Reactive oxygen species (ROS), by their very nature, are highly reactive, and it is no surprise that they can cause damage to organic molecules. In cells, ROS are produced as byproducts of many metabolic reactions, but plants are prepared for this ROS output. Even though extracellular ROS generation constitutes only a minor part of a cell’s total ROS level, this fraction is of extraordinary importance. In an active apoplastic ROS burst, it is mainly the respiratory burst oxidases and peroxidases that are engaged, and defects of these enzymes can affect plant development and stress responses. It must be highlighted that there are also other less well-known enzymatic or non-enzymatic ROS sources. There is a need for ROS detoxification in the apoplast, and almost all cellular antioxidants are present in this space, but the activity of antioxidant enzymes and the concentration of low-mass antioxidants is very low. The low antioxidant efficiency in the apoplast allows ROS to accumulate easily, which is a condition for ROS signaling. Therefore, the apoplastic ROS/antioxidant homeostasis is actively engaged in the reception and reaction to many biotic and abiotic stresses.

Calcium ions (Ca²⁺) are absorbed from the soil by the root cells before being distributed throughout the plant. The transport of Ca²⁺ to aboveground parts relies on the movement of xylem, which is influenced by the transpiration rate of individual organs. The final distribution of Ca²⁺ in tissues and cells depends on the effective function of channels and transporters that facilitate Ca²⁺ movement through plasma and specific intracellular membranes. Local fluctuations in free Ca²⁺ concentrations serve as a mechanism to elicit cellular responses, characterized by distinct calcium signatures, or to enable long-distance signaling in a cell-to-cell network. The specificity of individual Ca²⁺ transport proteins is primarily determined through studies in plants with induced changes in the expression of these proteins. Concurrently, it is recognized that plant cells contain significant reserves of Ca²⁺ both in the labile and in the permanently bound states, which influences various aspects such as biomechanical properties or the defensive capabilities of the cells. The diverse roles of Ca²⁺ in plant cells highlight the importance of a thorough understanding of Ca²⁺ metabolism.

During developmental leaf senescence in Arabidopsis, mitochondria simultaneously maintain primary energy processes and are the site of a number of catabolic pro

Ammonium nutrition has been suggested to be associated with alterations in the oxidation‐reduction state of leaf cells. Herein, we show that ammonium nutrition in Arabidopsis thaliana increases leaf NAD(P)H/NAD(P)⁺ ratio, reactive oxygen species content and accumulation of biomolecules oxidized by free radicals. We used the method of rapid fractionation of protoplasts to analyse which cellular compartments were over‐reduced under ammonium supply and revealed that observed changes in NAD(P)H/NAD(P)⁺ ratio involved only the extrachloroplastic fraction. We also showed that ammonium nutrition changes mitochondrial electron transport chain activity, increasing mitochondrial reactive oxygen species production. Our results indicate that the functional impairment associated with ammonium nutrition is mainly associated with redox reactions outside the chloroplast.