free radical inhibition, and def1 gene expression. MHAF showed increased plant height, PAL activity, and mpk6-1 and erf1 gene upregulation, while H₂O₂ increased POD activity and upregulated pr1a gene. These findings suggest possible stress response pathways that are activated and enhanced by the presence of these stress factors, both individually and in conjunction with one another, making it possible to use them as novel strategies for agricultural stress management.

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.