In the NFT system, lettuce roots remained for some time (interval between two consecutive recirculations) without NS, which increases the probability of plants experiencing water deficit, especially in the hottest periods of the day.
The Nutrient Film Technique (NFT) stands out as the most viable for the use in commercial facilities and it is common to adopt recirculation frequency of 0.25 h.
However, there are not many studies evaluating biochemical characteristics of lettuce in different hydroponic systems with different recirculation frequencies and with variable water quality, in the same experiment.
The NS recirculation frequencies were every 0.25 h in the NFT system and every 0.25, 2 and 4 h in the DFT system.
The salt stress decreased shoot dry matter production of plants regardless of the hydroponic system and NS recirculation frequency (Figure 2), the reductions were more pronounced in the NFT system (45%) compared to the DFT system (10, 38 and 30% for the frequencies of 0.25, 2 and 4 h, respectively).
For the DFT system the reduction of the frequency of recirculation of the nutrient solution impaired the dry matter production of lettuce 'Taina' when using brackish water.
The frequency treatments started on the sixth day after transplantation (DAT); in the initial period (until 5 DAT), two recirculations of the solution were performed during the day, one in the morning (08:00 a.m.) and the other in the afternoon (04:00 p.m.); each event lasted for 15 min.
Therefore, for the lowest number of nutrient solution recirculations during the day, there was approximately 90% reduction in electric energy consumption in comparison to control, with a potential lower wear-and-tear of the system and, consequently, longer life.
It has also been discussed the necessity of hydroponic systems less dependent on electric energy, since the NFT, the most usual system in the Brazilian hydroponics, is an active system that requires pumping for nutrient solution recirculation. In this context, systems such as Floating and Deep Film Technique (DFT) are promising for the rural communities of the semiarid region, where interruptions in water supply are frequent.
The present study used a hydroponic system with leveled channels in order to evaluate the cultivation of coriander subjected to different intervals of nutrient solution recirculation in interaction with the use of freshwater and brackish water, the existence of gradients of growth and yield along the leveled hydroponic channel and quantify water and energy consumption.
For each recirculation frequency, two timers were activated simultaneously, turning on 10 electric pumps.
Two levels of electrical conductivity (EC) of water (0.32 and 4.91 dS [m.sup.-1]) and four frequencies of recirculation of nutrient solution (intervals of 0.25, 2, 4 and 8 h) were studied.