Acta agriculturae Slovenica, 117/3, 1–14, Ljubljana 2021 doi:10.14720/aas.2021.117.3.1865 Original research article / izvirni znanstveni članek Foliar silicate application improves the tolerance of celery grown un- der heat stress conditions Fadl Abdelhamid HASHEM 1 , Rasha M. EL-MORSHEDY 1, 2 , Tarek M. YOUNIS 1 and Mohamed A. A. ABDRABBO 1 Received September 08, 2020; accepted July 28, 2021. Delo je prispelo 8. septembra 2020, sprejeto 28. julija 2021 1 Central Laboratory for Agricultural Climate, Agricultural Research Center, Egypt 2 Corresponding author, email: rmorshedy@hotmail.com Foliar silicate application improves the tolerance of celery grown under heat stress conditions Abstract: Temperature rise is one of the most challeng- ing climate change impacts that increase the intensity of heat stress. In this investigated the production of celery (Apium graveolens var. rapaceum F1 hybrid)) was tested during the late season. The experiment was carried out during the two suc- cessive summer seasons of 2019 and 2020 in Giza Governor- ate, Egypt. The experimental design is a split-plot, the main plots consist of three low tunnel cover treatments, and three spray treatments with three replicates in sub-main plots. Re- sults showed that the use of white net cover gave the highest vegetative growth and yield followed by the black net. Val- ues of plant yield were 951, 765, and 660 g/plant for white, black and without cover, respectively, in the first season. The foliar application of 3 mM of potassium silicate produced the highest vegetative growth and yield compared to the control treatment. Referring to the effect of spray foliar application of potassium silicate on yield 1.5 mM (S1), 3 mM (S2), and control were 892, 795, and 689 g/plant in the first season, re- spectively. The best combination that delivered the highest vegetative growth and yield was a cover low tunnel with a white net combined with S2 foliar application. Key words: celery; climate measures; physical protec- tion; vegetative growth; chemical analysis Foliarno dodajanje silikata izboljšuje toleranco zelene, ki ra- ste v razmerah vročinskega stresa Izvleček: Dvig temperatrure je eden izmed največjih izzivov podnebnih sprememb, katerega učinki povečujejo jakost vročinskega stresa. V tej raziskavi je bil v zvezi s tem preučevan pridelek zelene (Apium graveolens var. rapeceum F1 hibrid) tekom pozne rastne sezone. Poskus je bil izveden v dveh zaporednih poletnih sezonah 2019 in 2020 na območju upravne enote Giza, Egipt. Poskus je bil izveden kot poskus z deljenkami, kjer so obravnavanja na glavnih ploskvah ob- segale tri vrste nizke tunelske kritine in tri obravnavanja s škropljenjem s tremi ponovitvami na podploskvah. Rezultati so pokazali, da je uporaba bele mreže kot kritine dala največji prirast biomase in največji pridelek, čemur je sledila uporaba črne mreže. V prvi rastni sezoni so bili pridelki 951, 765, in 660 g/rastlino za belo, črno kritino in brez nje. V primerjavi s kontrolo je foliarno dodajanje 3 mM kalijevega silikata dalo največjo prirast biomase in največji pridelek. Glede na učinek foliarnega dodajanja kalijevega silikata (1,5 mM (S1), 3 mM (S2), in kontrola) na velikost pridelka so bile njegove vrede- nosti 892, 795, in 689 g/rastlino v prvi rastni sezoni. Najboljša kombinacija, ki je povzročila najboljšo rast in dala največji pridelek je bil nizek tunel pokrit z belo mrežo in s S2 foliar- nim obravnavanjem. Ključne besede: zelena; podnebne razmere; fizikalna za- ščita; vegetativna rast; kemijska analiza Acta agriculturae Slovenica, 117/3 – 2021 2 F . A. HASHEM et al. 1 INTRODUCTION The global climate is expected to witness an in- crease in temperature in the range 2–4 °C by the end of 21 st century (IPCC, 2007). More importantly, predic- tions based on global climate model analysis suggested that the tropical and subtropical regions of the world will be the worst to suffer from the forthcoming heat stress (Battisti and Naylor, 2009). Because of rising tem- perature, alterations in plant’s phenology such as spring and autumn, phenology was noticed across different plant species (Li et al., 2014). Improving micro-climatic conditions for horti- cultural plants and its influence on the plant growth and productivity were considered of the critical factors that control the need to continuous production all- over the year (El-Gayar et al., 2018). Studies conducted by Zakher and Abdrabbo (2014) showed that shading could increase plant growth and productivity through moderating of the harmful effects of high air tempera- ture during summer season for tomato plants. Plastic screen nets in the form of protected cultivation cover- ing materials were widely used for many purposes in the horticulture sector. For example, it was used to pro- tect crops from different farming negatively influencing factors such as heat waves, strong wind, flying insects, mamals, and birds (Al-Helal and AbdelGhany, 2010). Applying screen net regardless of its color had been proved to protect plant against environmental risks such as high air temperature, excessive solar radiation and wind, which improve microclimate for the grown crops through reduction heat, drought stresses, and moderation of extreme climatic events which led to im- proving crop yield and quality (Abul-Soud et al., 2014). The application of plastic net covers in crop production was a sufficient way to provide a cheap and reduced en- ergy consuming technology than polyethylene green- houses (Shahak, 2008). Abdrabbo et al. (2013) stated that open field treatment had a higher air temperature than white net, while black net had the lowest air tem- perature during summer season. Regarding the effect of cover net on relative humidity Hashem et al. (2011) stated that relative humidity increased under black net by 2–4 % compared with open field conditions. Vegeta- tive growth parameters such as plant height, number of leaves, leaf area and productivity under white net were expressively higher than that under open field (Medany et al., 2009). Treder et al. (2016) proved that covering the greenhouses with screen net increases light scat- tering without affecting the light spectrum which led to increase light efficiency that reflected on increasing growth and production measures. One of the most common applications for protect- ing plants from heat stress is foliar spraying with Si, which is approved to be a good option concerning the food productivity; consequently, using Si application was recommended as one of the acceptable practices to increase of vegetable plants productivity (Bakhat et al., 2018). Therefore, several stress factors such as heat waves, which affect vegetables and its productivity are managed by the foliar application of Si via mitigate the injurious impacts of stressors (Cooke and Leishman, 2016). Also, Si is considered as a growth regulator, which participates in the regulation of physiological pro- cesses in plants including seed germination, stomata closure, ion uptake and transport, membrane perme- ability, photosynthesis and plant growth rate according to Noura et al. (2019). This research was conducted to study the effect of protection of celery plant using black and white screen net as well as three foliar applications of potassium silicate and their interactions on vegeta- tive growth, and yield of celery during summer season. 2 MATERIALS AND METHODS 2.1 EXPERIMENTAL SITE This study was carried out in Dokki Location, Giza Governorate, Egypt, during the summer seasons of 2019 and 2020. Dokki location is located at latitude 30.03 and longitude 31.20 with an altitude of 23 m above sea level. Describing the climate of the region; it is dry during summer season, while warm and moderate rain during winter season. The soil of the experimental site is clay soil and having bulk density 1.16 g cm -3 , pH in soil paste (1:2.5) 7.81, EC e 2.39 dS m -1 , and field capacity 25.77 %. 2.2 EXPERIMENTAL PROCEDURE Seedlings of celery (Apium graveolens L. var. ra- paceum (Miller) Gaudin F1 hybrid) with one month from seed germination was used in the current applied study. Seeds were obtained from Takii and Co. LTD (Kyoto, Japan). Seedlings were transplanted into sub- strate system on 16 th and 18 th of March in the 2019 and 2020 seasons, respectively. The following measurements were performed for five labeled plants per replication for each treatment at the end of growing seasons: plant length, number of leaves per plant, base plant diameter, chlorophyll content as well as celery yield. Total of ni- trogen (N), phosphorus (P) and potassium (K) in leaves were measured, ascorbic acid (vitamin C) and soluble sugar were measured in the fresh leaves. Experimental Acta agriculturae Slovenica, 117/3 – 2021 3 Foliar silicate application improves the tolerance of celery grown under heat stress conditions plots were arranged in a split plots design with three replicates. Each experimental plot contained five raised beds (4 m length x 0.8 m width). The distance between each two beds was 0.50 m. The experimental design consists of main plots and sub-plots. The main plot is comprised of three cov- er treatments including white screen net, black screen net, and control (without cover). And the sub-plot con- tains 1.5 mM (S1), 3 mM (S2), and 0 mM (S0) of potas- sium silicate with S0 serving as control (sprayed with tap water). The silicate foliar applications were sprayed on the plant leaves three times, at 3, 5 and 15 weeks from cultivation, at a rate of 50 ml per plant for each. Three replicates were used in this study. Celery plants were irrigated using drippers with flow rate of 4 l h -1 and the distance between each two plants was 0.30 m. Chemical fertilizers (NH 4 ) 2 SO 4 (20.6 % N), K 2 SO 4 (48 % K 2 O) and P 2 O 5 (37 % P 2 O 5 ) were injected within ir- rigation water system at the rate of 80, 40 and 50 kg acre -1 respectively for fertigation purpose. The fertiga- tion was programmed to be three times weekly, and the duration of irrigation time depended highly upon the plant needs. All treatments received the same quantity of fertilizers. 2.3 PLANT ANALYSES Plant samples (outer leaves) were collected after six weeks from transplanting and dried in the oven at 70 °C for one day. Total nitrogen (N) in the dried leaves, digested by H 2 SO 4 /H 2 O 2 mixture, was measured using Kjeldahl method according to the procedure described by Chapman and Pratt (1961). Total phosphorus (P) was measured using spectrophotometer according to Watanabe and Olsen (1965) and total potassium (K) in leaves was measured using flame photometer as de- scribed by Jackson (1958). Total chlorophyll was meas- ured using chlorophyll meter SPAD-502Plus. Soluble sugar content was measured by photometer using the anthrone-sulfuric acid method (Yemm & Wills 1954). Ascorbic acid (vitamin C) was measured in the fresh leaves following 2, 6, dichlorophenol indophenol visual titration method (A.O.A.C., 1980). 2.4 CLIMATE MEASURES Light intensity, maximum and minimum tempera- ture as well as relative humidity were measured under different screen net cover treatments every day using digital climatic sensors. Digital thermo-hygrograph (model: TFA Dostman/Wertheim - Kat. Nr. 5002) was used to measure temperature and relative humidity. The digital thermo-hygrograph was allocated over pol- ystyrene trays in the middle of each treatment above the level of celery plants canopy and the maximum air temperature was recorded at 13:00, while the aver- age relative humidity was calculated by the average of maximum and minimum relative humidity every day. The average weekly maximum temperature and humid- ity was calculated using the daily climatic data. Light intensity was measured in each treatment daily above the celery plants canopy at mid-day (13:00) by portable Lux-meter (Model FMC- 10M). The average weekly light intensity was calculated from the measured data. 2.5 STATISTICAL ANALYSIS Analysis of data was done using SAS program (SAS, 2000). The differences among means for all traits were tested for significance at 5 % level using LSD ac- cording to Waller and Duncan (1969). 2.6 ECONOMIC ANALYSIS OF APPLIED TREAT- MENTS Economic analysis, after considering the cost of cover celery with screen net and potassium silicate, the incomes from celery yield was used (CIMMYT, 1988) according to the formulas: (Net Income = value of obtained yield – annual cost of screen net and potassium silicate application). (Relative increase in income (RII) = (net income / income of control) x 100.) The lifetime of screen net is five years. The cost of spray application of potassium silicate was considered. 3 RESULTS AND DISCUSSION 3.1 CLIMATIC DATA The average maximum air temperatures for the physical protection treatments showed that the use of screen net influenced maximum and minimum temperature (Figure. 1 and 2). Temperature tended to be lower under the black net cover by almost 3 °C compared to open field conditions. The white net reduced the maximum air temperature by almost 1 °C compared to ambient conditions. The minimum air temperature took the same trend, the lowest minimum air temperature was recorded under the black screen net; while the white slightly lower than the black Acta agriculturae Slovenica, 117/3 – 2021 4 F . A. HASHEM et al. Figure 2: The maximum air temperature under black net and white net compared to the open field of the two studied seasons of 2019 and 2020 Figure 1: The minimum air temperature under black net and white net compared to the open field of the two studied seasons of 2019 and 2020 Acta agriculturae Slovenica, 117/3 – 2021 5 Foliar silicate application improves the tolerance of celery grown under heat stress conditions Figure 3: The minimum relative humidity under black net and white net compared to the open field of the two studied seasons of 2019 and 2020 Figure 4: The maximum relative humidity under black net and white net compared to the open field of the two studied sea- sons of 2019 and 2020 Acta agriculturae Slovenica, 117/3 – 2021 6 F . A. HASHEM et al. considering the minimum temperature (about 0.5 °C), which is lower compared to open field conditions. The same trend of maximum and minimum temperature was obtained during both seasons. The maximum relative humidity took another trend; the open field had the lowest relative humidity during both seasons. Black net cover had the highest average maximum and minimum relative humidity followed by the white net cover during the two studied seasons (Figure. 3). Rela- tively, humidity under the black net cover was higher by 3 – 5 % than open field. Maximum average weekly light intensity under different physical protection treatments showed that open field conditions recorded the high- est light intensity followed by the white net cover while the black net screen net had the lowest light intensity during both seasons (Figure. 4). The obtained results of the low temperature under physical protection treat- ments according to the observation, achieved lower interception of sun radiation rays under screen cover than ambient conditions. Use of screen net especially black nets penetrated light intensity and increase rela- tive humidity by 2-5 %. Similar results were reported by Al-Helal and Abdel-Ghany (2010) and Abdrabbo et al. (2013) who indicated that covering the plants with black or white net led to the reduction of the tempera- ture because of the lessening of the radiation via re- flection or absorption by covered materials. Formerly addressed by Shahak et al. (2008) that using screen net led to decrease in air temperature around the cultivated plants in comparison with open field. In conclusion, the lowest recorded maximum temperature was achieved by black net 3.2 VEGETATIVE GROWTH The effect of different net covers on celery vegeta- tive growth characteristics, i.e., plant height, number of leaves per plant, base of plant diameter and chlorophyll content, were presented in Table 1 and 2. Data showed that using black net cover significantly increased the celery plant height, followed by a white net cover, while the lowest plant height was obtained without applying net cover, during the two studied seasons. Number of leaves per plant, base plant diameter and chlorophyll took different trends, the highest values were obtained by white net cover followed by black net procedure. The lowest number of leaves per plant, base of plant diam- eter and chlorophyll were obtained by open field. Regarding the foliar application using two concen- trations of potassium silicate, the highest plant height was obtained by S2 treatment followed by S1. The low- est plant height was obtained by control. Number of Figure 5: Light intensity under black net and white net compared to the open field of the two studied seasons of 2019 and 2020 Acta agriculturae Slovenica, 117/3 – 2021 7 Foliar silicate application improves the tolerance of celery grown under heat stress conditions leaves per plant, base of plant diameter and chlorophyll took the same trend. Regarding the interaction between different net covers and foliar application of potassium silicate, data illustrated that the highest plant height were obtained by black net cover combined with S2 foliar application. Number of leaves per plant, base of plant diameter and chlorophyll took another trend. The highest values were obtained by covering with white net combined with S2 foliar application. The lowest vegetative growth of celery plants was obtained by open field treatment combined with control. The same results were obtained by Tubana and Heckman (2015) who stated that application of silicon led to decrease the harmful effects of heat waves and then enhancing the plant growth and productivity. The same result was confirmed by Bakhat et al. (2018), they concluded that application of silicon compound as foliar application led to enhancement of plant growth and improving the ability of plants to combat the abi- otic stresses such as heat waves. On the other hand, the protection of celery plants from high temperature using black net led to increase the plant height due to low light intensity under the tunnel, which led to improve- ment of the plant elongation. Using white net led to decrease the stress from exposure to direct sun radia- tion without reducing the light intensity such as black net, which also led to increasing the reception of plant leaves to daylight and increasing photosynthesis and then the enhancement of plant growth parameter such as number of leaves and plant diameter (Medany et al., 2009). Moreover, use of screen net led to reduction of the daylight intensity but increasing the light usage ef- ficiency due to sun rays scattered when penetrate the screen later, which led to achieve the sun rays a new angels that led to reach for all plant leaves and then in- creases the total plant photosynthesis (Abul-soud et al., 2014). The screen net could also increase solar radia- tion scattering up to 50 %; that enhanced plant growth. On the other hand, dark net reduced radiation reach- ing crops canopy (Shahak et al., 2004). Low light inten- Treatment Plant height (cm) Number of leaves Base plant diameter chlorophyll Yield (g) cm SPAD g/plant Net covers treatment black 59.3 68.8 4.05 30.6 765 white 47.1 73.8 5.01 36.2 951 control 39.5 56.6 3.72 45.2 660 LSD 5% 3.07 2.37 0.37 2.42 5.82 Potassium Silicate treatment S0 35.8 55.9 3.33 42.5 689 S1 49.3 68.5 4.53 36.0 795 S2 60.8 74.8 4.92 33.5 892 LSD 5% 4.18 2.03 0.146 1.05 4.84 Interaction between cover net and slilicate spray black S0 42.0 58.7 3.66 33.5 653 S1 59.7 68.4 4.22 31.8 718 S2 76.1 79.3 4.27 26.4 925 white S0 33.4 66.9 3.90 42.8 863 S1 48.9 72.3 5.41 29.6 988 S2 59.1 82.1 5.70 36.3 1003 control S0 32.0 41.9 2.42 51.2 553 S1 39.3 64.9 3.96 46.7 680 S2 47.1 63.0 4.77 37.8 748 LSD 5% 1.95 2.19 0.256 2.47 7.03 Table 1: Different efficiencies of different net covers on celery vegetative growth characteristics during the first growing season of 2019 S0 (sprayed with tap water), S1 (1.5 mM of potassium silicate), S2 (3 mM of potassium silicate), SPAD (Unit for determines the amount of chlo- rophyll present by measuring absorbance two wavelength regions), LSD 5% (Significance at 5 % level) Acta agriculturae Slovenica, 117/3 – 2021 8 F . A. HASHEM et al. sity under black net resulting from netting affected the micro-climatic conditions and reduce the plant growth especially in the winter season because of low light intensity (Hashem et al., 2011). Furthermore, silicon application increased the chlorophyll content of plant leaf and enhanced the antioxidant system in plants that were exposed to abiotic stress which led to better pho- tosynthesis (Al-aghabary et al., 2004) 3.3 CELERY YIELD Presented data in Table (1 and 2) shows that there were significant effects considering the used cover on celery plants wither it is the black or white screen net, the celery yield was improved during both seasons. The indicated data from using white net coverage led to in- creasing in celery yield significantly; secondly came the black net coverage, while the lowest mass of celery was obtained by control treatment. Effect of foliar application treatments of potassium silicate on celery yield was significantly noticeable dur- ing both seasons. The high concentration of potassium silicate (S2) treatment gave the highest celery yield fol- lowed by low concentration of potassium silicate (S1). The lowest celery yield was obtained by control treat- ment during both seasons. Concerning the interaction effect of screen net coverage and foliar application of potassium silicate, it was statistically significant; the highest celery yield was obtained by white net cover combined with S2 foliar application during the two season followed by black screen net cover combined with S2 foliar application. The lowest celery yield was obtained by control (with- out cover) treatment combined by without foliar ap- plication. The same results were obtained by Piotr et al. (2009) who mentioned that using cover screen led to enhance celery stalks quality, however the dark screen cover decreased dry matter content and obtained yield. Zakher and Abdrabbo (2014) studied the growing veg- Treatment Plant height (cm) Number of leaves plant diameter chlorophyll Yield (g) cm SPAD g/plant Net covers treatment black 61.0 67.7 5.19 26.9 808 white 56.6 76.4 5.55 34.4 912 control 41.6 49.8 3.50 38.3 677 LSD 5% 2.75 3.42 0.26 2.35 6.04 Potassium Silicate treatment S0 50.4 60.3 4.19 35.8 674 S1 52.8 61.3 4.86 34.4 798 S2 55.9 72.3 5.19 29.5 924 LSD 5% 1.792 1.032 0.253 1.985 5.07 Interaction between cover net and slilicate spray black S0 56.0 63.0 4.89 27.5 723 S1 60.3 69.4 5.11 28.4 830 S2 66.7 70.6 5.56 24.9 870 white S0 54.7 70.6 5.44 36.5 740 S1 56.7 76.4 5.43 35.4 933 S2 58.3 82.3 5.78 31.4 1063 control S0 40.7 47.3 2.22 43.3 560 S1 41.3 37.9 4.04 39.5 630 S2 42.7 64.2 4.22 32.2 840 LSD 5% 1.03 1.84 0.209 1.83 6.93 Table 2: Different efficiencies of different net covers on celery vegetative growth characteristics during the second growing season of 2020 S0 (sprayed with tap water), S1 (1.5 mM of potassium silicate), S2 (3 mM of potassium silicate), LSD 5 % (Significance at 5 % level) Acta agriculturae Slovenica, 117/3 – 2021 9 Foliar silicate application improves the tolerance of celery grown under heat stress conditions etable crops during the summer using shading net; shading led to decrease air temperature, which reduces plant growth and lower yield percentage. Another study was conducted considering the production of celery during summer season by using screen net; the results indicated that white screen net led to increasing the plant growth, celery quality and productivity (Siwek et al., 2009). As of the effect of silicon, it enhanced plant tolerance during summer season. Bakhat et al. (2018) reveled that silicon improved plant growth and pro- ductivity under abiotic stresses conditions. Cooke and Leishman (2016) had the same results under heat waves stresses compared to plants without silicon foliar ap- plication. Moreover, foliar application of potassium silicate led to the increase of potassium concentration in celery leaves under different screen net coverage; Po- tassium (K) is necessary for the function of all living cells and is thus present in all plant tissues. K is a vital element for plant growth and productivity as well as the quality of produced vegetables (Marschner, 2012). Shen et al. (2009) concluded that foliar application by silicon compound led to relieve of high-temperature stress in vegetables. In addition, silicon application protects cul- tivated vegetables against the ultraviolet-B radiation by increasing photosynthesis and antioxidant levels. High level of ultraviolet-B radiation produces a wide physi- ological damage to plants, which had been implanted during summer season. 3.4 CHEMICAL ANALYSIS OF CELERY OUTER LEAF Table 3 and 4 shows that the concentration of N, P , Table 3: Different efficiencies of different net covers on chemical analysis of celery outer leaf during the first growing season of 2019 S0 (sprayed with tap water), S1 (1.5 mM of potassium silicate), S2 (3 mM of potassium silicate), N (Nitrogen), P (phosphorous), K (potassium). LSD 5% (Significance at 5 % level Treatment % N P K Soluble sugars content Vitamin C % % mg.kg -1 fresh mg / 100g fresh Net covers treatment black 1.49 0.27 3.11 6.59 2.52 white 1.78 0.39 3.80 6.97 2.86 control 1.85 0.49 4.48 8.23 3.44 LSD 5% 0.11 0.08 0.26 0.39 0.22 Potassium Silicate treatment S0 1.95 0.44 3.30 8.05 3.28 S1 1.62 0.36 3.63 7.20 2.92 S2 1.55 0.34 4.45 6.53 2.62 LSD 5% 0.07 0.03 0.19 0.66 0.15 Interaction between cover net and slilicate spray black S0 1.88 0.32 2.70 7.72 2.98 S1 1.30 0.24 2.90 6.59 2.64 S2 1.29 0.23 3.73 5.46 1.93 white S0 1.84 0.46 3.57 7.30 2.96 S1 1.86 0.37 3.54 6.93 2.80 S2 1.63 0.34 4.29 6.67 2.81 control S0 2.13 0.54 3.65 9.13 3.89 S1 1.70 0.47 4.46 8.08 3.31 S2 1.72 0.46 5.33 7.47 3.11 LSD 5% 0.06 0.04 0.38 0.21 0.15 Acta agriculturae Slovenica, 117/3 – 2021 10 F . A. HASHEM et al. K, soluble sugar content and vitamin C in celery leaves cultivated under the tested treatments during the two studied seasons. It indicated that, in general, the treat- ment of covered celery plants was sufficient to give high values of the studied macronutrient percentages N, P and K as well as soluble sugar content and vitamin C in the celery leaf. Plants that were covered by white net gave the lowest N, P, K, soluble sugar content and vita- min C, while the highest values were obtained by con- trol treatment; due to effect of heat stress for plants at ambient conditions which reduced the photosynthesis and metabolism, leading to storing the nutrient in the plant tissues (Abul-Soud et al., 2014 and Zakher and Abdrabbo, 2014). On the other hand, the appropriate microclimate under white and black screen net led to enhancement of the plant primary metabolism and then improves the ability of plant roots to absorb water and fertilizer from soil without stress which led to the enhancement of growth parameters (Hashem et al., 2011; Medany et al., 2009). Regarding the using of potassium silicate foliar ap- plication treatments, data in Table 3 and 4 indicated that using 3.0 mM of potassium silicate increased K per- centage in celery leaf more than the other treatments. N and P took another trend, as the control treatment had the highest percentages in celery’s outer leaves. It may be due to the role of potassium silicate in improving the ability of plants to combat the stresses during summer and enhancing the machinery and metabolism, which reflected on increasing plant mass making dilution ef- fect of nutrients in its tissues (Abd El-Rahman et al., 2018 and Zakher and Abdrabbo, 2014). Table 4: Different efficiencies of different net covers on chemical analysis of celery outer leaf during the second growing season of 2020 S0 (sprayed with tap water), S1 (1.5 mM of potassium silicate), S2 (3 mM of potassium silicate), N (Nitrogen), P (phosphorous), K (potassium). LSD 5% (Significance at 5 % level) Treatment % N P K Soluble sugars content Vitamin C % % mg.kg -1 fresh mg / 100g fresh Net covers treatment black 1.41 0.29 3.74 6.18 2.59 white 1.70 0.36 4.47 7.44 3.17 control 1.90 0.37 5.04 8.30 3.45 LSD 5% 0.16 0.06 0.29 0.40 0.12 Potassium Silicate treatment S0 1.90 0.39 3.73 8.42 3.52 S1 1.65 0.34 4.41 7.31 3.08 S2 1.46 0.29 5.10 6.19 2.61 LSD 5% 0.13 0.03 0.56 0.93 0.22 Interaction between cover net and slilicate spray black S0 1.76 0.36 3.08 7.68 3.24 S1 1.16 0.27 3.49 5.76 2.41 S2 1.31 0.23 4.66 5.10 2.12 white S0 1.75 0.39 4.02 7.98 3.38 S1 1.83 0.35 4.59 7.65 3.27 S2 1.53 0.33 4.80 6.68 2.86 control S0 2.20 0.41 4.11 9.59 3.95 S1 1.95 0.40 5.16 8.52 3.56 S2 1.55 0.32 5.84 6.78 2.85 LSD 5% 0.07 0.03 0.204 0.11 0.11 Acta agriculturae Slovenica, 117/3 – 2021 11 Foliar silicate application improves the tolerance of celery grown under heat stress conditions Cover Potassium Silicate Avge. yield kg/plant Yield Grass in- come Cover cost Annual cover cost Applied Potassium Silicate cost Potassium Silicate cost Spray cost Total treat- ments Net in- come Incremental income Ton/ Acre Acre *L. E/ Acre *L. E/ Acre Liter/ Acre Cost/ Acre *LE/ Acre cost *LE *L. E/ GH *L. E / GH Black net S0 653 15.7 31320 21000 4200 0 0 150 4350 26970 600 S1 718 17.2 34440 21000 4200 3 360 150 4710 29730 3360 S2 925 22.2 44400 21000 4200 6 720 0 4920 39480 13110 White net S0 863 20.7 41400 21000 4200 0 0 150 4350 37050 10680 S1 988 23.7 47400 21000 4200 3 360 150 4710 42690 16320 S2 1003 24.1 48120 21000 4200 6 720 0 4920 43200 16830 control S0 553 13.3 26520 0 0 0 0 150 150 26370 0 S1 680 16.3 32640 0 0 3 360 150 510 32130 5760 S2 748 17.9 35880 0 0 6 720 0 720 35160 8790 Table 5: Economic analysis for using cover net and potassium silicate during the first growing season of 2019 Average price 2 LE/ kg cover cost 5 LE/ m 2 The greenhouse applied by one acre net cover Potassium Silicate cost 120 L.E/Liter Average currency change rate = (1 USD = 16 L.E.) Acta agriculturae Slovenica, 117/3 – 2021 12 F . A. HASHEM et al. Cover Potassium Silicate Avge. yield kg/plant Yield Grass income Cover cost Annual cover cost Applied Potassium Silicate cost Potassium Silicate cost Spray cost Total treat- ments Net income Incremental income Ton/ Acre Acre L. E/ Acre L. E/ Acre Liter/ Acre Cost/ Acre LE/ Acre cost LE L. E/ GH L. E / GH Black net S0 723 17.4 34720 21000 4200 0 0 150 4350 30370 3640 S1 830 19.9 39840 21000 4200 3 360 150 4710 35130 8400 S2 870 20.9 41760 21000 4200 6 720 0 4920 36840 10110 White net S0 740 17.8 35520 21000 4200 0 0 150 4350 31170 4440 S1 933 22.4 44800 21000 4200 3 360 150 4710 40090 13360 S2 1063 25.5 51040 21000 4200 6 720 0 4920 46120 19390 control S0 560 13.4 26880 0 0 0 0 150 150 26730 0 S1 630 15.1 30240 0 0 3 360 150 510 29730 3000 S2 840 20.2 40320 0 0 6 720 0 720 39600 12870 Table 6: Economic analysis for using cover net and potassium silicate during the second growing season of 2020 Average price 2 LE/ kg cover cost 5 LE/ m 2 The greenhouse applied by one acre net cover Potassium Silicate cost 120 L.E/Liter Average currency change rate = (1 USD = 16 L.E.) Acta agriculturae Slovenica, 117/3 – 2021 13 Foliar silicate application improves the tolerance of celery grown under heat stress conditions 3.5 ECONOMIC ANALYSIS Cost of using nets for protect celery plants were 21000 Egyptian pound (L.E.) per acre, for white or black nets during the two studied seasons (Tables 5 and 6). We consider a lifetime for the cover screen net of 5 years then the annual cost of covering with net was 4200 L.E. The cost of spray potassium silicate was also considered in this analysis. The other costs of pro- duction were not considered such as labor, inputs, ir- rigation, etc., because these are the same for the tested treatments (under white and black screen net as well as open field) on one acre of celery. Compared to con- trol, the benefits (total gross profit) of using the differ - ent treatments were higher than cultivate in open field. White net combined with application of potassium silicate S2 was superior in yield of two years, compar- ing with the other treatments during both seasons; the white net combined with S1 came in the second order; the lowest values was obtained by open field treatment combined with absent potassium silicate application. Regarding the relative increase in income compared to control treatment; the white net with S1 and S2 gave the highest values; use of potassium silicate S2 came in the third option. The lowest relative increase in income was obtained by without screen cover combined with absent potassium silicate treatment. From the above we can conclude that using of physical or chemical pro- tection led to the improvement of the profitability of celery during the early season compared to the control treatment. 4. CONCLUSIONS This research provided evidence on how to pro- duce winter leafy crops such as celery plants during the early summer season providing high quality produc- tion by applying the required physical and chemical protection for plants. The results showed that using of screen net coverage increased plant growth and provid- ed high quality yield compared to the situation without appropriate coverage. Also, the acquired results gave a recommendation for the usage of potassium silicate as foliar application for plant protection from heat waves. This study confirmed that silicon has a beneficial effect of foliar application. Using white screen net combined by foliar application of potassium silicate 3 mM gave the highest yield of celery and enhanced the vegetative growth and yield during the late season. 5. REFERENCES A.O.A.C., (1980). Association of Official Methods of Analytical Chemists, Official Methods of Analysis 13 th ed., Washington, D.C., U.S.A. Abd El-Rahman, N. 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