Drought is one of the important abiotic stresses, where plants constantly do not receive sufficient rainfall required to complete their metabolic activities (Zhang et al., 2022). The unavailability of water reduces 9-10% of the total crop productivity worldwide (Lesk et al., 2016). Monsoon rainfall has decreased by roughly 6% from 1951 to 2015 as per the climate change assessment report by the Ministry of Earth and Science (Krishnan et al., 2020). Low rainfall makes the dry land areas more vulnerable to runoff losses leading to drought proneness. A study from South Africa during 2017-2021 revealed that 25000 agricultural sector-based workers lost their jobs due to the adverse impact of drought on the economy (Orimoloye et al., 2022).

Drought impacts seed germination, the number of tillers, spikes, grain per plant, grain weight, plant stand and grain yield (Oumarou Abdoulaye et al., 2019; Ning et al., 2020; Gui et al., 2021a; Gui et al., 2021b; Sheteiwy et al., 2021a; Sheteiwy et al., 2021b; Ben-Jabeur et al., 2022; Sial et al., 2022; Sheteiwy et al., 2022). The drought also impairs the nutrient absorption ability of plants from the upper soil horizon (Abdelaal et al., 2021). It also modulates the physiological processes of the plants such as photosynthesis, respiration, leaf water potential, mineral absorption, circadian rhythm and hormone regulation etc. (Seleiman et al., 2021; Wang et al., 2021; Ghani et al., 2022; Hemati et al., 2022). Drought stress increases the production of reactive oxygen species (ROS) such as superoxide (O₂ ^-), singlet oxygen (O₂), hydrogen peroxide (H₂O₂), and hydroxyl radicals (OH^-) to levels that are frequently greater than the plant’s scavenging capability (Chitara et al., 2017; Kumari et al., 2021; Sachdev et al., 2021). ROS damages cells and cellular components, impair physiological and biochemical processes and can even cause plant death (Xie et al., 2019). Owing to ROS-induced oxidative stress increased electrolyte leakage followed by lipid peroxidation and plasmalemma damage. Lipid peroxidation causes the breakdown of polyunsaturated lipids in ketones and malondialdehyde (MDA) (Ju et al., 2018). Excessive ROS generation causes site-specific amino acid modification, peptide chain fragmentation, changed electric charge and enhanced protein proteolysis. ROS causes deoxyribose oxidation, strand breaks, nucleotide loss, a variety of changes to the bases of the nucleotides, and DNA-protein crosslinks (Sharma et al., 2019; Ahmed and Lingner, 2020).

Finger millet (Eleusine coracana (L.) Gaertn) is a millet crop widely produced in tropical and subtropical areas of Asia and Africa. It contains a very high amount of calcium (344 mg/100 g). The millet seed coat is an edible component of the kernel and has a high concentration of dietary fiber, phytochemicals such as polyphenols (0.2–3.0%) and high gallic acid (Hadimani and Malleshi, 1993; Chethan and Malleshi, 2007). Finger millet is important for pregnant and breastfeeding women and children’s nutrition. It plays a significant role in the economies of marginal farmers. Furthermore, finger millet straw is an excellent animal feed, containing up to 60% digestible elements. The seed coat also has anti-cancer and anti-diabetic properties owing to its high polyphenol content.

Nutrients play a significant role in plant growth and development. The scarcity of nutrients in the plants caused an irreversible change. Presently used fertilizers especially nitrogenous fertilizers in crops are less efficient due to their losses in the form of volatilization, surface runoff, leaching and gaseous form. The scarcity of nutrients are more severe when plants are suffering from water deficiency because most of the mineral uptake from the soil to plant cell take the same pathway as a water flow. The application of nanomaterials to crops would revolutionize farming practices by reducing the adverse environmental effects of modern agricultural activities and improving nutrient use efficiency (NUE), grain quality and crop yield (Liu and Lal, 2015; Mahil and Kumar, 2019). Nanotechnology can provide a workable solution to control the difficulties associated with increasing N-based fertilizer usage efficiency (Zulfiqar et al., 2019). It is anticipated to cause a paradigm shift in NUE, resulting in increased agricultural productivity (Ladha et al., 2005). In this context, sequential research was undertaken by synthesizing CaP-U NPs. In comparison to bulk counterparts, CaP-U NPs exhibit greater reactivity and surface area. Considering the above facts, the current study was conducted to reveal the Plant growth promoting potential of urea-doped calcium phosphate nanoparticles (CaP-U NPs) in finger millet (Eleusine coracana (L.) Gaertn.) under irrigated and drought stress conditions: an emerging fertilization technique under climate change scenario.

Nanofertilizer such as U-ACP nanoparticles were used as a nitrogen source for Vitis vinifera L. (Gaiotti et al., 2021). N-doped ACP NPs with half the absolute N-content than in conventional urea treatment promote the formation of an equivalent amount of root and shoot biomass, without nitrogen depletion (Carmona et al., 2021). The high nitrogen use efficiency (up to 69%) and a cost-effective preparation method support the sustainable real usage of N-doped ACP as a nano fertilizer. In a field experiment, the use of calcium phosphate NPs doped with urea (U-ACP) for the fertilization of Triticum durum plants, indicated that yields and quality of the crops treated with the nanoparticles at reduced nitrogen dosages (by 40%) were unaltered in comparison to positive control plants, which were given the minimum N dosages to obtain the highest values of yield and quality in fields. In light of these reports here bring to light the possibility of using engineered nanoparticles to deliver nitrogen to plants more safely and efficiently. However, further research is still needed to secure the most suitable application protocols for real agricultural practices (Ramírez-Rodríguez et al., 2020b). Nano-Urea applied to Pennisetum glaucum L. at 30 and 45 DAS, significantly increased plant height, dry matter accumulation, chlorophyll content and nitrogen content (Sharma et al., 2022).

Indeed, drought continues to ravage different regions of the globe, with devastating consequences on soil nutrient bioavailability and crop productivity. Nano NPK improved photosynthetic rate, stomatal conductance, CO₂ concentration, water use efficiency and relative water content. The chemical composition (plant pigments, total carbohydrates, total phenolic, tannin, total flavonoids, oil constituents, macro and micro-elements) with indigenous hormones (gibberellic acid GA3 and abscisic acid ABA) and antioxidant enzymes (peroxidase and superoxide dismutase) were also positively affected (Mahmoud and Swaefy, 2020). Furthermore, Ca2+ improved maize photosynthesis (45%), stomatal conductance (47%), and accumulation of total soluble sugars (20%) along with the decline in H₂O₂ content (23%) (Naeem et al., 2018). hydroxyapatite nanoparticles foliar application in Adansonia digitata provide a significant increase in plant growth characteristics (Soliman et al., 2016).

Mechanistically, Research has shown that increased nitrogen (N) improves crop drought tolerance and significant impact on photosynthesis. The nitrogen in plants influenced the water conductivity increasing the accumulation of osmoprotectants and antioxidants (Chang et al., 2016; Zhong et al., 2017; Song et al., 2019). But the majority of supplied N is lost through leaching, volatilization and denitrification leading to a reduction in crop N usage efficiency (Hussain et al., 2019; Pan et al., 2021). The objectives of the present study are to: i) synthesize and characterize urea-doped calcium phosphate nanoparticles (CaP-U NPs); ii) determine whether CaP-U NPs can mitigate the impact of drought stress on the performance of finger millet; and iii) evaluate whether using a lower dose of CaP-U NPs. Collectively, all effects were compared with those of bulk urea to determine the significance of nanoscale size.

Data and results were represented in means, which were statistically examined by SPSS (Statistical package for the social science) software comparing variance (ANOVA) function. Duncan’s multiple range test was used to compare the treatment mean values at the P ≤ 0.05 significant level. Principal component analysis (PCA) and Pearsion correlation were done using Origin and R-square (version 4.1.2) to demonstrate the correlation between the various plant growth parameters and defense enzyme and their relationship with the different treatments.

Water scarcity significantly influences plant performance by reducing growth, development and other physiological processes by higher accumulation of ROS levels, which causes cell dysfunction in the plants. However, in the present study, applying CaP-U NPs increased the growth, photosynthetic pigments as well as antioxidant enzyme activity by reducing the ROS level under drought-stress conditions. N accumulation in the foliar parts of the plants has a positive correlation with root water conductivity ( Figure 10 ). The nitrogen in plants influenced the water conductivity, which is regulated by the expression of the aquaporin gene mainly nodulin 26-like protein (NIPs) and tonoplast intrinsic proteins (TIPs). However, over-expression of the aquaporin genes could enhance plant drought tolerance (Ren et al., 2015; Li et al., 2016). Generally, NIPs were observed to facilitate the transport of water and the efflux of N, as well as the entry of N into cells via the plasma membrane, followed by vacuolar loading through TIPs. Vacuolar loading is beneficial for the storage of excess N, and vacuolar unloading can remobilize the N under N starvation conditions (Ding et al., 2018).

Nano-fertilizers application significantly improved uptake through pores or uptake could be facilitated by complexation with molecular transporters, through the creation of new pores, or by exploitation of endocytosis or ion channels (Rico et al., 2011). Nano-fertilizers are nutrient carriers capable of holding bountiful nutrients due to their high surface area and releasing it slowly (Abdel-Aziz et al., 2016). Nano fertilizers control the release of nutrients from the fertilizer granules to improve nutrient utilization efficiency (NUE) while preventing the nutrient ions from getting fixed or lost in the environment (Subramanian et al., 2015; Chhipa, 2017). Nanoscale fertilizers could lead to the more effective delivery of nutrients as their small size may allow them access to various plant surfaces and transport channels. Leaf surfaces are nano- and microstructured surfaces containing cuticular pores and stomata. A study on the penetration of two different sizes particles (43 nm or 1.1 μm diameter) into leaves of Vicia faba L. indicated that the nano-sized particles could penetrate the leaf interior through the stomatal pores (Eichert et al., 2008). The stomatous leaf surfaces of V. faba and Prunus cerasus had an average pore radius ranging from 25 to 100 nm. Once within the plant, cell-to-cell transport within a plant could be facilitated by the plasmodesmata (Zambryski, 2004). Plasmodesmata are nanoscale channels, 50-60 nm in diameter, enabling cell-to-cell communication and transport (Gunning and Steer, 1996). Chitosan-NPK (10%) fertilizer application gives a significant increase in crop index and harvest index as compared to conventional fertilizer used in wheat crops (Abdel-Aziz et al., 2016). Although in Abelmoschus esculentus, the application of commercial fertilizer needed more quantity (5 g/week) compared to manufactured nano urea (50 mg/week) (Tarafder et al., 2020). Zeolite Based Nitrogen Nano-fertilizers increase yield and nitrogen content in Maize plants (Manikandan and Subramanian, 2016). Urea nano fertilizers can reduce a minimum of 25% of the recommended dose of conventional urea fertilizer, which may be introduced as a more sustainable and economical agricultural practice (Singh et al., 2023). A study was conducted in the Haryana state of India with a total area of 1225 acres and found that an average yield was recorded 5.35% higher in wheat, 24.24% in sesame and 8.4% in mustard by applying nano fertilizers of nitrogen and zinc along with the organic farming practice (Kumar et al., 2022). If N nano fertilizer was applied at the rate of 60 kg ha⁻¹ on Sunflowers would produce the highest seed yield (17.6%) and oil yield (28.7%) as compared to conventional N fertilizer (Handayati and Sihombing, 2019). Half doze of HA-N nano fertilizer shows a similar effect of urea in rice (Bhavani et al., 2020).

CaP-U NPs are primarily used as a regulator rather than a nutrient source because they have the ability to control the release of urea and other nutrients. This controlled release feature allows for more efficient use of the nutrients by the plants. Furthermore, the calcium phosphate component of CaP-U NPs can also increase the availability of essential nutrients such as phosphorus and calcium. This can enhance plant growth and improve crop yields. In the present study, the formation of the CaP-U NPs was confirmed by the color change of the solution. FE-SEM determined the morphology of CaP-U NPs. In FE-SEM Neither urea nor calcium phosphate exhibit any areas of phase separation, indicating that the nanocomposite is successfully encapsulated between the two nano matrices. According to FE-SEM analysis, nano urea particles were of different sizes and possessed a fiberlike structure. These findings are similar to Tarafder et al. (2020) and Kottegoda et al. (2017). According to TEM analysis, the rods are covered with urea at the nanoscale. Carmona et al. (2021) found the existence of irregularly shaped Nano-U-ACP in TEM imaging. Nano NPK TEM analysis confirmed the precipitation of amorphous round-shaped nanoparticles with sizes in the 10-25 nm range (Ramírez-Rodríguez et al., 2020a). The shape of luminous europium-calcium phosphate nanoparticles was round, with sizes far below 100 nm under TEM analysis (Ortiz-Gómez et al., 2020). CaP NPs had a diameter of 26 nm as measured by TEM photomicrographs (Morgan et al., 2008).

Rajonee et al. (2016) designed a nitrogen delivery system wherein hexadecyl trimethyl ammonium bromide (HDTMABr) was used as a surfactant for modifying zeolite, which was then loaded with nitrogen. A pot experiment with Ipomoea aquatica was carried out to evaluate the efficiency of this synthesized nanocomposite and the results showed that nitrogen uptake by the plants treated with nanocomposite was higher than those treated with conventional urea. CaP-Urea NPs have a larger specific surface area and reactivity than their crystalline counterparts (bulk) material. In lettuce, foliar spray of the urea and dry yeast extract significantly improved vegetative growth characteristics, pigment content, chemical content as well as head yield compared to untreated control (Abd El Galil et al., 2021).

In the present study, urea is loaded onto the CaP surface and hence released more slowly than highly soluble traditional fertilizers; this helped in plants’ gradual nitrogen absorption. CaP also shows high solubility in neutral or slightly acidic environments and hence can transfer more of these essential ions to the plant, allowing them to be used as multi-nutrient nano fertilizer (Giroto et al., 2017; Kopittke et al., 2019). Calcium plays a significant role in the development of the structural part and in the biochemical process, i.e., signaling of the plants (Sharma et al., 2017). Calcium also gives resistance to plants against fungal infection since it plays a crucial role in stabilizing and strengthening the cell wall (Cesco et al., 2020). In soybean (Glycine max), the application of the hydroxyapatite nanoparticles as a rich source of phosphorus was reported to be remarkably efficient in increasing growth rate and seed yield by 32.6 and 20.4%. In the current research, the well-characterized CaP-U NPs were used for foliar treatment of the finger millet under irrigated and drought conditions. Experimental results revealed that CaP-U NPs significantly improved all the growth-related parameters viz., shoot and root length, shoot and root dry weight, shoot and root fresh weight and leaf area of finger millet at both 0.1 and 0.5% concentrations under irrigated and drought conditions. Biochemical parameters such as chlorophyll a, chlorophyll b, total proline content, total phenol content, superoxide dismutase (SOD), peroxidase (POD), ascorbate peroxidase (APX) etc. Also significantly improved by the foliar treatment of CaP-U NPs under irrigated and drought conditions.

The maximum content of chlorophyll a (2.2 and 2.6-fold increase) ( Figure 5A ) and b (2.2 and 4.8-fold increase) ( Figure 5B ) was recorded in CaP-U NPs treated plants, both under irrigated and drought conditions. Previous research has found that by controlling the biosynthesis of PSI, PSII, and LHCs, Cyt b6f, ATP synthase, and photosynthetic enzymes, N supply and allocation within the leaf have a significant impact on photosynthesis. Out of the total nitrogen (N) in leaf cells, 75% is present in the chloroplast (Onoda et al., 2017). About 24% of the N in leaves goes to thylakoids, and 75% of that nitrogen goes to light-harvesting proteins (Zhong et al., 2019). N is a part of the chlorophyll molecule. One molecule of Chl contains 4 molecules of N (Evans and Clarke, 2019), which helps in photosynthesis to absorb sunlight energy, promoting plant growth and grain yield (Li et al., 2008). Drought stress reduced plant photosynthesis by destroying the pigment system. Therefore, retaining green leaves during drought in plants is a significant indicator of drought tolerance.

N has a close relationship with stomatal conductance and/or movement. As the main N source for plants, nitrate could regulate stomatal movements (De Angeli et al., 2006). The leaf analysis from nano fertilizer-treated plots found 17.04% more nitrogen, 16.31% phosphorus, and 67.50% potassium compared to the control; the total chlorophyll content rose to 30.68%, and the net rate of photosynthesis increased to 71.7% (Ha et al., 2019). The number of leaves, plant height, and leaf area of the coffee seedlings under greenhouse conditions were also improved by the use of Chitosan-based NPK nano fertilizer (Ha et al., 2019). In Sesame indicum L., the combined application of potassium nano fertilizer and urea could relieve water stress and adverse effects (Mahdavi Khorami et al., 2020). The maximum shoot length was recorded in CaP-U NP at 0.5% with 1.4- and 1.7-fold increase ( Figure 3A ), while maximum root length was also recorded in CaP-U NP at 0.5% with 1.9 and 2-fold increase ( Figure 3B ), respectively. Nitrogen differentially regulates cell elongation and division (Luo et al., 2021). Generally, nitrogen-efficient plants developed a strong root system, which has large root biomass, root volume, root absorption surface area, root active absorption area and a high root oxidation capacity (Xiong et al., 2021). The deposition of lignin and suberin in the roots is controlled by N (Gao et al., 2017). In a study on C. sativus, foliar application of CaP-U NPs provided approximately a three-fold increase in the shoot length, as compared to the control, as well as increased nitrogen (N), calcium (Ca) and phosphorus (P) accumulation in both root and shoot (Feil et al., 2021). In wheat crops, foliar application of the chitosan nanoparticles loaded with nitrogen, phosphorus and potassium (NPK) increased shoot length and grain yield substantially (Abdel-Aziz et al., 2016). The leaf area of the plants was significantly impacted under drought conditions. The maximum leaf area was recorded in CaP-U NP at 0.5% with 1.4 and 2.3-fold increases respectively ( Figure 3C ). The maximum plant fresh weight was recorded in CaP-U NP at 0.5% with 1.6 and 2.3-fold increases ( Figures 4A, B ), while the dry biomass of plants decreased due to severe water stress. The maximum plant dry weight was recorded in CaP-U NP at 0.5% with 3.3 and 2.8-fold increases ( Figures 4C, D ) respectively, as compared to control, under irrigated and drought conditions. Pradhan et al. (2021) synthesized Urea Hydroxyapitide (UH) NPs and tested them only for the germination of rice seed (IR-36) and observed that UH NPs substantially increase seedling growth, fresh weight and dry weight of treatments compared to control. Applying CaP-U NPs to Triticum durum Desf., significantly increased the fresh and dry weight of the shoot (Ramírez-Rodríguez et al., 2020b). Rice plants fertilized with exogenous urea–chitosan nanohybrid (i.e., 500 mg/L) + 60% classical urea, significantly enhanced the growth and yield-related traits (Elshayb et al., 2022). In Sorghum bicolor (L.) Moench, application of the calcium nitrate-gelatin (CNG) coated urea showed maximum dry matter accumulation, high average plant chlorophyll content and apparent nitrogen recovery (ANR) of 71.14% in shoot and 4.5% in roots, respectively (Khan et al., 2021). In Solanum tuberosum L., application of the nano-tri combination (N+Mo+B) increased chlorophyll content, the yield of the dry vegetative part, starch content, total protein and ascorbic acid (Al-juthery and Al-Maamouri, 2020). In hydroponics also, Cucumis sativus L. supplemented with urea provides a maximum growth of root-shoot length and biomass of the plant (Carmona et al., 2021). In addition, Pisum sativum L. treated with SiO₂ NPs significantly increased their relative water content by 29% and specific leaf area by 17% compared to the non-treated control (Sutulienė et al., 2021). Applying the Chitosan nanoparticles in Zea mays significantly enhanced the plant height, leaf area, number of leaves and concentration of organic acids regulators of stress tolerance mechanisms (Khati et al., 2017).

During water scarcity in plants, the activity of the osmoprotectants is increased, which regulates the osmotic potential and increases the stability of membranes and metabolic enzymes of the cells by ROS scavenging (Ahanger et al., 2014). Many researchers suggested a positive correlation between proline accumulation and plant stress. Proline, an amino acid, plays a significant role in alleviating various stress conditions in plants. Besides acting as an excellent osmolyte, proline plays three major roles during stress, i.e., as a metal chelator, an antioxidative defense molecule, and a signaling molecule (Ghosh et al., 2022). Research suggested that proline is 300 times more soluble in water than other amino acids and thus acts as a comparatively non-toxic osmolyte. Nitrogen deficiency in Phaseolus vulgaris has declined the proline level by stimulating proline dehydrogenase. However, the proline level was raised under adequate nitrogen due to the activation of ornithine δ-aminotransferase (Sánchez et al., 2002). Proline helps to maintain the structural integrity of the plant cell, as well as the scavenging of reactive oxygen species (ROS) under drought stress. The maximum total proline content was recorded in CaP-U NP at 0.5% with 1.5-fold increase ( Figure 5C ) and total phenol with a 1.6-fold rise ( Figure 5D ) compared to control under drought conditions. No significant changes were observed between control and treated plants under irrigated conditions. The regulation of antioxidant enzymatic activity is a natural response of plants to oxidative stress caused by various external biotic and abiotic stress factors (Mohammadi et al., 2021). To deal with oxidative damage, plants have evolved an excellent defensive strategy of antioxidant enzyme activities such as SOD, POD, CAT, and APX (Xie et al., 2019). SOD catalyzes superoxide (O^-) elimination by dismutation into oxygen (O₂) and hydrogen peroxide (H₂O₂). The plant’s ability to cope with drought stress is associated with low MDA and H₂O₂. Therefore, under drought conditions, the minimum MDA and H₂O₂ content were recorded ( Figures 7A, B ) in CaP-U NP-treated plants at 0.5% as well as significantly improved SOD (1.9-fold) ( Figure 6A ), POD (1.4-fold) ( Figure 6B ) and APX (3.3-fold) ( Figure 6C ) activity under drought conditions compared to the control. Here, finger millet plants treated with CaP-U NPs showed a good resistance system to alleviate the damage caused by oxidative stress. The enhanced antioxidant enzymatic activities such as SOD, POD and CAT and reduction in MDA and H₂O₂ content indicated an increased redox defense system in response to drought stress. N fertilizer significantly increases leaf superoxide dismutase (SOD) and peroxidase (POD) activities of rice (Jalloh et al., 2009) growing under Cd stress and maize under water stress conditions (Zhang et al., 2007; Ahmad et al., 2022). Foliar application of Chitosan NPs under greenhouse conditions enhanced enzyme activity such as chitosanase, peroxidase and polyphenol oxidase (Sathiyabama and Manikandan, 2021). Chitosan (CHNPs) nanoparticles enhanced phenylalanine ammonia-lyase (PAL), peroxidase (POX), polyphenol oxidase (PPO), catalase (CAT) and β-1, 3 glucanase (GLU) activity in tomato during bacterial wilt infection (Narasimhamurthy et al., 2022). The foliar application of Fullerenol Nanoparticles reduced drought impact by increasing APX in sugarbeets (Beta vulgaris L.) (Borisev et al., 2016). During stress conditions, ascorbic acid (AA) acts as a reducing agent, which reduces H₂O₂ to H₂O and dehydroascorbate (DHA) in chloroplasts and cytosols, respectively (Sharma and Dubey, 2004). The cytosolic APX plays a crucial role in protecting plants from drought and heat stress (Koussevitzky et al., 2008). A heme-containing enzyme, GPX removes excess H₂O₂ in cytosol and vacuole (Sreenivasulu et al., 2004). The foliar application of chitosan nanoparticles (Cs NPs) on L. iberica. during water stress conditions provided the increased activity of superoxide dismutase (SOD), ascorbate peroxidase (APX) and peroxidase (POD) (Javanmard et al., 2022). Foliar and soil treatment of Chitosan NPs (30, 60, and 90 ppm)increased the proline level, catalase (CAT), and superoxide dismutase (SOD) activity during drought stress conditions in Hordeum vulgare L. (Behboudi et al., 2018). Si NPs alleviate the stress in finger millet by up-regulating the activity of antioxidant enzymes like APX, CAT, SOD and GPX (Mundada et al., 2021). The foliar application of Fullerene nanoparticles reduced drought impact by decreasing MDA in sugar beets (Borisev et al., 2016). Application of CaP-U NPs significantly improved morphological and physiological traits more compared to similar or higher doses of bulk urea. This improvement in the plants could be linked to the higher N availability from the CaP-U NPs treatment, as compared to the control. Foliar application of CaP-U NPs on finger millet showed positive effects in terms of improved leaf relative water content (LRWC), stomatal conductance, chlorophyll contents, photosynthetic rate, nitrogen assimilation, increased carbohydrate production and N metabolism under drought stress.

The present study aimed to synthesize and characterize CaP-Urea NPs to assess their potential role in plant growth promotion and defense activation in finger millet under drought conditions. Color changes in visual observation and morphology of NPs by FE-SEM, HR-TEM, and XRD analysis confirmed the synthesized nanoparticles as CaP-U NPs. A broader and valuable outcome of the present work is that a lower dose of CaP-U NPs seems superior in enhancing crop growth, relative to a higher bulk urea dose. Foliar application of the CaP-U NPs increased plant growth indices and activated plant defense under adverse climatic conditions compared to urea as bulk application. Furthermore, CaP-U NPs at 0.5 and 0.1% were found to be slightly more effective in growth indices and defense activation. This investigation demonstrates the roles of nanotechnology in agriculture, one of which is to minimize the input of chemicals into the environment while sustaining crop growth. In future, further studies on yield and nutritional quality improvement using these novel nano-particles under field studies challenged with environmental stresses would provide a deeper understanding of the overall field performance of CaP-U NPs and their potential for commercialization as nano-fertilizer.

The original contributions presented in the study are included in the article/supplementary materials. Further inquiries can be directed to the corresponding author.

DM: conceptualization, methodology, and writing and original draft preparation. PC: supervised the research work and reviewed and edited the manuscript and provided inputs for framing of the manuscript. MC: provided technical assistance and editing of the manuscript. VU: reviewing of the manuscript. JS: provided technical assistance in XRD analysis. All authors contributed to the article and approved the submitted version.