Calcium oxalate crystals are widespread among animals and plants. In land plants, crystals often reach high amounts - up to 80% of dry biomass. They are formed within specific cells, and their accumulation constitutes a normal activity rather than a pathological symptom, as occurs in animals. Despite their ubiquity, our knowledge on the formation and the possible role(s) of these crystals remains limited. We show that the mesophyll crystals of Amaranthus hybridus (pigweed) exhibit diurnal volume changes with a gradual decrease during daytime and a total recovery during the night. Moreover, stable carbon isotope composition indicated that crystals are of non-atmospheric origin. Stomatal closure (under drought conditions or exogenous application of abscisic acid) was accompanied by crystal decomposition and by increased activity of oxalate oxidase that converts oxalate into CO2. Similar results were also observed under drought stress in Dianthus chinensis, Pelargonium peltatum and Portulacaria afra. Moreover, in pigweed, despite closed stomata, the leaf metabolic profiles combined with chlorophyll fluorescence measurements indicated active photosynthetic metabolism. In combination, calcium oxalate crystals in leaves can act as a biochemical reservoir that collects non-atmospheric carbon, mainly during the night. During the day, crystal degradation provides subsidiary carbon for photosynthetic assimilation, especially under drought conditions. This new photosynthetic path, with the suggested name "alarm photosynthesis", seems to provide a number of adaptive advantages such as water economy, limitation of carbon losses to the atmosphere and a lower risk of photoinhibition, roles that justify its vast presence in plants.

Silver nanoparticles (AgNPs) were synthesized using hydroalcoholic extracts of dittany (Origanum dictamnus), sage (Salvia officinalis), sea buckthorn (Elaeagnus rhamnoides, syn. Hippophae rhamnoides), and calendula (Calendula officinalis) as reducing agents. AgNPs synthesized using NaBH₄ and citric acid were used as control. The impact of the origin of the extract and preparation conditions (light, temperature, reaction time) on the properties of the synthesized AgNPs was investigated. The structure, morphology, composition, physicochemical characteristics, and colloidal stability were characterized using dynamic laser scattering (DLS), ultraviolet-visible spectrophotometry (UV–/Vis), XRD, X-ray fluorescence (XRF), TEM, and FTΙR. The reduction of total phenolic and flavonoid content of the extracts after the reaction of AgNPs synthesis was also determined. Low IC50 values for all types of AgNPs revealed good antioxidant activity, attributable to the phenolic and flavonoid content of their surface. The results suggest that plant extract selection is important to the green synthesis of AgNPs because it affects the kinetics of their synthesis as well as their morphology, physicochemical characteristics, and colloidal stability. In vitro permeation studies on porcine skin revealed that AgNPs remained at the upper layers of stratum corneum and did not penetrate the skin barrier after 4 h of cutaneous application suggesting the safety of their application on intact skin for a relatively short time.