PEER-REVIEW ARTICLE PEER-REVIEWED ARTICLE bioresources.cnr.ncsu.edu Mosa et al. (2025). “Oranges & anti-sunburn spray,” BioResources 20(4), 9606-9624. 9606 Influence of Kaolin, Calcium Oxide, and Boron Trioxide Sprays to Reduce Sunburn and Enhance Fruit Productivity and Quality in Murcott Mandarin Walid F. A. Mosa ,a,* Waleed A. A. Alsakkaf,b Lidia Sas-Paszt ,c and Hayssam M. Ali ,b,* Climate change has increasingly disrupted the growth and development of many fruit crops. Among the associated challenges, sunburn caused by excessive light and solar radiation is a major physiological disorder affecting citrus and other fruit species. This study evaluated the potential of kaolin (KL), calcium oxide (CaO), and boric acid (B2O3) to mitigate sunburn and improve fruit set, yield, and quality in Murcott mandarin. Foliar sprays were applied at concentrations of 2000, 3000, and 4000 ppm KL, either alone or in combination with CaO and B2O3 at 0 CaO + 0 B2O3, 500 ppm CaO + 50 ppm B2O3, and 1000 ppm CaO + 100 ppm B2O3. Applications were performed four times during each season (mid-March, early July, early August, and early September) in 2023 and 2024. The results demonstrated that foliar application of KL, CaO, and B2O3 significantly increased fruit set, yield, and both physical and chemical quality attributes by reducing sunburn incidence across both study seasons. The most effective treatments were 4000 ppm KL + 500 ppm CaO + 50 ppm B2O3 and 4000 ppm KL + 1000 ppm CaO + 100 ppm B2O3. DOI: 10.15376/biores.20.4.9606-9624 Keywords: Fruit quality; Kaolin; Productivity; Sunburn Contact information: a: Plant Production Department (Horticulture-Pomology), Faculty of Agriculture, Saba Basha, Alexandria University, Alexandria 21531, Egypt; b: Department of Botany and Microbiology, College of Science, King Saud University, Riyadh– 11451, Saudi Arabia; c: The National Institute of Horticultural Research, Konstytucji 3 Maja 1/3, 96-100 Skierniewice, Poland; * Corresponding author: walidmosa@alexu.edu.eg; hayhassan@ksu.edu.sa INTRODUCTION Fruit trees are highly vulnerable to environmental stresses, and the severity of these stresses has intensified in recent years due to climate change (Rachappanavar et al. 2022). Elevated temperatures impair photochemical activity and reduce light energy conversion efficiency, with excessive heat and radiation damaging the photosystems (Sun et al. 2018). Photosynthesis is particularly sensitive to high temperatures, leading to rapid declines under intense light conditions (Ahammed et al. 2018). During summer, when temperatures exceed 45 °C, sunburn damage can cause yield losses of up to 40% (Sarooghinia et al. 2020). The combined effects of strong solar radiation, high temperature, and low humidity trigger physiological disorders that markedly decrease fruit yield and quality (Kalcsits et al. 2017). Sunburn typically appears as peel discoloration and surface burns, progressing to large necrotic black patches on the fruit skin (Pareek et al. 2015). mailto:walidmosa@alexu.edu.eg https://orcid.org/0000-0003-1954-502X https://orcid.org/0000-0003-4076-4032 https://orcid.org/0000-0001-6801-4263 PEER-REVIEWED ARTICLE bioresources.cnr.ncsu.edu Mosa et al. (2025). “Oranges & anti-sunburn spray,” BioResources 20(4), 9606-9624. 9607 Fertilizers and spraying of particle suspensions have emerged as promising strategies to enhance photosynthesis, improve leaf development, and promote overall plant vigor, while simultaneously reducing nutrient losses compared with conventional fertilizers. Their high efficiency not only improves yield and fruit quality but also minimizes negative environmental impacts such as soil pollution and nutrient leaching (Usman et al. 2020; Jakhar et al. 2022). Due to their small particle size and high absorption capacity, nanoparticles can improve nutrient delivery and uptake efficiency (Yadav et al. 2023). Kaolin (KL) has been widely studied as a protective agent against abiotic stress. When applied as a foliar spray, KL forms a reflective barrier on leaves and fruits that lowers tissue temperature, mitigates solar radiation stress, reduces transpiration, and ultimately enhances fruit yield and quality (Glenn and Puterka 2010). KL reduces sunburn injury by reflecting ultraviolet and infrared radiation, without interfering with stomatal function or photosynthesis (Ergun 2012; Glenn 2012). In olive trees, KL application under water- limited conditions improved photosynthetic rates (Denaxa et al. 2012). Similarly, in mango, KL coatings alleviated high irradiance stress, decreased leaf temperature and vapor pressure deficit, and increased photosynthesis, gas exchange, fruit set, and total yield (Chamchaiyaporn et al. 2013). Other studies reported that KL improves plant growth, dry matter accumulation, and water use efficiency under stress, while reducing transpiration, tissue temperature, and leaf thickness (Segura-Monroy et al. 2015). As a natural clay mineral, KL acts as both a reflective agent and an antitranspirant, enhancing photosynthetic efficiency and stress tolerance (Ramírez-Godoy et al. 2018). It has been shown to reduce canopy temperature (Luciani et al. 2020), improve gas exchange under drought and salinity (Abdallah 2019), and alter biochemical traits such as soluble solids and anthocyanin levels in grapes (Cataldo et al. 2022; Brito et al. 2019). Saure (2005) emphasized that calcium is crucial for membrane stability and cell integrity. It helps prevent chlorophyll degradation, improves trees’ nutritional quality by increasing chlorophyll content, leaf area, and water uptake, and enhances fruit nutrient levels. This, in turn, boosts photosynthetic efficiency, positively impacting vegetative growth and yield. Calcium, which is commonly applied as CaCl2 or Ca(NO3)2, plays essential roles in plant growth, development, and stress responses. It supports photosynthesis by activating key metabolic enzymes (Akhtar et al. 2010), regulates auxin function, and promotes cell division, elongation, and pollen tube growth (Fageria 2009). In grape, calcium contributes to flowering, fruit set, cluster weight, and cell expansion (Bonomelli and Ruiz 2010). Preharvest CaCl2 sprays reduce postharvest decay and physiological disorders (Madani et al. 2014), while enhancing antioxidant levels and secondary metabolites (Ghesmati et al. 2017; Vighi et al. 2019). Calcium strengthens the cell wall through the formation of Ca-pectate complexes, improving fruit firmness and delaying ripening (Zhang et al. 2019; Jaime-Guerrero et al. 2024). In addition, calcium regulates ethylene production, respiration, chlorophyll development, and senescence (Mounika et al. 2021). Calcium has shown promise in improving fruit yield (Gao et al. 2019), enhancing stress tolerance under salinity and drought (Nasrallah et al. 2022), and improving storage and shelf life (Zhou et al. 2018; Huang et al. 2023). Boron (B) is another essential nutrient that plays critical roles in photosynthesis, stomatal conductance, carbohydrate metabolism, and reproductive development. Adequate boron supply improves photosynthetic efficiency, pollen germination, tube elongation, flowering, and fruit set (Sotiropoulos et al. 2002; Marschner 2012). Boron oxide (B₂O₃) is recognized as an essential factor in regulating the translocation of sugars and carbohydrates PEER-REVIEWED ARTICLE bioresources.cnr.ncsu.edu Mosa et al. (2025). “Oranges & anti-sunburn spray,” BioResources 20(4), 9606-9624. 9608 within plant systems. In olive (Olea europaea L.), boron deficiency markedly diminishes pollen viability, fruit set, and the normal differentiation of reproductive organs. Furthermore, insufficient boron availability compromises the mechanical stability of the leaf cuticle, frequently resulting in epidermal fissuring. Such structural impairments adversely affect stomatal functionality, thereby reducing their efficiency in the foliar uptake of nutrient solutions (Khayyat et al. 2007). Boron deficiency reduces cell division and photosynthetic activity, while affecting structural integrity of cell walls by limiting Ca–pectin binding (Ardic et al. 2009; Gupta and Solanki 2013). In various fruit crops, borax application improves fruit set, sugar transport, the metabolism of carbohydrates, nucleic acids, indole acetic acid, and phenols, seed development, and overall productivity (Xu et al. 2015; Mohammed et al. 2018; Shireen et al. 2018; Bons and Sharma 2023). It also enhances vitamin C and soluble protein content, maintains cell membrane integrity, and increases metabolic activity (Milagres et al. 2019; Xu et al. 2021; Li et al. 2023). Given the importance of these nutrients, the present study was conducted to evaluate whether foliar application of kaolin, calcium, and boron compounds could mitigate sunburn damage in Murcott mandarin while improving fruit yield and quality. Materials and Methods Experimental site and applied treatments The experiment was conducted during the 2023 and 2024 seasons on eight-year- old Murcott mandarin (Citrus reticulata) trees grafted onto Citrus volkameriana rootstock, spaced 4 × 5 m apart. The orchard was located in El-Nubaria, Beheira Governorate, Egypt, with trees grown in sandy soil under drip irrigation. Soil characteristics are presented in Table 1. Sixty uniform trees were selected and subjected to the same cultural practices throughout both seasons. Treatments consisted of foliar sprays of kaolin (KL: 2, 3, or 4 g/L), calcium oxide (CaO: 0, 0.5, or 1 g/L), and boric acid (B2O3: 0, 50, or 100 mg/L), as well as an untreated control (Table 2). Sprays were applied four times each season: mid- March, early July, early August, and early September. Treatments were arranged in a randomized complete block design (RCBD) with five replicates (one tree per replicate), giving a total of 60 trees. Spray solution volume was 20 L per tree, prepared with tap water. Fruit Set %, fruit drop %, and fruit number At full bloom in April, the total number of flowers was recorded, and fruit set (%) was calculated (Eq. 1). Fruit set % = 𝑁𝑜.𝑜𝑓 𝑠𝑒𝑡 𝑓𝑟𝑢𝑖𝑡𝑠 𝑁𝑜.𝑜𝑓 𝑓𝑙𝑜𝑤𝑒𝑟𝑠 × 100 (1) The fruit drop percentage was calculated by Eq. 2. Fruit drop % = 𝑁𝑜.𝑜𝑓 𝑓𝑟𝑢𝑖𝑡𝑙𝑒𝑡𝑠 𝑎𝑡 𝑖𝑛𝑖𝑡𝑖𝑎𝑙 𝑠𝑒𝑡 − 𝑁𝑜.𝑜𝑓 ℎ𝑎𝑟𝑣𝑒𝑠𝑡𝑒𝑑 𝑓𝑟𝑢𝑖𝑡𝑠 𝑁𝑜.𝑜𝑓 𝑓𝑟𝑢𝑖𝑡𝑙𝑒𝑡𝑠 𝑎𝑡 𝑖𝑛𝑖𝑡𝑖𝑎𝑙 𝑠𝑒𝑡 × 100 (2) Fruit Yield (ton/ha) Fruit yield (t/ha) was calculated by multiplying the average fruit weight per tree by the number of trees per hectare. PEER-REVIEWED ARTICLE bioresources.cnr.ncsu.edu Mosa et al. (2025). “Oranges & anti-sunburn spray,” BioResources 20(4), 9606-9624. 9609 Table 1. Soil Analysis Sand 90.8% Silt 6.55% Clay 2.65% Textural class Sandy Organic matter 0.275% pH 8.26 CaCO3 4.8% EC (dS/m) 1.2 Soluble anions meq/L) HCO3 - 3.12 CL- 4.57 SO4 -- 4.91 Soluble cations (meq/L) Ca++ 2.22 Mg++ 1.5 Na+ 3.66 K+ 4.42 Available nutrients (mg/kg) Macronutrients N 117.5 P 15.2 K 297.5 Micronutrients Fe 1.9 Zn 0.38 Mn 1.85 Table 2. Weather Data during the Experimental Seasons of 2023 and 2024 Year 2023 2024 2023 2024 2023 2024 2023 2024 Months Minimum Temperature (°C) Maximum Temperature (°C) Average Relative Humidity (%) Total Precipitation (mm) January 11.89 12.20 20.35 19.84 72.73 66.88 31.70 28.20 February 10.61 11.50 18.87 20.30 68.77 71.17 48.70 13.90 March 12.75 12.55 23.43 23.27 63.57 64.55 19.00 8.90 April 14.39 15.70 26.33 28.03 59.06 63.53 16.00 2.80 May 17.36 18.07 29.53 29.92 59.56 55.40 0.40 0.00 June 20.98 22.39 32.63 35.66 60.92 58.55 2.40 0.30 July 23.70 24.64 35.93 35.61 61.01 60.53 0.00 0.00 August 24.36 24.73 35.00 35.54 60.61 59.61 0.00 0.00 September 24.05 23.87 34.80 33.45 60.11 60.04 0.60 0.00 October 21.30 20.47 30.15 29.77 65.05 62.07 4.30 0.00 November 18.05 17.01 26.89 24.72 63.62 65.22 5.00 6.80 Table 3. Applied Treatments T1 0 KL + 0 CaO + 0 B2O3 T2 0 KL + 500 ppm CaO + 50 ppm B2O3 T3 2000 ppm KL + 0 CaO + 0 B2O3 T4 0 KL + 1000 ppm CaO + 100 ppm B2O3 T5 3000 ppm KL + 0 CaO + 0 B2O3 T6 2000 ppm KL + 500 ppm CaO + 50 ppm B2O3 T7 4000 ppm KL + 0 CaO + 0 B2O3 T8 2000 ppm KL + 1000 ppm CaO + 100 ppm B2O3 T9 3000 ppm KL + 500 ppm CaO+ 50 ppm B2O3 T10 3000 ppm KL + 1000 ppm CaO + 100 ppm B2O3 T11 4000 ppm KL + 500 ppm CaO + 50 ppm B2O3 T12 4000 ppm KL + 1000 ppm CaO + 100 ppm B2O3 PEER-REVIEWED ARTICLE bioresources.cnr.ncsu.edu Mosa et al. (2025). “Oranges & anti-sunburn spray,” BioResources 20(4), 9606-9624. 9610 Physical fruit quality Fruit length and diameter (cm) were measured using a digital Vernier caliper. Average fruit weight was recorded from 10 representative fruits. Fruit volume (cm3) was determined by water displacement in a 1000 mL graduated cylinder. Firmness (kg/cm2) was measured with a Magness–Taylor pressure tester. Sunburn incidence (%) was determined using Eq. 3: Fruit sun scalds % = Number of sun scald fruits 𝑡𝑜𝑡𝑎𝑙 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑓𝑟𝑢𝑖𝑡𝑠 × 100 (3) Chemical fruit quality Total soluble solids (TSS %) in the juice of fruits were measured by using a hand refractometer (ATAGO, Tokyo, Japan). Total acidity (%) was determined as citric acid/100 milliliters of fruit juice (AOAC 2005). The phenol-sulfuric acid method was used to estimate the total sugars by using 1.0 mL of sample treated with 1.0 mL of 5% phenol and 5.0 mL of concentrated H2SO4 and measured at 485 nm. Reduced sugars were estimated by using the 3,5-dinitro salicylic acid (DNS) method by using 2.0 mL of the sample and 1.5 mL of DNS at 80 °C for 10 min and measuring at 510 nm (Lam et al. 2021). The content of ascorbic acid was determined by titration using 2, 6-dichloro phenol indophenol (Nielsen 2017). Fruit carotene content was measured using the method of (Aquino et al. 2018) at a wavelength of 440 nm. Nutritional Status After harvesting the fruits in the 2023 and 2024 seasons, 40 leaves from the middle part of the shoots were collected from each tree (Arrobas et al. 2018). After washing the leaves very well with tap water, they were washed again with distilled water, dried at 70 °C until constant weight, and ground and digested using H2SO4 and H2O2 until the solution became clear. The nitrogen content was determined using the micro Kjeldahl method (Wang et al. 2016). The phosphorus content was measured using the Vanadomolybdo method (Wieczorek et al. 2022). Potassium content was determined using a flame photometer (Asch et al. 2022). Atomic absorption spectrophotometry was used to measure the content of Ca, and B. Statistical Analysis Data were analyzed by one-way analysis of variance (ANOVA) using CoHort Software 6.311 (Pacific Grove, CA, USA). Treatment means were compared using the least significant difference (LSD) test at the 0.05 probability level (Snedecor and Cochran 2021). RESULTS Fruit set percentage, fruit weight, fruit number, and overall yield were significantly improved by foliar spraying of KL in combination with calcium and boron compared with the control (Table 4; Fig. 1). The greatest increases were observed with treatment T12, followed by T11, T10, and T9, which all showed significant improvements over untreated trees in both seasons. PEER-REVIEWED ARTICLE bioresources.cnr.ncsu.edu Mosa et al. (2025). “Oranges & anti-sunburn spray,” BioResources 20(4), 9606-9624. 9611 Table 4. Effect of the Foliar Spraying of Kaolin, CaO, and B2O3 on the Fruit Set %, Fruit Weight, and Number in Murcott Mandarin during 2023 and 2024 Treatments Fruit Set (%) Fruit Weight (g) Fruit Number 2023 2024 2023 2024 2023 2024 T1 3.75f 4.38e 173.25g 174.88h 302.67f 305.33e T2 3.9f 4.56e 174.06g 176.68gh 311.67e 311.33d T3 4.19e 4.81d 175.04g 176.60gh 312.67e 318.67c T4 4.29de 4.90cd 179.67f 178.76g 316.33de 321.67c T5 4.32de 4.89cd 182.82e 182.16f 319.33cd 327.00b T6 4.36cde 4.88cd 183.54de 184.71ef 324.00bc 327.00b T7 4.39cde 4.89cd 184.96d 186.19de 324.67bc 329.00ab T8 4.49bcd 5.03c 186.93c 188.54cd 326.33ab 329.33ab T9 4.57bc 5.08bc 187.43c 190.85bc 328.67ab 329.33ab T10 4.59bc 5.27ab 187.36c 190.53bc 329.67ab 332.33a T11 4.66b 5.31a 191.67b 192.62b 329.67ab 331.67ab T12 5.11a 5.39a 198.20a 198.66a 331.00a 333.00a LSD0.05 0.21 0.20 1.79 2.85 5.23 4.67 Note: No significant differences between the treatments that have the same letters in each column. The explanation for the treatments is demonstrated in Table 2. Fig. 1. The effect of the foliar spraying of KL, CaO, and B2O3 on the fruit yields in kg per tree and in ton per hectare in Murcott mandarin during 2023 and 2024 seasons Fruit volume, length, and diameter were also significantly enhanced by KL + CaO + B2O3 applications (Table 5). Among the treatments, T12 produced the highest values, while T11 and T10 also markedly improved these parameters relative to the control. i h h g f ef de cd c c b a j i h g f ef de cd bc b b a T1 T2 T3 T4 T5 T6 T7 T8 T9 T10 T11 T12 YIELD(KG/TREE)2023 2024 i h h g f ef de cd c c b a j i h g f ef de cd bc b b a T1 T2 T3 T4 T5 T6 T7 T8 T9 T10 T11 T12 YIELD (TON/HECTARE)2023 2024 PEER-REVIEWED ARTICLE bioresources.cnr.ncsu.edu Mosa et al. (2025). “Oranges & anti-sunburn spray,” BioResources 20(4), 9606-9624. 9612 Table 5. Effect of the Foliar Spraying of Kaolin, CaO, and B2O3 on the Fruit Volume, Length, and Diameter in Murcott Mandarin during 2023 and 2024 Treatments Fruit Volume (cm3) Fruit Length (cm) Fruit Diameter (cm) 2023 2024 2023 2024 2023 2024 T1 180.58g 182.11h 5.31e 5.26 e 6.15e 6.18f T2 181.39g 183.78gh 5.38de 5.34de 6.17de 6.20ef T3 182.37g 184.00gh 5.42de 5.36cde 6.18de 6.20ef T4 187.34f 186.23g 5.45de 5.41cde 6.20cde 6.20ef T5 189.82e 189.36f 5.47de 5.47cd 6.23cd 6.23def T6 190.54de 192.18ef 5.54cd 5.49cd 6.24c 6.24de T7 192.62cd 193.62de 5.67bc 5.52bcd 6.29b 6.26cd T8 193.60c 196.01cd 5.67bc 5.56bc 6.31b 6.26cd T9 194.43c 198.15bc 5.75b 5.69b 6.31b 6.32bc T10 194.36c 200.06b 5.79ab 5.93a 6.31b 6.34b T11 198.67b 197.93bc 5.84ab 5.95a 6.35b 6.34b T12 205.53a 205.96a 5.93a 5.98a 6.42a 6.41a LSD0.05 2.21 2.82 0.16 0.18 0.05 0.06 Note: No significant differences between the treatments that have the same letters in each column Fruit juice percentage and firmness increased significantly in trees treated with KL + CaO + B2O3 (Table 6). Treatment T12 achieved the greatest improvement, whereas T11, T10, and T9 also produced positive effects compared with untreated trees in both seasons. In contrast, sunburn incidence was markedly reduced by T12, T11, and T10, with moderate reductions observed in T9 and T8 compared to the control. Table 6. Effect of the Foliar Spraying of Kaolin, CaO, and B2O3 on Sunburn and Fruit Juice Percentages, and Fruit Firmness in Murcott Mandarin during 2023 and 2024 Treatments Sunburn (%) Juice (%) Fruit Firmness (kg/cm2) 2023 2024 2023 2024 2023 2024 T1 13.90a 13.50a 48.10f 50.10g 2.20c 2.23c T2 13.00b 12.60b 49.49ef 51.60fg 2.23bc 2.23c T3 12.50bc 12.00bc 49.87ef 51.62fg 2.24bc 2.24c T4 12.00cd 11.79c 51.13de 52.87ef 2.23 bc 2.26c T5 11.80cd 11.43cd 51.53de 53.00ef 2.26b 2.26c T6 11.74d 10.79de 51.67de 53.67def 2.26b 2.27c T7 10.47e 10.30e 52.37cd 54.33de 2.26b 2.27c T8 10.29ef 9.53f 52.43cd 55.27de 2.26b 2.28c T9 10.26ef 9.30f 54.33c 56.20cd 2.27b 2.33b T10 9.67f 8.43g 54.43c 58.33c 2.34a 2.36b T11 8.90g 8.30g 56.80b 62.33b 2.35a 2.37b T12 8.25g 7.83g 61.91a 65.00a 2.38a 2.45a LSD0.05 0.67 0.75 2.17 2.43 0.04 0.05 Note: No significant differences between the treatments that have the same letters in each column Chemical fruit quality also responded positively to kaolin + CaO + B2O3. Total soluble solids (TSS), vitamin C, and carotene content were significantly increased, with T12 again being the most effective treatment (Table 7). T11, T10, T9, and T8 also showed substantial improvements compared with untreated trees. At the same time, fruit acidity was significantly reduced by all treatments, with T12 producing the greatest reduction across both seasons. PEER-REVIEWED ARTICLE bioresources.cnr.ncsu.edu Mosa et al. (2025). “Oranges & anti-sunburn spray,” BioResources 20(4), 9606-9624. 9613 Table 7. Effect of the Foliar Spraying of Kaolin, CaO, and B2O3 on Fruit Total Soluble Solids, and Acidity Percentages, Vitamin C, and Carotene in Murcott Mandarin during 2023 and 2024 Treatment TSS (%) Acidity (%) VC (mg/100 mL Juice) Carotene (mg/100g) 2023 2024 2023 2024 2023 2024 2023 2024 T1 8.69h 9.38f 1.35a 1.23a 29.93e 30.47h 2.78e 2.81f T2 9.00gh 9.43f 1.24b 1.23a 30.68de 31.43gh 2.90de 3.07e T3 9.24fg 9.93e 1.167c 1.20ab 31.13cde 32.22fg 2.95d 3.17e T4 9.39efg 10.21de 1.137cd 1.157b 32.19cd 32.05fg 3.02cd 3.14de T5 9.52ef 10.49d 1.12cd 1.10c 31.32cde 33.14ef 3.02cd 3.26cd T6 9.88de 11.11c 1.10de 1.08cd 32.21cd 34.33de 3.14bc 3.28bc T7 10.19cd 11.18c 1.10de 1.06cd 32.18cd 34.60cd 3.15bc 3.40ab T8 10.23bcd 11.29c 1.09de 1.08cd 32.91bc 35.82bc 3.17b 3.42a T9 10.37bcd 11.52bc 1.05e 1.07 cd 32.17cd 36.53b 3.19ab 3.43a T10 10.53bc 11.83b 0.98f 1.04de 34.46ab 36.63b 3.23ab 3.41a T11 10.73b 11.94b 0.91g 1.00e 35.02a 36.87b 3.28ab 3.46a T12 11.45a 12.40a 0.88g 0.86f 36.16a 38.30a 3.32a 3.48a LSD0.05 0.46 0.46 0.05 0.04 1.67 1.22 0.13 0.12 Note: No significant differences between the treatments that have the same letters in each column Sugar fractions were strongly affected by the treatments (Table 8). T12 and T11 significantly increased total sugars, reducing sugars, and non-reducing sugars, while T10, T9, T8, and T7 also enhanced sugar content relative to the control. Leaf mineral composition was similarly improved (Table 9). Foliar sprays of KL + calcium + boron significantly increased leaf concentrations of N, P, K, Ca, and B compared with untreated trees. T12 consistently produced the highest nutrient contents, followed by T11, T10, T9, and T8. Table 8. Effect of the Foliar Spraying of Kaolin, CaO, and B2O3 on Fruit Content from the Total, Reduced, and Non-reduced Sugars in Murcott Mandarin during 2023 and 2024 Treatments Total Sugars (%) Reduced Sugars (%) Non-reduced Sugars (%) 2023 2024 2023 2024 2023 2024 T1 5.47e 6.13d 3.05d 3.55f 2.42de 2.58d T2 5.47e 6.28d 3.10d 3.58f 2.37e 2.70 bcd T3 5.48e 6.33d 3.15d 3.77de 2.33e 2.57d T4 5.57de 6.43d 3.12d 3.70ef 2.453de 2.73bcd T5 5.80c 6.45d 3.25d 3.77de 2.55d 2.68cd T6 5.75cd 6.55d 3.20d 3.88cde 2.54d 2.68cd T7 6.28b 7.03c 3.55bc 3.88cde 2.73c 3.15a T8 6.36b 7.05c 3.57bc 3.93cd 2.79c 3.11ab T9 6.47b 7.11bc 3.53bc 4.04bc 2.94b 3.07abc T10 6.48b 7.47ab 3.50c 4.05bc 2.98b 3.43a T11 6.87a 7.58a 3.73ab 4.17b 3.14a 3.41a T12 6.93a 7.66a 3.81a 4.51a 3.12a 3.15a LSD0.05 0.21 0.4 0.19 0.17 0.12 0.38 Note: No significant differences between the treatments that have the same letters in each column PEER-REVIEWED ARTICLE bioresources.cnr.ncsu.edu Mosa et al. (2025). “Oranges & anti-sunburn spray,” BioResources 20(4), 9606-9624. 9614 Table 9. Effect of the Foliar Spraying of Kaolin, CaO, and B2O3 on Leaf Content from Nitrogen, Phosphorous, Potassium, Calcium, and Boron in Murcott Mandarin during 2023 and 2024 Treatments N% P% K% Ca% B (ppm) 2023 2024 2023 2024 2023 2024 2023 2024 2023 2024 T1 1.62f 1.76g 0.52f 0.52g 1.43e 1.48g 1.37g 1.35h 43.33g 43.00g T2 1.62f 1.78g 0.53f 0.53g 1.43e 1.50fg 1.37g 1.37gh 45.00fg 44.67g T3 1.64f 1.80fg 0.52f 0.57f 1.45e 1.50fg 1.41fg 1.38gh 45.67fg 45.67fg T4 1.70e 1.83ef 0.53f 0.58ef 1.45e 1.53f 1.40fg 1.43fg 46.33efg 46.33fg T5 1.73d 1.83ef 0.54f 0.60e 1.46e 1.58e 1.44ef 1.45ef 47.33ef 45.67fg T6 1.74d 1.84ef 0.54f 0.63d 1.52d 1.66d 1.45def 1.50de 49.33de 48.33ef T7 1.82c 1.86de 0.58e 0.63d 1.52d 1.68d 1.43f 1.54cd 51.67cd 51.33de T8 1.83c 1.90cd 0.59de 0.64d 1.60c 1.74c 1.48de 1.52d 54.67c 54.33d T9 1.84c 1.94bc 0.62cd 0.65d 1.61c 1.83b 1.49cd 1.59c 60.33b 60.00bc T10 1.88b 1.99b 0.64bc 0.68c 1.68b 1.84b 1.53c 1.66b 59.33b 58.33c T11 1.93a 2.04a 0.66b 0.72b 1.72a 1.87a 1.59b 1.67b 62.00ab 62.00b T12 1.93a 2.06a 0.70a 0.76a 1.73a 1.90a 1.65a 1.73a 65.00a 66.00a LSD0.05 0.03 0.04 0.03 0.02 0.03 0.04 0.04 0.06 3.27 3.10 Note: No significant differences between the treatments that have the same letters in each column PEER-REVIEWED ARTICLE bioresources.cnr.ncsu.edu Mosa et al. (2025). “Oranges & anti-sunburn spray,” BioResources 20(4), 9606-9624. 9615 DISCUSSION In this study, foliar application of kaolin (KL), calcium oxide (CaO), and boron trioxide (B2O3) significantly reduced fruit sunburn, enhanced fruit set, and improved both yield and fruit quality of Murcott mandarin trees. The combined treatment (T12) was consistently superior across both seasons, indicating a strong synergistic effect of these elements on productivity and fruit quality traits. The positive effects of KL observed here agree with previous findings that KL enhances crop performance under heat stress. KL forms a reflective particle film that lowers fruit and leaf surface temperature, mitigates water loss, and optimizes physiological processes such as stomatal conductance and photosynthesis (Glenn and Puterka 2010; Dinis et al. 2018; Brillante et al. 2016). Studies on pear, olive, grapevine, and citrus similarly reported that KL sprays reduce sunburn, improve fruit set, and enhance both physical and biochemical quality traits (Colavita et al. 2011; Brito et al. 2018; Gullo et al. 2020; Terán et al. 2024). These benefits stem not only from reduced heat load but also from improved water use efficiency and the stimulation of primary and secondary metabolism, which contribute to higher fruit size, juice content, vitamin C, carotene, and sugar levels. Applying KL particle film promotes both primary and secondary metabolic processes in grapevines, thereby enhancing berry quality. In addition, it has been employed as a short-term strategy for mitigating climate change, as its reflective properties toward ultraviolet and infrared radiation help decrease leaf temperature and improve photosynthetic performance by alleviating photoinhibition (Conde et al. 2018). KL helps reduce water loss from transpiration and maintains a relatively open rate of stomata, thereby ameliorating the plant water content (Brito et al. 2019). KL can lower leaf temperature by up to 6.7 °C, promoting stomatal opening, which in turn boosts photosynthesis and improves plant tolerance to intense light and heat (de Abreu et al. 2022). Applying KL at concentrations of 2%, 4%, and 6% significantly improved fruit set percentage, yield, fruit weight, and firmness, while simultaneously reducing sunburn incidence in pomegranate (Al-Saif et al. 2022). Foliar KL application provides key nutrients and improves soil moisture retention, while its reflective properties lower leaf temperature and reduce water loss, collectively supporting cell division, elongation, and overall growth (Vani et al. 2023). KL alleviates environmental stress by minimizing leaf damage and abscission, protecting carotenoids, elevating proline, chlorophyll, and gas exchange, and regulating key hormones like abscisic acid, salicylic acid, indole-3-acetic acid, and jasmonic acid to strengthen stress tolerance (Terán et al. 2024). KL sprays at 3 and 6 % on apple improved tree crown and volume, increased fruit weight, fruit length, firmness, and fruit soluble solids (Faghih et al. 2021), enhanced leaf area, chlorophyll, photosynthesis, gas exchange, internal CO₂, water-use efficiency, potassium status, and kernel yield/quality, while reducing leaf temperature, fruit drop, sunburn, and cracking in Persian walnut (Gharaghani et al. 2018). Spraying KL at 3 and 6 % on ‘Tardy Nonpareil’ Almond trees improved shoot growth, prevented premature leaf abscission, water use efficiency and tree yield, reduced leaf temperature by up to 3.25 °C, and it increased the photosynthesis rate and stomata conductance, tree yield, and nut attributes, under water deficit (Gharaghani et al. 2023). Calcium oxide application also was found to play a role in enhancing fruit quality in this study. As background, it is known that each type of calcium salt produces a different pH in distilled water, influencing its solubility. Calcium citrate has a higher solubility than calcium carbonate in water, the difference is minimal at pH 7.5. The partial pressure of PEER-REVIEWED ARTICLE bioresources.cnr.ncsu.edu Mosa et al. (2025). “Oranges & anti-sunburn spray,” BioResources 20(4), 9606-9624. 9616 CO2 reduced the solubility of calcium carbonate at pH 7.5. Soluble calcium calculations aligned with in-vivo data on absorbed calcium. The data highlight the influence of pH and CO2 on the solubility of calcium salts in the presence of bicarbonate secretions in the intestine (Goss et al. 2007). Marschner (2012) noted that calcium plays a key role in binding proteins and polysaccharides in the cell wall, enhancing flowering, fruit maturation, and the transport of carbohydrates from leaves to fruits. Calcium oxide spray enhances cell wall permeability, improving water and nutrient transport from leaves to fruits, which boosts fruit growth, maintains moisture balance between the peel and internal tissues, and preserves cell wall elasticity (Korkmaz and Askın 2015). Foliar CaCl2 applications have been shown to improve fruit firmness, size, and storability by strengthening cell walls, maintaining water balance, and delaying senescence (Michailidis et al. 2020). The present results align with reports from pomegranate, apple, and kiwifruit where Ca sprays improved yield, fruit weight, soluble solids, and vitamin C while reducing acidity and sunburn (Ramírez-Godoy et al. 2018; Denaxa et al. 2023; Fahmy et al. 2023). The improvements in firmness and reduced sunburn incidence observed in the present study further support Ca’s protective role under environmental stress. CaCl2 application in fruits helps maintain firmness and cell turgor, while preventing physiological disorders (Jain et al. 2019). The spraying of 100 and 200 CaCO3 notably increased the stem thickness and plant height, which are essential for supporting the overall structure and stability of the plant, and this improvement can be attributed to Ca’s crucial role in maintaining cell wall integrity and promoting cell division and elongation (Nangare et al. 2020; Mogazy et al. 2022). Applying CaCl2 at 1.5 and 2.0% on apple significantly improved fruit physical attributes such as fruit firmness, fruit diameter, length, weight, and size, fruit content from anthocyanin, phenols, TSS, acidity, starch content, and fiber content (Ranjbar et al. 2020). Spraying pomegranate trees cv. Wonderful with CaCl2 at 2 and 3 % noticeably improved the fruit weight, number and fruit yield, fruit dimensions, as well as fruit content from soluble solids, and vitamin C, while it significantly reduced the fruit acidity, fruit drop percentages and fruit sunburn percentages (Fahmy et al. 2023). Calcium oxide (CaO), commonly called quicklime, is a white to grayish solid obtained by heating calcium carbonate to release carbon dioxide. At room temperature, it readily reabsorbs carbon dioxide from the air and reacts vigorously with water to form calcium hydroxide, Ca(OH)2, releasing heat in the process—hence the term “quick,” or living, lime. This exothermic reaction has been used in portable heat sources. One of the oldest manufactured chemical products, quicklime is widely applied in construction, industrial neutralization processes, and occasionally as fertilizer, though calcium carbonate is generally preferred for agriculture. When water acts on CaO, it produces calcium hydroxide, also known as slaked lime. Only a small amount dissolves to yield limewater, while the remainder forms a suspension called milk of lime. Calcium hydroxide serves as an industrial alkali and a key ingredient in mortars, plasters, and cement. It also plays roles in the kraft paper process and in sewage treatment as a flocculant (Hanusa 2025). Boron supplementation also contributed significantly to fruit set, retention, and overall productivity. Boron is essential for pollen germination, tube growth, sugar transport, and cell wall integrity, flower fertilization and ultimately improving the productivity (Davis et al. 2003; Bybordi and Malakouti 2006; Lewis 2019). Deficiency often results in flower drop, reduced fertilization, and lower leaf growth, root elongation, and yields (Han et al. 2008; Wang et al. 2015). The present findings agree with earlier reports by Ali et al. (2017), they noted that the external spraying of B2O3 at 0.2 and 0.3% improved the fruit set %, retained fruit number, fruit weight, pulp %, fruit productivity, PEER-REVIEWED ARTICLE bioresources.cnr.ncsu.edu Mosa et al. (2025). “Oranges & anti-sunburn spray,” BioResources 20(4), 9606-9624. 9617 fruit dimensions, and fruit content of chlorophyll, carotene, and vitamin C in mango and lowered the fruit drop percentage. Boron also has a vital role in cell structure and cell wall integrity (Lewis 2019) and in maintaining plasma membrane functions (Brown et al. 2002). The reduction in fruit drop from borax spraying may also be attributed to B’s indirect role in auxin synthesis, which delays the formation of the abscission layer during the early stages of fruit development, ultimately increasing the percentage of fruit retention (Gupta et al. 2022). Mosa et al. (2015) noted that the spraying of apple with B2O3 at 0.2% markedly improved the shoot diameter, shoot length and leaf area, fruit set %, productivity, while it minimised the fruit acidity and fruit drop percentages. Spraying mandarin trees with borax at 0.04% boron remarkably improved the total flower and fruit set percentage and significantly reduced the flower and fruit drop percentages as compared to untreated trees (Ruchal et al. 2020). Sajid et al. (2024) reported that spraying B2O3 at 1% on three sweet cherry cultivars increased leaf area, fruit diameter, pulp %, total soluble solids, total sugars, fruit number, yield in kg, fruit set %, while it reduced the percentages of fruit drop and fruit acidity compared to untreated trees. Taken together, the combined application of KL, CaO, and B2O3 compounds improved both physiological efficiency and fruit quality traits in Murcott mandarin. KL primarily reduced heat and water stress, CaO enhanced cell wall strength and nutrient transport, and B2O3 optimized reproductive success and carbohydrate allocation. The synergistic effects of these treatments translated into higher fruit number, size, firmness, juice content, vitamin C, carotene, sugars, and leaf mineral concentrations. These findings highlight the potential of integrated foliar sprays as an eco-friendly strategy to enhance citrus productivity and mitigate the negative impacts of abiotic stress. Further work should investigate the economic feasibility of large-scale adoption and the long-term effects of repeated applications on soil-plant interactions. CONCLUSIONS 1. Foliar application of kaolin (KL) together with calcium oxide (CaO) and boron trioxide (B2O3) effectively mitigated the adverse effects of high temperature and solar radiation, reducing fruit sunburn and thereby increasing the proportion of marketable Murcott mandarin fruits. 2. The combined application of KL, CaO, and B2O3 compounds improved photosynthesis, fruit set, yield, and overall fruit quality. 3. The treatment with 4000 ppm KL + 1000 ppm CaO + 100 ppm B2O3 consistently produced the best results across all measured traits. ACKNOWLEDGMENTS The authors would like to extend their sincere appreciation to the Ongoing Research Funding Program (ORF-2025-123), King Saud University, Riyadh, Saudi Arabia. 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