Pectins, e.g., calcium pectinate, constitute the main structural fraction of the middle lamella and maintain tissue integrity. electron microscopy and an X-ray microanalyzer were used and showed a beneficial effect of calcium preparations around the ultrastructure of fruit epidermis and hypodermis cells, manifested in the presence of a normally developed cell wall with a regular middle lamella, preserved continuity of cytoplasmic membranes, and stabilized cell structure. In the selected elements of apical epidermis cells, the highest level of Ca2+ ions was detected in the middle lamella, cell wall, plasmalemma, and cytoplasm. The highest increase in the Ca2+ content in these cell constituents was recorded in treatment IV, whereas the lowest value of the parameters was noted in variant III. L. plants. This effect was associated with improvement in stomatal conductance and thermostability of the oxygen-evolving complex (OEC), which might be due to lower accumulation of reactive Cilliobrevin D oxygen species. Ca2+ pretreatment of heat-stressed tobacco plants decreased the contents of H2O2 and superoxide radical anion (O2??), enhanced the induction of heat shock protein 70 (HSP70), and increased glutathione reductase (GR) activity, whilst the activities of superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), and peroxidase (POD) were either enhanced or inhibited, compared to the high-temperature treatment. The findings reported by Cilliobrevin D Zhang et al. [14] exhibited that Ca2+ participated in the Cilliobrevin D nitric oxide (NO)-induced tolerance to low temperature (11 C/7 C) by modulating leaf gas exchange, PSII-related processes, carbohydrate metabolism, and expression of chlorophyll synthesis-related genes in L. seedlings leaves. Hajihashemi et al. [15] described the protective role of pretreatment of (quinoa) seeds with CaCl2, H2O2, and sodium nitroprusside (SNP) at concentrations of 5, 5, and 0.2 mM, respectively, which limited the adverse effect of salt stress on seed germination. This beneficial effect was manifested by a significant increase in the germination rate, relative germination rate, and germination index, as well as enhanced protein and amino acid contents. Moreover, the stimulated amylase activity resulted in starch breakdown and increased content of water-soluble sugars, which have an osmoprotectant role in overcoming salt stress. Calcium is usually a component or an activator of many important enzymes. As part of respiratory enzymes, it determines fruit firmness. Therefore, low calcium content in stored fruit contributes to high intensity of respiratory processes, which leads to rapid turgor loss [8,9]. Ca2+ ions regulate the hormonal balance in plants. They reduce the synthesis of ethylene, delay fruit aging, and are involved in carbohydrate metabolism by increasing starch accumulation [6,16,17]. At the cellular level, calcium is usually a structural component of cell walls linking the long chains of -D-galacturonic acid in protopectins. Pectins, e.g., Rabbit Polyclonal to KLHL3 calcium pectinate, constitute the main structural fraction of the middle lamella and maintain tissue integrity. The role of pectins is particularly important in the fruit structure. These compounds form a specific scaffold, which is usually solid and strong at high calcium content, but fragile and weak at low Ca2+ concentrations. Hence, fruit with low calcium content will be small with poorly developed flesh, low firmness, and a thin epidermis. Calcium dehydrates cytoplasmic colloids, thereby increasing their viscosity and reducing hydrophilicity. It stabilizes cell walls and cytoplasmic membranes and regulates their permeability and selectivity. It also influences cell division, growth, and function, and determines proper pollen germination, pollen tube growth, and seed formation [8,16,18,19]. 1.2. Foliar Feeding Besides the basic mechanism of nutrient uptake via the root system, certain amounts of essential elements can also be taken up by shoots, leaves, and fruit, as applied in practice. This type of nutrient supply is used especially in orchard and indoor crop cultivation. Nutrients supplied through foliar feeding penetrate through ectodesmata (approx. 30 nm in diameter) present in the outer cell wall of the shoot epidermis. These routes of penetration of secreted substances to the wall are involved in the formation of Cilliobrevin D cuticle and surface waxes [20,21]. To facilitate the penetration of nutrients through the cuticle, brokers increasing adhesion of liquids to herb cells are added to working solutions [22]. Foliar feeding is usually based on chelated fertilizers; however, calcium ions form such compounds with difficulty [20,23,24,25]. A thick cuticle and wax layer around the fruit surface is a barrier that prevents sprayed nutrients from penetration [26,27,28]. A thick apple fruit cuticle built of overlapping wax platelets contributes to lower susceptibility of the epidermis to damage and russeting formed by the cork tissue. In turn, a thin cuticle built of a hardly flexible uniform tissue promotes fruit russeting [29,30,31,32,33,34]. The susceptibility of apples to russet is usually carried by genes responsible for the structure of the outer fruit layers, in particular the cuticle [35,36]. 1.3. Calcium Content Estimation of the.