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Watermelon (Citrullus lanatus) belongs to the plant family Cucurbitaceae. It is cultivated in many regions of Brazil, particularly in rainfed areas; thus, it is subject to the problems arising from irregular rainfall. Hydrogel is a soil conditioner used to retain and provide water to plants for long periods of time. Therefore, incorporating hydrogel into the soil should increase water availability for watermelon cultivation, and may be an alternative method to reduce the risk of water deficit. This study evaluated the effect of different concentrations of hydrogel on the initial growth of watermelon cv. ‘Crimson Sweet’ under different shading conditions. The experiment was carried out in a greenhouse with completely randomized design with a 4×3 factorial scheme. The treatments consisted of four hydrogel doses (0.0; 1.5; 3.0; and 6.0 g L substrate), three shading conditions (under full sun; black shade net, and red shade net, both with 50% shading), and five replicates per treatment. The variables evaluated were plant height; stem diameter; shoot, root, and total dry matter; and leaf area. The best hydrogel dose for watermelon cultivation was 1.5 g L, but the higher doses impaired plant growth, regardless of shading conditions. The black net improved biomass accumulation in ‘Crimson Sweet’ watermelon, whereas the red net induced plant etiolation. keywords: Citrullus lanatus. Soil conditioner. Shade nets. CRESCIMENTO INICIAL DE MELANCIEIRA EM DIFERENTES CONCENTRAÇÕES DE HIDROGEL E CONDIÇÕES DE SOMBREAMENTO RESUMO – A melancieira é uma cucurbitácea cultivada em várias regiões do Brasil, principalmente, em áreas de sequeiro, sujeito aos riscos da irregularidade das chuvas. O hidrogel é um condicionador do solo utilizado para reter e disponibilizar água para as plantas por períodos prolongados. Assim, a incorporação do hidrogel no solo visando aumentar a disponibilidade de água para o cultivo da melancieira pode ser uma alternativa para diminuir os riscos de déficit hídrico. Diante do exposto, objetivou avaliar o efeito do hidrogel no crescimento inicial de melancieira cultivar ‘Crimson Sweet’, em diferentes condições de sombreamento. O trabalho foi realizado em casa de vegetação, organizado experimentalmente no delineamento inteiramente casualizado, em esquema fatorial 4x3, sendo o primeiro fator quatro doses de hidrogel (0,0; 1,5; 3,0; 6,0 g L de substrato) e o segundo fator três ambientes de sombreamento (sol pleno; malha de cor preta e malha fotoconversora de cor vermelha, ambas com 50 % de sombreamento), e cinco repetições por tratamento. Procedeu-se as análises das seguintes variáveis: altura das plantas, diâmetro do caule, massa seca da parte aérea, massa seca da raiz, massa seca total e área foliar. A melhor concentração dentre aquelas testadas para o cultivo de melancieira foi de 1,5 g L de hidrogel, sendo que doses superiores não favorecem o crescimento das plantas, independentemente das condições de sombreamento. O uso da malha preta favoreceu o acúmulo de massa em plantas de melancia ‘Crimson Sweet’ enquanto a malha vermelha induziu ao estiolamento das plantas. Palavras-chave: Citrullus lanatus. Condicionador de solo. Malhas de sombreamento.  Corresponding author Received for publication in 02/21/2019; accepted in 10/09/2019. Paper extracted from research of the conclusion work of agronomy course of the first author. Department of Plant Production (Horticulture), Universidade Estadual Paulista “Júlio de Mesquita Filho”, Jaboticabal, SP, Brazil; brenojp93@gmail.com – ORCID: 0000-0002-3419-6609. Engineering Unit, Agência de Fomento do Estado da Bahia, Salvador, BA, Brazil; r.gustavoaraujo@gmail.com – ORCID: 0000-00028396-5646. Center for Agricultural, Environmental and Biological Sciences, Universidade Federal do Recôncavo da Bahia, Cruz das Almas, BA, Brazil; anacletosantos86@gmail.com – ORCID: 0000-0003-4629-3948, gilvandas218s2@hotmail.com – ORCID: 0000-0003-4624-1339, fran-eng@hotmail.com – ORCID: 0000-0002-3800-5646. WATERMELON INITIAL GROWTH UNDER DIFFERENT HYDROGEL CONCENTRATIONS AND SHADING CONDITIONS B. J. PEREIRA et al. Rev. Caatinga, Mossoró, v. 32, n. 4, p. 915 – 923, out. – dez., 2019 916 INTRODUCTION Watermelon (Citrullus lanatus) is a cucurbit cultivated in various regions of Brazil and accounts for around 105,491 ha of cultivated land. The largest producing states in the country are Rio Grande do Sul (346,310 tons), São Paulo (291,846 tons), Goiás (267,896 tons), Bahia (239,177 tons), and Tocantins (232,907 tons) (IBGE, 2018). The cultivation of this vegetable is of great economic and social importance because it is mainly carried out by family farmers. Some of the challenges faced by growers are producing quality fruits while maintaining high production levels throughout the year, which is difficult because watermelon is mainly grown in dryland areas (OLIVEIRA et al., 2015b). This means that it is subject to irregular rainfall, which influences its yield. It has been suggested that hydrogel incorporation into the soil may be an alternative method that could be used to reduce the risk of water deficit or, in cases where irrigation is needed, to reduce the volume of water used. The more efficient use of water should lead to improvements in crop yields (MONTESANO et al., 2015; LOPES et al., 2017). Hydrogel is a synthetic or natural soil conditioner that retains and provides water to plants for extended periods (AHMED, 2015). When incorporated into the soil, it significantly improves soil physical properties by increasing soil moisture and decreasing water evaporation rates (CÂMARA et al., 2011; YANG et al., 2014). Previous studies have shown that hydrogel improves the growth of yellow passion fruit (FAGUNDES et al., 2015), lettuce (SANTOS et al., 2015), jatropha (DRANSKI et al., 2013), and eucalyptus (BERNARDI et al., 2012) seedlings. However, no studies have analyzed hydrogel effects on watermelon. Cultivation in protected environments is a technology that improves the physical protection and acclimatization of plants. It generates micrometeorological changes that alter environmentplant relationships (REGO; POSSAMAI, 2006; SHAHAK, 2008). The use of colored shade nets at sites subject to high temperatures and light intensity reduces the amount of radiant energy, which positively affects physiological processes in most plants (ARAÚJO et al., 2006; HENRIQUE et al., 2011). However, there have been few studies about how shading affects watermelon plants. Furthermore, there seems to be no reports about the association between hydrogel and shade nets when they are both used to cultivate plant species. Therefore, this study evaluated the effect of different concentrations of hydrogel on the initial growth of watermelon under different shading conditions. MATERIALS AND METHODS The experiment was carried out between November, 2017 and January, 2018 in a greenhouse at the Soil and Plant Mineral Nutrition sector of the Federal University of Recôncavo da Bahia UFRB (12°39’32” S; 39°5’9” W) in Cruz das Almas City, Bahia State, Brazil. The experiment had a completely randomized design with a 4×3 factorial scheme. The first factor was the four hydrogel doses (0.0; 1.5; 3.0; 6.0 g L substrate) and the second factor was the three shading environments (full sun; black net; and red net, both with 50% shading). There were five replicates per treatment. Watermelon cultivar ‘Crimson Sweet’ seeds were sown in 200-cell plastic trays containing washed sand. The trays were arranged on benches in an arched-roof greenhouse covered with transparent, low-density polyethylene film. It also had anti-aphid screens along the sides. At 14 days after sowing, when the first permanent leaf pair had formed, the seedlings were transplanted to 2.6 dm3 pots at a sowing rate of one plant per pot. The substrate used consisted of 90% washed sand and 10% expanded vermiculite. The plant nutritional needs were met by applying Hoagland and Arnon (1950) solution, diluted to 50% after the first week of application. Hydrogel at different doses was added and homogenized into the dry substrate. The commercial product acrylamide potassium acrylate copolymer (class E, Plant Gel) from FORTH was used. According to the manufacturer, the amount of water absorbed is 200 to 400 times its initial weight and its size increases by up to 100 times. Pot capacity (equivalent to field capacity) was determined at the UFRB Soil Physics Laboratory. In the laboratory, three replicates of the pots with the same volume used in the experiment were filled with the substrate. A cylindrical PVC tube, which was 20 cm in height and 2.54 cm in diameter, was inserted into the center of the pot and filled with the substrate (washed sand + 10% vermiculite). The substrate was lightly tapped with a spatula to reduce the voids. Then, about 100 mL of water (corresponding to the volume of the tube) was added and the tops of both the tube and the pot were covered with film paper to prevent evaporative water loss. Excess water was drained into the dry substrate, and the water retained in the substrate within the tube was used to determine the field capacity. After 24 hours, a substrate fraction was taken from the middle part of the tube, placed in a capsule, and weighed to obtain the wet substrate weight (WSW). Subsequently, the capsule containing the substrate was placed in an oven at 105°C for 24 hours and then weighed to obtain the dry substrate weight (DSW). Gravimetric moisture (μg%) was obtained using the formula: WATERMELON INITIAL GROWTH UNDER DIFFERENT HYDROGEL CONCENTRATIONS AND SHADING CONDITIONS B. J. PEREIRA et al. Rev. Caatinga, Mossoró, v. 32, n. 4, p. 915 – 923, out. – dez., 2019 917 The value was 3.0%. Then, the gravimetric moisture value was converted to volumetric moisture by multiplying the value by the substrate density (sd = 1.8) using the following formula: