Cilantro Photosynthetic Parameters in Response to Different Flows of Nutrient Solutions Prepared with Brackish Waters Dominant in Na+, Cl−, or Ca2+

Although the NFT (nutrient film technique) solution application rate for cilantro is known for fresh water, the application rate is still debatable when using brackish water. The application rate alone influences flow velocity dynamics, which, when associated with nutrient solution salinity, can impact plant development when saline water is used. Knowledge of how to best combine solution salinity and application rates will help decide if brackish water can be used to produce cilantro under hydroponic conditions. Thus, two trials were conducted in sequence from November 2019 to February 2020 under a protected environment. Cilantro cv. Verdão was submitted to four levels of electrical conductivity of nutrient solutions (ECns of 1.7, 3.0, 4.5, and 6.0 dS m−1) combined with four flow rates (1.0, 2.0, 3.0, and 4.0 L min−1). Because Na+ and Ca2+ are predominant ions in brackish waters in the crystalline and sedimentary regions in the Brazilian Semiarid region, the first study used brackish waters dominated by NaCl and the second study used waters dominated by CaCl2. We measured gas exchange and other photosynthetic parameters in plants cultivated with nutrient solutions high in Cl− and prevalent in Na+ or Ca2+, each combined with different application rates. We concluded that the increment in salinity decreased the gas exchange of cilantro plants, especially when the brackish waters were dominant in Ca2+ and Cl−. Up to an ECns of 4.5 dS m−1, plants maintained their leaf chlorophyll concentrations, although with reduced gas exchange. Salt stress compromised chlorophyll a fluorescence, affecting important parameters such as initial, maximum, and variable fluorescence. Besides the effects of salinity on chlorophyll a and b concentrations, the quantum and maximum yields of photosystem II remained stable, indicating that photosystem II may have adapted to the saline conditions applied in this study. The variation in application rates was unable to attenuate the deleterious effects of salinity, regardless of the ionic prevalence. We conclude that cilantro plants can be cultivated under hydroponic conditions, using currently accepted flow rates, with nutrient solutions of up to 3.0 dS m−1 without severe damage to plant photosynthetic parameters.