Photosynthetic Parameters of Melons in Response to NO3− and NH4+ as N Sources and Irrigation with Brackish Water High in Na+, Ca2+, and Cl−

High levels of dissolved salts in irrigation water sources limit melon cultivation in northeastern Brazil. In this context, nitrogen fertilization has been employed as one strategy to alleviate the effects of salt stress on plants. This study aimed to evaluate the effect of different nitrogen sources on cantaloupe melon cultivation under fertigation and irrigation with water of the same salinity and different cationic concentrations (Na+ and Ca+). The research consisted of two experiments, each following a randomized complete block design in a 4 × 2 factorial arrangement with four replicates. The treatments included four levels of electrical conductivity of the nutrient solution (2.0; 3.0; 4.0; and 5.0 dS m−1) and two nitrogen sources of different origins: NO3− [Ca(NO3) and KNO3] and NH4+ [CH4N2O and NH4H2PO4]. The following factors were chlorophyll pigments, chlorophyll a fluorescence, and fruit weight. Nitrogen fertilization with NH4+ mitigated salt stress by increasing the synthesis of chlorophyll a and carotenoids in plants irrigated with NaCl-based saline water. Furthermore, there was no influence of nitrogen sources on chlorophyll a fluorescence. Finally, NO3− fertilization reduced the effects of salt stress on the leaf mass ratio, specific leaf area under Ca2+ fertigation, and relative growth rate of leaf area in melons under cationic prevalences of Na+ or Ca2+ (associated with Cl−).