Salinity and boron (B) toxicity are two distinct yet related issues that can affect the growth and development of plants (Lu et al. 2023). In order to meet the increasing food demand, ways to enhance productivity in saline and B toxic soils should be explored. Salinity refers to the presence of high levels of salt in the soil or irrigation water. It can be caused by natural factors such as the proximity to coastal areas or human activities like excessive use of fertilizers or poor irrigation practices. High salt concentration in the soil creates an osmotic imbalance, drawing water out of the plant's roots and reducing its ability to take up water (Khatri and Rathore 2022). This leads to water stress and subsequent wilting. Salinity disrupts the balance of essential ions, particularly sodium (Na) and potassium (K), in plant cells. Excessive Na uptake can lead to toxicity, while reduced K levels hinder important physiological processes (Wakeel 2013). Salinity interferes with nutrient uptake by plants, especially for essential nutrients like nitrogen, phosphorus, and magnesium. This can result in nutrient deficiencies, affecting plant growth and yield. High salt levels can lead to the accumulation of toxic ions, such as chloride (Cl) or Na, in plant tissues. These ions can disrupt cellular processes, damage cell membranes, and interfere with metabolic functions. Salinity can inhibit photosynthesis, the process by which plants convert sunlight into energy (Khatri and Rathore 2022). This results in reduced growth and diminished yield. Boron is an essential micronutrient for plants, but excessive levels can lead to toxicity. Boron plays a crucial role in cell wall formation and stability. However, excessive B levels can disrupt the structural integrity of the cell walls, leading to impaired growth and weakened plant tissues. High B concentrations can interfere with the uptake of other essential nutrients, such as calcium and magnesium (Long and Peng 2022). This imbalance can disrupt various physiological processes and hinder plant growth. Boron toxicity can disrupt several metabolic processes, including carbohydrate metabolism and protein synthesis. This can negatively impact plant growth, development, and overall productivity. One characteristic symptom of B toxicity is the development of necrotic spots or browning on the edges and tips of leaves. This is often accompanied by stunted growth and reduced vigor. Boron toxicity can also interfere with flower and seed development, leading to abnormal reproductive structures and reduced seed set (Brdar-Jokanovic 2020).
Salinity and B toxicity are generally simultaneous stress conditions in plants (Pandey et al. 2019). Since B transport in plants is mainly dependent on transpiration, in saline soils with high osmotic pressure, transpiration decreases, resulting in reduced B transport, and consequently, plants can be less affected by B toxicity (Mohammed et al. 2016). However, due to specific ion toxicity caused by salinity or osmotic stress, the biomass of plants decreases, leading to a relative increase in the amount of boron in the plant (Pandey et al. 2019; Karimi and Tavallali 2017).
Boron and Ca have similar features, which were including low mobility, has a structural role on cell wall with low cytosolic concentration, and growth alterations at the deficiency. The amount and availability of either of these elements impacts on the distribution (Ramón et al. 1990) and the requirement of the other for optimal plant growth (Teasdale and Richards 1990; Bonilla et al. 2004; Etasami et al. 2021). It's important to note that while both B and Ca are essential nutrients, their application should be based on the specific needs of the plants being cultivated and the analysis of the soil to avoid deficiencies or excesses that can negatively impact plant growth. In addition, Ca plays crucial structural and signaling roles in plant development. Besides, it has a role in the cell wall structure, extension function, osmoregulation, extension with cell wall stiffness, and regulation of some enzymes, improves fruit density and decline the pathogen affect (Hamza et al. 2021; Mogazy et al. 2022).
Reactive oxygen species (ROS) production is one of the responses of plants to abiotic stresses such as salinity, drought and others, which plant developed antioxidant enzymes to overcome the excessive ROS production in salinity stressed-plant cells (You and Chan 2015). The effect of nano-fertilizer on increasing the amount of antioxidant enzymes activity which have been verified in many studies (Abdoli et al. 2020; Gaafar et al. 2020; Gonz´alez-García et al. 2021; Mushtaq et al. 2020; Rani et al. 2016). The researchers showed that nano-fertilizer have the properties of certain antioxidant enzymes and hence support the plant to cope with the oxidative stress conditions such as nano iron, zinc, calcium, silicon and others. Nano fertilizers possess distinct physico-chemical properties that confer advantages over traditional chemical fertilizers. With small particle dimensions (< 100 nm), nano fertilizers can effectively penetrate plant systems (Zahedi et al. 2020; Seleiman et al. 2021; Gil-Díaz et al. 2022). Due to their ultra-small size, nano fertilizers exhibit a high surface area (Babu et al. 2022; Hussain et al. 2022), which enhances their absorption and retention capacity compared to conventional bulky chemical fertilizers. The increased surface area of nano fertilizers enables them to contain higher nutrient loads and release these nutrients gradually in accordance with crop demands, without causing any negative consequences (Siddiqi and Husen 2017; Babu et al. 2022; Gil-Díaz et al. 2022). In contrast, common synthetic fertilizers often suffer from low plant uptake efficiency, leading to nutrient wastage through leaching, volatilization, and gaseous emissions, contributing to environmental degradation (Dimkpa et al. 2015a, b; Benlamlih et al. 2021; Yadav et al. 2023). The slow and gradual nutrient release characteristic of nano fertilizers mitigates these negative environmental outcomes. As a result, nano fertilizers show promise in sustainable agriculture practices by improving nutrient utilization efficiency and minimizing environmental impacts.
In saline and B toxic conditions, plants Ca uptake decreases due to the antagonistic effect caused by Na (Acosto-Motos et al. 2017). Additionally, the effectiveness of Ca applications in preventing B toxicity is known (González-Fontes et al. 2014). Studies have explored obtaining nano-Ca from eggshells (Habte et al. 2019; Jalu et al. 2021). However, there is no research on the effects of nano-Ca on plants grown in saline-B toxic soils. In this study, nano-Ca was obtained from eggshell waste and its impact as an alternative Ca source on plant growth, Ca, B, Na, and Cl uptake, as well as oxidative stress mechanisms under saline and B toxic conditions were investigated.