Impaginato 593 Adv. Hort. Sci., 2019 33(4): 593­603 DOI: 10.13128/ahsc­8228 Effect of palm leaf biochar on melon plants (Cucumis melo L.) under drought stress conditions S. Bagheri 1, M.R. Hassandokht 2 (*), A. Mirsoleimani 3, A. Mousavi 4 1 Department of Horticultural Sciences, Faculty of Agriculture, Tehran Science and Research Branch University, Tehran, Iran. 2 Department of Horticultural Sciences, Faculty of Agriculture, University of Tehran, Karaj, Iran. 3 Department of Plant Production, Faculty of Agriculture and Natural Resources of Darab, Shiraz University, Shiraz, Iran. 4 National Institute of Genetic Engineering and Biotechnology, Tehran, Iran. Key words: macro­elements, melon, micro­elements, morphological traits, proline. Abstract: In order to investigate the effect of palm leaf biochar on some charac­ teristics of Cucumis melo L. under drought stress, a split plot experiment was conducted in a completely randomized block design with three replications for two consecutive years. The main plot was irrigation level (60, 85, and 100% water requirement) and subplot was biochar in four levels (0, 0.18, 0.24, and 0.36 kg/m2). Results showed that treatment of 0.24 kg/m2 biochar and 100% water requirement increased the characteristics of water use efficiency as 88%, shoot fresh weight as 77%, shoot dry weight as 32%, root fresh weight as 100%, root dry weight as 84%, root length as 54%, and average fruit weight 84% com­ pared to treatment without biochar and 60% water requirement. The highest level of leaf N, Mn and K, shoot length, leaf area, leaf number, fruit diameter and fruit flesh thickness in the treatment of 0.36 kg/m2 biochar and 100% water requirement were higher 58%, 48%, 65%, 18%, 50%, 95%, 43% and 55%, than to of treatment without biochar and 60% water requirement respectively and had no significant difference with the treatment of 0.24 kg/m2 biochar and 85% water requirement. The highest rates of Fe, Zn and Cu were related to 0.36 kg/m2 biochar and 60% water requirement as 60, 44 and 66% respectively com­ pared to treatment without biochar and 100% water requirement. The biochar­ free treatment with 60% water requirement accounted for the highest amount of proline due to high stress, and the proline content reduced with increasing biochar and decreasing stress in treatments. Generally, the treatments of 0.24 and 0.36 kg/m2 of biochar increased most of the characteristics, however no significant difference was observed between these treatments. Moreover, in 85% water requirement the drought stress conditions could compensate with the application of biochar. Thus, using 0.24 kg/m2 of biochar and 85% of water requirement, recommended for the best result. 1. Introduction Melon (Cucumis melo L.) is from cucurbitaceae family that requires (*) Corresponding author: mrhassan@ut.ac.ir Citation: BAGHERI S., HASSANDOKHT M.R., MIRSOLEIMANI A., MOUSAVI A., 2019 ­ Effect of palm leaf bio‐ char on melon plants (Cucumis melo L.) under drought stress conditions ­ Adv. Hort. Sci., 33(4): 593­603. Copyright: © 2019 Bagheri S., Hassandokt M.R., Mirsoleimani A., Mousavi A. This is an open access, peer reviewed article published by Firenze University Press (http://www.fupress.net/index.php/ahs/) and distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Data Availability Statement: All relevant data are within the paper and its Supporting Information files. Competing Interests: The authors declare no competing interests. Received for publication 23 June 2019 Accepted for publication 24 October 2019 AHS Advances in Horticultural Science http://creativecommons.org/licenses/by/4.0/ http://creativecommons.org/licenses/by/4.0/ http://creativecommons.org/licenses/by/4.0/ Adv. Hort. Sci., 2019 33(4): 593­603 594 warm weather and high light to grow (Sangeetha et al., 2006). Drought is one of the most important envi­ ronmental stresses that adversely impact plant growth and crop production. More than 45% of worldʼs agricultural lands are permanently exposed to drought and 38% of the world’s population resides in those places (Ashraf and Foolad, 2007). Therefore, the majority of efforts will be focused on producing more crops in water shortage conditions in the future (Sinaki et al., 2007). In arid and semi­arid conditions, which consist of the major part of Iran, the lack of sufficient and proper vegetation causes reduction of the return of plant remnants and organic matter to the soil (Tate, 2000). Most of the soils in the arid and semi­arid areas in Iran contain less than 1% organic matter (Asghari, 2011). The organic matter shortage reduces the stability of the soil structure and its flak­ ing, eventually creating a hard and dense soil (Hemmat et al., 2010). The use of organic fertilizers such as animal manure is a way of increasing the organic matter content in agricultural soils, however the application of this material cannot meet the needs of these soils (Mesa and Spokas, 2011). Therefore, in order to improving the soil, the use of organic resources such as agricultural waste, com­ post, urban waste, and sewage sludge is necessary, so that while increasing agricultural products, sus­ tainable development can be achieved in agriculture (Yin Chan and Xu , 2009; Nazmi et al., 2012). In recent years, biochar has been used as a soil reformer, an organic carbon source, and somehow a method for carbon sequestration in agricultural soils. Biochar is a char produced from plant biomass and agricultural waste like wheat straw, corn, rice, which is produced during the thermochemical process of pyrolysis; this process is referred to the slow burning of organic matter under low or lack of oxygen condi­ tion (Glaser and Birk, 2012). It has been reported in several studies that biochar is a useful reformer to improve the soil physical and chemical characteristics and is effective in preserving soil organic matter, increasing fertilizer efficiency, and enhancing crop production, especially in the soils of subtropical and tropical areas that have long been cultivated (Van Zwieten et al., 2010). Biochar enhances the water holding capacity of the soil (Basso et al., 2013) and change the particle size distribution and porosity of the soil due to its high specific surface area (SSA) (Sun et al., 2014) and also is a direct source for K, Ca, P, Zn, and Cu (Chan et al., 2008). In addition, biochar increase the soil nutrient availability due to increas­ ing the cation­exchange capacity (CEC), changing the soil pH. Using a biochar produced from rice plant residues increased the plant fresh and dry weight, root fresh and dry weight, stem length, and leaf num­ ber in lettuce and cabbage plants (Carter et al., 2013). Addition of biochar increased the soil pH, EC, organic carbon, CEC and N, P, K, Na, Ca, and Mg con­ centration of the soil and also the P, N, and K con­ tents of the lettuce plants in this soil (Nigussie et al., 2012). Depending on the variety and farming condi­ tions during the year, each palm produces about 15­ 25 dry leaves, each weighting 1.5 to 2.5 kg. The gen­ eralization of this amount of plant residues to several million palms in Iran leads to a great deal requiring the management of productivity and optimal use. These wastes can be converted into biochar and then used in soil. In recent years, many areas of Iran have been faced with water shortages and droughts, thus increasing soil water holding capacity by adding organic matter and biochar to soil can increase the potential of land use in these areas. Therefore, the present study was accomplished aiming to exploit the palm leaf biochar in order to increase soil organic matter and diminish the adverse effects of drought stress and investigate its effect on some characteris­ tics of melon plants. 2. Materials and Methods This experiment was carried out in 2016 and 2017 in an agricultural farm in Zarrindasht region of Fars province, Iran, with a longitude of 54°, 20ʹ and a lati­ tude of 28°, 20ʹ with an altitude of 1021 m from the sea level. In this experiment, the Samsouri Varamin early variety melon was used. The remains of palm leaves from Zarrindasht orchards were collected, air dried, and crushed and then packed in aluminum sheets to limit the oxygenation and packs were placed in the oven for four hours at 560 °C to pro­ duce biochar (Hall et al., 2008). Table 1 shows some chemical properties of biochar used in the experi­ ment. This experiment was conducted in the split Table 1 ­ Some chemical characteristics of biochar used in the experiment pH (1:7) EC (dS m­1) (1:7) Mn (ppm) Cu (ppm) Zn (ppm) Fe (ppm) K (%) P (%) N (%) 9 7.5 0.74 0.09 0.83 983.2 32.4 2 1.39 Bagheri et al. ‐ Effect of palm leaf biochar on melon plants 595 plot form in completely randomized block design with three replications. The main plot was irrigation level in three levels (60, 85, and 100% water require­ ment) and biochar as subplot in four levels (0, 0.18, 0.24, and 0.36 kg/m2). I1B1 = Without biochar with 60% water requirement; I1B2 = 0.18 kg m2 biochar with 60% water require­ ment; I1B3 = 0.24 kg m2 biochar with 60% water require­ ment; I1B4 = 0.36 kg m2 biochar with 60% water require­ ment; I2B1 = Without biochar with 85% water requirement; I2B2 = 0.18 kg m2 biochar with 85% water require­ ment; I2B3 = 0.24 kg m2 biochar with 85% water require­ ment; I2B4 = 0.36 kg m2 biochar with 85% water require­ ment; I3B1 = Without biochar with 100% water require­ ment; I3B2 = 0.18 kg m2 biochar with 100% water require­ ment; I3B3 = 0.24 kg m2 biochar with 100% water require­ ment; I3B4 = 0.36 kg m2 biochar with 100% water require­ ment. In the year before planting, the farm was fallow plowed well and leveled. Before planting, a soil sam­ ple prepared and some its chemical properties were evaluated (Table 2). Biochar was mixed with soil at 10 cm depth and then seeds were planted at an appro­ priate depth on the rows at distance of 2.5 m and 0.5 m on the row. The drip irrigation was applied, so that, a dripper was placed beside each plant in order to measure the amount of water consumed by the plant. In the 4 to 5 leaf stage, 50 kg/ha of nitrogen, 40 kg/ha of phosphorus and 40 kg/ha of potassium were added to the soil from sources of urea, potassium sulfate and Triple Super Phosphate respectively. To estimate of the plant water requirement, the meteorological data including minimum and maxi­ mum temperature, minimum and maximum humidi­ ty, solar radiation, and wind speed were taken from the Zarrindasht Meteorological Office (Table 3). Then, the amount of evapotranspiration of melon plant was measured and the daily water requirement of the plant was obtained using the appropriate formulas for two years. For estimation of potential evapo­ transpiration parameters (ETOs) and water requirements by the proposed method of FAO using meteorological data and field surveys related to agronomic calendar and different stages of plant growth. It is then calculated by introducing the vegetation coefficient (Kc) according to plant type, stage and duration of growth and its effect on (ETO), evapotranspiration (ETc). Finally, by reducing the effective rainfall, the net requirement of irrigation water (In), which is the soil moisture deficiency, is estimated to be offset by irrigation. Etc = Eto*Kc, where: ETc = Actual evapotranspiration of the plant (mm/day) ETo = Reference evapotranspiration (mm/day) Kc = Plant coefficient. Before the melon fruit ripens, parameters of stem length, plant length, leaf area, and number of leaves per plant were measured. At harvest time, the para­ meters of total yield, average fruit weight, shoot fresh weight, shoot dry weight, root fresh weight and root dry weight determined by scale. Fruit length, fruit diameter and root length determined by a ruler and fruit skin thickness, fruit flesh thickness, deter­ mined by a caliper after slicing the fruits. Since it was not possible to separate the leaf at all stages to mea­ Table 2 ­ Some chemical characteristics of the farm soil pH (1:7) EC (dS m­1) (1:7) N (%) P (%) K (%) 7.8 0.54 0.05 0.122 0.014 Year Month Mean minimum temperature (°C) Mean maximum temperature (°C) Precipitation (mm) Potential evapotranspiration (mm) 2016 Mar. 6.4 29.8 30 187.6 Apr. 12.2 42.8 0.2 314.9 May. 17.6 44.2 0 397.8 2017 Mar. 9.8 37.2 25.9 185.2 Apr. 15.2 39.8 3.2 300 May. 17.8 44.8 0 387.4 Table 3 ­ Some meteorological characteristics during the two years of experiment Adv. Hort. Sci., 2019 33(4): 593­603 596 sure the leaf area, first several leaves were separated and their length, width, and length by width were calculated. Then, the area of the leaves was mea­ sured using the graph paper (mm) and the surface area relation was obtained using the Excel software. The following relation, which has the highest regres­ sion coefficient (R2), was used to calculate the leaf area: Y= 1.03 x + 44 where: Y (cm2) = Leaf area X (cm2) = Length (cm) * Width (cm) Leaf proline content were determined by Bates method (Bates et al., 1973). Leaf samples washed with distillated water, dried at 65°C for 48 h in an oven and ground. Total N in the leaves was deter­ mined by micro­kjeldahl method (Bremner, 1996). The grounded leaf samples were ashed at 550°C and digested with 2 N hydrochloric acid. P concentration in the extracts was determined by the yellow color method and K using flame photometer (Helmke and Sparks, 1996). Concentrations of Fe, Zn, Mn and Cu were determined by an atomic absorption spec­ trophotometer (PG 990, PG Instrument Ltd. UK) as well. The water use efficiency was calculated as a cor­ relation between plant yield and plant water use dur­ ing the treatment period (Liu et al., 2015). WUE = Y/V Where: WUE, Y, and V were water consumption effi­ ciency in kg/m3, plant yield in kg per plant, and total water consumption in m3, respectively. Statistical analysis was performed using the SAS software (Statistical Analysis System) (V9) (SAS Institute Inc. Cary, NC, USA). Differences among the mean values were detected by Least Significant Differences (LSD) test at %5 level. 3. Results The results revealed that the effects of drought stress and biochar and also the interaction of them were significant on water use efficiency and all physi­ ological characteristics (Table 4). I3B3 and I3B4 treat­ ments increased 88% and 76% in water use efficiency respectively compared to I1B1 treatment, however there was no significant difference compared to treatments of I3B2, I2B3 and I2B4 (Table 5). Source of changes Degree of freedom Mean square Water use efficiency Shoot fresh weight Shoot dry weight Root fresh weight Root dry weight Shoot length Root length Leaf area Leaf number Fruit diameter Fruit flesh diameter Average fruit weight Total yield r (replication) 2 125.59 42833.5 * 3253.9 ** 53.3 ** 1.0 ** 112.5 ** 35.5 ** 115.1 1626.1 ** 10.2 0.07 60257.037 ** 1506425.93 ** Stress (a) 2 103.08 ** 228545.6 ** 6954.6 ** 158.6 ** 2.9 ** 184.2 ** 72.8 ** 4135.6 ** 3603.2 ** 351.9 ** 2.5 ** 597026.20 ** 14925655.00 ** r (year) 2 5.69 9183.5 15.3 1.02 0.1 1.2 0.2 76.5 2.2 4.1 0.09 29931.55 748288.89 Biochar (b) 3 37.19 ** 229634.7 ** 3071.4 ** 81.0 ** 1.1 ** 117.8 ** 20.0 ** 6129.9 ** 3079.3 ** 82.0 ** 0.6 ** 225506.06 ** 5637651.49 ** a*b 6 7.44 ** 59484.6 ** 358.1 ** 10.0 * 0.3 ** 60.9 ** 4.2 * 1087.7 ** 618.2 ** 25.5 ** 0.15 * 38834.35 ** 970858.94 ** a*r ( year ) 8 4.9 11845.3 367.0 4.0 0.1 28.5 2.3 608.7 47.4 6.3 0.2 18493.98 462349.70 Error 36 1.43 13001.4 69.7 4.19 0.0825 13.47 1.777 244.9 117.4 4.78 0.0561 5284.20 132105.19 Table 4 ­ Results of analysis of variance (ANOVA) of biochar on some properties of melon plants under drought stress Table 5 ­ Effects of biochar and drought stress on some properties of melon plants under drought stress Treatment Water use efficiency Shoot dry weight (g) Root dry weight (g) Shoot length (cm) Root length (cm) Leaf area (cm2) Leaf number per plant Fruit diameter (cm) Fruit flesh thickness (cm) Average fruit weight (g) I1B1 7.19 c 188.3 e 1.71 d 75 d 10.2 e 167.7 d 76.5 e 36 e 2.15 e 565.2 d I1B2 7.75 c 193.3 de 2.52 bc 77 cd 12.7 d 186.9 cd 91.5 de 43.3 d 2.76 cd 612.7 cd I1B3 7.92 c 206 cd 2.34 bc 76.8 cd 13.4 cd 188.3 cd 103.4 bcd 43.3 d 2.64 d 624.6 cd I1B4 7.88 c 205.2 cd 2.06 cd 77 cd 14.1 bcd 190.2 cd 106.5 bcd 43.4 d 2.84 bcd 637.5 cd I2B1 7.68 c 200.8 cde 2.5 bc 77 cd 14.7 bcd 193.1 cd 98.5 cd 46.4 cd 2.81 bcd 603.7 cd I2B2 8.73 bc 216 bc 2.68 ab 81.2 bcd 14.2 bcd 198.8 c 104.2 bcd 46.1 cd 2.83 bcd 687.9 cd I2B3 11.77 ab 238.7 a 3.14 a 85.5 ab 15.8 abc 212.2 bc 119.7 ab 48.2 bc 3.23 ab 929.3 ab I2B4 12.01 ab 209.8 bc 2.44 bc 79.6 bcd 15.2 abc 207.5 bc 110 bcd 46.5 cd 3.19 abc 934.7 ab I3B1 9.02 bc 213.3 bc 2.77 ab 78.1 cd 14.5 bcd 202.4 c 103 bcd 47 c 3.14 abc 723.1 c I3B2 12.01 ab 222.7 b 2.68 ab 80.8 bcd 15.9 abc 203.1 c 106 bcd 46.5 cd 3.16 abc 922.7 b I3B3 13.55 a 250 a 3.15 a 83.4 abc 17.5 a 233.9 ab 117.4 abc 50.7 ab 3.3 a 1057 a I3B4 12.69 a 249.2 a 2.71 ab 88.5 a 16.1 ab 252.7 a 132.5 a 51.7 a 3.35 a 994.7 ab Bagheri et al. ‐ Effect of palm leaf biochar on melon plants 597 well as the interaction of them on leaf proline con­ tent and all chemical characteristics except for the interaction of P were significant (Table 6). The treat­ ment of I3B4 increased the N (Fig. 4), K (Fig. 5), and Mn (Table 7) as 58, 65, and 48%, respectively, com­ pared to the treatment of I1B1. Regarding N ele­ Interaction of the treatments indicated that the treatment of I3B3 increased shoot fresh weight (Fig. 1), shoot dry weight (Table 5), root fresh weight (Fig. 2), root dry weight and root length (Table 5) by 77, 32, 100, 84, and 71% compared to the treatment of I1B1, respectively. The highest shoot length, leaf area, leaf number per plant, fruit diameter, and fruit flesh thickness (Table 5) were associated with I3B4 treatment, which increased these characteristics 18, 50, 95, 43, and 55% compared to I1B1 treatment respectively and there was no significant difference compared to the I3B3 treatment. Regarding the shoot length, leaf number per plant, and fruit flesh thickness (Table 5), there was no significant differ­ ence between I3B4 treatment with the treatment of I2B3. The treatment of I3B3 increased the average fruit weight (Table 5) and total yield (Fig. 3) by 84% compared to the treatment of I1B1, however there was no significant difference compared to the treat­ ments of I3B4 and I2B3 with increase rates of 73, 63, and 62%, respectively. The biochar­free treatment with 60% water requirement also accounted for the lowest rates in all characteristics. The results indicat­ ed that the effects of drought stress and biochar as Fig. 1 ­ Effects of the interaction of biochar and drought stress on shoot fresh weight of melon. Fig. 3 ­ Effects of the interaction of biochar and drought stress on yield of melon. Fig. 2 ­ Effects of the interaction of biochar and drought stress on root fresh weight of melon. Table 6 ­ Results of analysis of variance (ANOVA) of chemical characteristics of melon leaf Source of variation Degree of freedom Mean square N P K Fe Zn Cu Mn Proline r (replication) 2 1.0 ** 0.004 * 0.2 * 3505.7 ** 144.7 * 4 68.2 * 1.5 * Drought stress (a) 2 3.1 ** 0.06 ** 2.1 ** 14716.9 ** 1321.4 ** 24.9 ** 970.7 ** 166.2 ** r (year) 2 2.3 0.02 1.2 20.0 39.9 0.2 5.8 0.04 Biochar (b) 3 1.2 ** 0.01 ** 1.0 ** 34202.0 ** 836.3 ** 63 ** 363.5 ** 13.7 ** a*b 6 0.3 ** 0.008 NS 0.2 ** 9413.6 ** 184.6 * 11.8 ** 200.9 ** 2.2 ** a*r ( year ) 8 0.08 0.001 0.07 1356.1 46 0.66 53.2 2.5 Error 36 0.0734 0.00123 0.0446 623.32 76.479 3.135 15.31 0.528 * and ** indicate significant difference in 1 and 5% level respectively; NS= not significant. 598 Adv. Hort. Sci., 2019 33(4): 593­603 ment, the treatments of 0.36, 0.24 and 0.19 Kg/m2 and 100% water requirement was not significantly different in comparison to treatments of I2B3 and I2B4 (Fig. 4). In terms of K, there was no significant difference between the treatments of I3B3 and I3B4 (Fig. 5). In addition, the Mn content in the treatments of I3B3 and I3B4 was not significantly different in compared to I2B3 and I2B4 treatments and the low­ est rate was also related to the biochar­free treat­ ment with 60% water requirement (Table 7). The treatment of 0.36 Kg/m2 increased the P level by 20% compared to the treatment without biochar and accounted for the highest rate, although it was not significant compared to the treatment of 0.24 Kg/m2 (15%) (Figs. 6 and 7). Mean comparison of drought stress treatments suggested that 100% and 85% water requirement increased the leaf P content 36% and 10% compared to the treatment of 60% water requirement respectively and were significantly dif­ ferent compared to each other (Figs. 6 and 7). The interaction of drought stress and biochar treatments showed that the treatment of 0.36 Kg/m2and 60% water requirement increased leaf Fe (Fig. 8), Zn, and Cu (Table 7) by 60, 44, and 66%, respectively, com­ pared to treatment without biochar with 60% water requirement, with the lowest rate being associated Fig. 4 ­ Effects of the interaction of biochar and drought stress on melon leaf N content. Treatment Zn (ppm) Cu (ppm) Mn (ppm) I1B1 44.07 bcd 9.51 d 44 d I1B2 46.87 bcd 10.18 d 47.17 d I1B3 54.97 ab 14.38 ab 51.51 bcd I1B4 63. 7 a 15.75 a 51.98 bcd I2B1 38.83 cde 9.98 d 49.68 cd I2B2 35.2 de 10.55 d 50.9 bcd I2B3 51.3 abc 11.92 bcd 56.88 abc I2B4 45.5 bcd 13.98 ab 59.1 ab I3B1 25.27 e 8.41 d 50.9 bcd I3B2 42.37 bcd 11.35 bcd 52.73 bcd I3B3 40.03 bcd 11.01 bcd 59.37 ab I3B4 43.53 bcd 10.85 cd 65.34 a Fig. 5 ­ Effects of the interaction of biochar and drought stress on melon leaf K content. Table 7 ­ Mean comparison interaction of biochar and drought stress on some characteristics of melon leaf Fig. 6 ­ Effects of drought stress on melon leaf P. In each column, mean values with the same letters do not have a significant difference in 1% probability level of the Duncan’s test. Fig. 7 ­ Effects of biochar on melon leaf P. Bagheri et al. ‐ Effect of palm leaf biochar on melon plants 599 with the biochar­free treatment with 100% water requirement. For Fe, the treatments of I1B2, I1B3 and I1B3 had not significantly difference to each other and with compared to I2B3 and I2B4 (Fig. 8). The leaf Zn content of the treatments of I1B3 and I1B4 and I2B3 was not significantly different (Table 7). In the case of Cu, the treatments of I1B3 and I1B4 were not significantly different from I2B4 (Table 7). The lowest level of proline was related to I3B3, and it was not significantly different from the treatment of I3B4. Moreover, the highest rate of proline was relat­ ed to the biochar­free treatment and 60% drought stress and was significantly different from other treatments (Fig. 9). 4. Discussion and Conclusions Reduction in water resources affects the physio­ logical processes of the plant and hence reducing the growth and yield. In this experiment, the water shortage of root media was compensated with the application of biochar and hence increasing in water holding capacity, and the water use efficiency of plant improved without decreasing growth and by increasing the nutrient supply and hence total yield. The treatments of I3B3 and I3B4 increased water use efficiency by 88 and 76% compared to the treatment of I1B1 respectively, but there was no significant dif­ ference compared to the treatments of I3B2, I2B3 and I2B4. In this experiment, no significant difference was observed between 85% and 100% water require­ ment, particularly in 0.24 and 0.36 Kg/m2, indicating the fact that biochar application in 85% water requirement significantly reduced plant water use and hence, a significant effect on the water use effi­ ciency of the plant. Akhtar et al. (2014) reported that the use of biochar obtained from rice bran and flaxseed increased water use efficiency in all irriga­ tion treatments compared to the biochar­free condi­ tions. Uzoma et al. (2011) indicated that the applica­ tion of 10, 15, and 20 tons cow manure biochar per hectare significantly increased the water use efficien­ cy of corn plants in a sandy soil. I3B3 treatment increased the shoot fresh weight, shoot dry weight, root fresh weight, root dry weight and root length by 77, 32, 100, 84, and 71%, respectively compared to the treatment of I1B1. The treatment of I3B3 had no significant difference in comparison to the treat­ ments of I3B4 and I2B3. Roots grew well in biochar beds, which can be due to the improvement of the physical and chemical conditions of the soil and therefore, reducce the soil resistance to the root growth (Chan et al., 2008). Biochar can improve water permeability of the soil and facilitate root infiltration and increase root weight and length. The highest shoot length, leaf area, and leaf num­ ber per plant were related to I3B4 in compared to I1B1 treatment, however treatment of I3B4 was not significantly different from the treatment of I3B3. In this experiment, decrease in irrigation and increase in plant stress caused decrease in the shoot length, leaf area, and leaf number per plant, and with increased irrigation and biochar application, and hence decreasing stress, these values increased. In the stress conditions, the plant size reduces due to reduced transpiration, hence reducing its leaf cells Fig. 8 ­ Effects of the interaction of biochar and drought stress on melon leaf Fe. Fig. 9 ­ Effects of the interaction of biochar and drought stress on leaf proline content of melon plants. Adv. Hort. Sci., 2019 33(4): 593­603 600 and leaf size. With increasing water use efficiency and thus decreasing stress, the biochar leads to an increase in the leaf area and leaf number per plant (Olympios, 1992). The shoot length increases because of the effect of biochar in increasing avail­ able P that causes increase in root growth and absorption of nutrients (Hossain et al., 2010). Moreover, Sang and Gio (2012) showed that the biochar with increasing chlorophyll content, leads to the improvement of photosynthesis, carbohydrate synthesis and biomass production; the result of which include the increase in the leaf area, leaf num­ ber per plant, hence increase in weight and length of the root and shoot of the plant. The treatment of I3B4 increased fruit diameter and fruit flesh diameter by the rate of 43 and 55 in compared to the treat­ ment I1B1, although there was no significant differ­ ence between the treatments of I3B3. In this experi­ ment, fruit diameter, fruit flesh diameter and hence the yield increased with the application of biochar. This is due to the nutritional elements available in the palm leaves (direct) and also the improvement of soil physical, chemical and biological characteristics (indirect) by biochar (Major et al., 2010). Biochar sig­ nificantly leads to increase in the organic carbon and soil fertility (Kumar et al., 2013), increased growth and crop yield (Spokas et al., 2010), and increased plant dry matter (Van Zwietn et al., 2007). The treat­ ment of I3B3 increased the average fruit weight and plant yield by 84% compared to the I1B1 treatment, however it was not significantly different compared to the treatment of I3B4, I2B4, and I2B3. Other researchers (Zhang et al., 2010) also attributed the increase in corn growth and yield in biochar treat­ ments to increased availability of the nutritional ele­ ments and improved physical properties of the soil, such as decreasing the apparent density. Furthermore, biochar improves soil chemical proper­ ties including functional groups and CEC (Kharea et al., 2013), in addition to increased plant access to nutrients and improved plant growth (Lehmann and Joseph, 2009). Uzoma et al. (2011) indicated that biochar application increased the growth and yield of corn compared to control, and had a significant effect on shoot length and number of leaves in different stages of corn growth in sandy soil. In this experi­ ment, addition of 0.19, 0.24, and 0.36 Kg/m2, espe­ cially the treatments of 0.24 and 0.36 Kg/m2and with­ out stress, increased other vegetative characteristics. Moreover, the application of biochar with 85% water requirement did not significantly change these char­ acteristics, but increased stress (60% water require­ ment) decreased plant vegetative properties. N is considered as a mobile element, so N level reduces in conditions of water shortage. Accordingly, it can be conclusively claimed that the addition of biochar to the soil, by increasing water retention, decreases the nitrate leaching from the soil and increases the avail­ ability of N in the soil, and this effect is stable for at least five months (Clough et al., 2013). Generally, there are varying reports on the effect of drought stress on nutrient content in plant species. The decreased rate of N in water shortage conditions (Muni Ram and Singh, 1995; Alam, 1999) and its strengthening under drought stress have been reported (Abdel Rahman et al., 1971). In this experi­ ment, the treatments of I1B4, I1B3 and I1B2 increased N content of the plant as 34, 32 and 10%, the treatments of I2B4, I2B3 and I2B2 increased N content of the plant as 52, 54, and 28%, and finally the treatments of I3B4, I3B3 and I3B2 increased N content of the plant by 58, 50 and 41% compared to treatment I1B1. It is concluded that the plant N con­ tent increases with application and increasing the biochar level and decreasing drought stress. Results showed that biochar utilization increased leaf P content under stress and non­stress condi­ tions. The effects of organic matter on increasing P availability in the soils depend on their phosphorus content. Due to the low amount of absorbable phos­ phorus in palm leaf biochar, this increase can be attributed to acids released from organic matter. These acids reduce the P stabilization in the soil and transform it into an absorbable form. The absorption of nutrients and available water by plant roots are closely related to each other. Water relations affect all physiological processes related to the solubility and availability of nutrients (Alam, 1999). In this experiment, application of 0.36, 0.24, and 0.19 Kg/m2 increased 20, 15, and 5% of plant P, respectively, in comparison to the control (without biochar). Moreover, the treatments of 100% and 85% of water requirement increased the plant’s P rate as respec­ tively 36% and 10% in comparison to the treatment of 60% of water requirement. It can be concluded that the leaf P content increased with increasing biochar level and decreasing drought stress. Biochar application increased K under stress and non­stress conditions. Increasing the soluble K due to the appli­ cation of biochar depends on their composition, especially their K content, the rate of K release, and the effect of organic molecules on the release of K from soil minerals (Jalali, 2011; Najafi­Ghiri, 2015). At the presence of higher water rate, univalent ions Bagheri et al. ‐ Effect of palm leaf biochar on melon plants 601 such as K in the soil solution increase relatively more than bivalent ions such as Ca and Mg, however as the soil becomes dry gradually, clay colloids absorb K (univalent ions) more strongly to their surface and prevent the separation of these ions (Kafi et al., 2009). In addition, since the overall growth of the plant, including the absorption activity of roots reduces due to stress, they will not be able to absorb K from the surface of clay colloids and, hence, the rate of absorption of these elements decreases (Radin and Eidenbock, 1984). In the present experi­ ment, treatments of I1B4, I1B3 and I1B2 increased the K rate as 13, 18, and 9%, also, treatments of I2B4, I2B3 and I2B2 increased the K rate as 39, 27, and 23%, and finally treatments of I3B4, I3B3 and I3B2 increased the K content by 65, 47, and 30%, respec­ tively. This results leads to the conclusion that the addition of biochar reduces stress and, as a result, increases the K content of the plant. Biochar applica­ tion increased Fe, Zn, and Cu under stress and Mn under non­stress conditions. The researchers have suggested that drought stress stops the activity of older roots and only the tip of the roots absorb nutri­ ents, hence the bivalent cations such as iron are absorbed more than the univalent ones and adsorp­ tion of the anions is limited (Martins et al., 2003). In the case of Zn and Cu elements, maybe in conditions of drought stress, continuous wetting and drying in the soil leads to the release of these elements from the clay layers and their concentration increases in the soil, hence increasing the adsorption phenome­ non (Logan et al., 1997). Mn and Fe have an inverse relationship with each other in terms of absorption by the plant, that is, increasing the Mn absorption decreases the Fe absorption (Martins et al., 2003). Changes in the availability of micro elements in the soil are affected by the characteristics of organic mat­ ter and soil. The nutrients of organic matters are released through its decomposition. Although various mechanisms are responsible for increase or decrease of retaining nutrients in the soil (Sposito, 1984), stud­ ies have shown that adding biochar to the soil is effective on the capability of use of ions due to affecting ion exchangable capacity and microbial activity (Atkinson et al., 2010). In an experiment, drought stress increased soil Zn and Cu and reduced Mn (Alizadeh et al., 2008). Drought stress increased Zn, Fe, and Cu content in the sage plant (Sodaeizadeh and Mansouri, 2014). In an experiment, biochar application increased Fe and Mn elements in ama­ ranth plant (Habibi et al., 2017). In this experiment, 60 and 85% water requirement increased the Fe con­ tent (76 and 107%), Zn (74 and 13%), and Cu (13 and 18%) compared to the 100% water requirement (without stress) and the treatments of 0.24 Kg/m2 in 60 and 85% water requirement and 0.36 Kg/m2in 60 and 85% stress, increased the Fe rate as 33%, 48%, 60%, and 32%, the Zn as 24, 16, 44, and 3%, and Cu as 51, 25, 65, and 47% respectively, compared to the treatment of I1B1. Moreover, in the present experi­ ment, the Mn level increased with the use of biochar and decrease in the drought stress, so that treat­ ments of I1B4, I1B3 and I1B2 increased the Mn by 18, 17, and 7%, the treatments of I2B4, I2B3 and I2B2 increased the Mn by 34, 29, and 15%, and eventually, the treatments of I3B4, I3B3 and I3B2 increased the Mn by 38, 44, and 19%, respectively. The use of biochar reduced proline content under stress and non­stress conditions. This finding sug­ gests that biochar decrease the water evaporation and keeping moisture in the root media, because of its large pores on its surface or improving the soil tex­ ture, and can improve root growth and hence reduce stress. Under drought stress conditions, the water potential of the leaf decreases substantially, which, solutions such as proline accumulation in the leaf in order to adapt to the osmotic conditions. Proline decreases in leaves under stress due to decreased synthesis and increased oxidation. It was observed that drought stress caused reduction in leaf water capacity of grape and, thus, increased proline and proline rate was reduced through the use of biochar in cultural media (Rasouli and Golmohammadi, 2009). In this experiment, the proline content decreased with increasing biochar treatments from 0.19 to 0.36 Kg/m2 and the increase of water require­ ment from 60 to 100%, with the lowest amount of proline being related to 0.24 and 0.36 Kg/m2and 100% water requirement. The results of this study revealed that adding palm leaf biochar to the soil especially in drought stress conditions reduces the water consumption rate and improve plant growth and yield. Treatments of 0.24 Kg/m2and 100% water requirement increased the shoot fresh weight, root fresh weight and plant yield compared to without biochar and 60% water requirement. 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