Evaluation of salt and root-zone temperature stresses in leafy vegetables using hydroponics as a clean production cultivation technique in northeastern Brazil

Abstract

Hydroponics has represented an emerging solution for regions with freshwater scarcity. Besides requiring a lower volume of water for production, this cultivation technique allows the use of brackish waters with high salinity levels. Under normal cultivation conditions (i.e., without any environmental control), plants are exposed to other abiotic stresses, such as temperature, which can further enhance the salinity effect. Given the above, three experiments were conducted using a nutrient film technique (NFT) hydroponic system: from April to May (autumn) with endive (Cichorium endivia L., cv. Gigante Barbarella), from May to July (autumn/winter) with lettuce (Lactuca sativa L., cv. Rubinela and cv. Veneranda), and from August to October (winter/spring) with chicory (Cichorium intybus L., cv. Pão de Açúcar and cv. Radiche Todo Ano), in 2021. The trials aimed to evaluate the isolated and/or combined effect of salt and root-zone temperature stresses on yield, water use efficiency, and visual quality of these vegetables. In all experiments, plants were subjected to stresses for 25 days in a randomized block design with a 2 × 4 factorial arrangement: two levels of electrical conductivity of water –ECw (0.30 and 5.50 dS m^–1 for endive and chicory; 0.25 and 6.50 dS m⁻¹ for lettuce) combined with four root-zone temperatures – RZTs (ambient-ARZT, ARZT + 2 °C, ARZT + 4 °C, and ARZT + 6 °C, corresponding to the mean temperatures of 24.72, 26.90, 28.77, and 30.73 °C for endive; 24.20, 26.00, 28.00, and 29.99 °C for lettuce; 27.12, 28.93, 31.01, and 32.86 °C for chicory). The isolated effects of RZT stresses or ECw levels on endive traits were observed, with leaf fresh matter losses of approximately 17 and 44% at higher ARZT + 6 °C (mean of 30.73 °C) and salinity (ECw 5.50 dS m^–1), respectively. Shoot fresh matter losses of around 32 and 52% (Veneranda and Rubinela lettuces, respectively) and 65% (Radiche Todo Ano chicory) occurred only due to salt stress. The interaction affected the shoot fresh matter of Pão de Açúcar chicory, with approximate losses of 80% at higher ARZT + 6 °C (mean of 32.86 °C) and salt stress (ECw 5.50 dS m^–1) than ARZT and ARZT + 2 °C and without salt stress (ECw 0.30 dS m^–1). Despite these reductions, visual quality of plants was not depreciated. Therefore, the present study suggested cultivating leafy vegetables such as lettuce, chicory, and endive using brackish waters, even under high temperatures of nutrient solutions.