# Smart Hydroponic Cultivation System for Lettuce
## 2 Materials and Methods
### Monitoring and Control System
protruding from the bottom and placed inside hydroponic net pots. The seedlings were distributed into treatment groups of 10 plants each. During the vegetative growth stage (Figure 6D), the plants remained in the growth chamber for the following 26 days. Finally, during the harvest stage (Figure 6E), on the 25 th day after transplanting (DAT), the nutrient solution was replaced with tap water to prevent bitterness in the lettuce at the time of consumption. In the plant growth chamber, the air temperature was regulated at 20 \(\pm\) 2 \({}^{\circ}\)C, while the RH was maintained at 60 \(\pm\) 10% RH. The ambient \(CO_{2}\) concentration was kept close to 400 parts per million (ppm). A photoperiod of 14 h per day was applied and the PPF was stabilized at 200 \(\pm\) 20 \(\upmu\)mol m\({}^{-2}\) s\({}^{-1}\). PPF measurements were performed by placing the quantum sensor at the height of the tallest true leaf. The height of the full-spectrum LED lamps was adjusted every three days depending on the growth stage in order to maintain the desired light intensity. The nutrient solution used in the experiment followed a commercial [PERSON] formulation (Soluponics Lechuga, Inverfarms S.A. de C.V., Queretaro, Mexico) [39], containing the essential micro- and macronutrients required for proper lettuce growth. To prepare the solution, the pH of 20 L of water was initially adjusted to 6.0 using 70% nitric acid (\(HNO_{3}\)), followed by the individual dissolution of mother solution bags A and B in 10 L of water each. The treatments applied consisted of nutrient solution concentrations of 100% (T100), 75% (T75) and 50% (T50). For the T100 solution, water with an adjusted pH of 6.0 was utilized and 5 mL of Steiner A and 5 mL of Steiner B were added per liter, resulting in an EC of approximately 1970 \(\upmu\)s cm\({}^{-1}\). For T75 and T50, the same procedure was followed, adjusting the concentrations to obtain an EC of approximately 1621 \(\upmu\)s cm\({}^{-1}\) (T75) and 1272 \(\upmu\)s cm\({}^{-1}\) (T50), by adding 3.75 mL and 2.5 mL per liter, respectively. To ensure the stability of these values, daily manual measurements of pH, EC, TDS and temperature of the nutrient solution were taken at 8:00 a.m. using a multiparameter water quality meter (model C-600, VIHELM, Shenzhen, China), as shown in Figure 7.

Figure 7: Manual measurement of physicochemical parameters of the nutrient solution using a water quality meter (model C-600, VIHELM, Shenzhen, China). ### Variables Obtained from the Cultivation Experiment

During the cultivation experiment, the Soil-Plant Analysis Development (SPAD) index was measured every 3 days between 8:00 AM and 9:00 AM using a portable chlorophyll meter (model MC-100, Apogee Instruments Inc., Logan, UT, USA) on the secondary leaves of the third and fourth leaf of each plant. The use of SPAD units is justified by their ability to provide fast, non-destructive and reliable measurements that correlate well with traditional destructive methods [40]. On 29 June 2024, 26 days after transplanting, seven lettuce plants from each treatment group were harvested to evaluate phenotypic variables. The lettuces were carefully removed from both the hydroponic net pot and the foam pad. The fresh weight of the entire plant, including the root system, was recorded, followed by individual measurement of each component. A tape measure was used to determine plant height and root length. The stem diameter at the intersection point with the root was measured using a Mitutoyo (model 500-196-30, Mitutoyo Corporation, Kanagawa, Japan) digital caliper. The number of leaves was determined by manual counting. To obtain dry weight, each lettuce and its corresponding root system were individually placed in Kraft paper bags and dried in an incubator (model FE-131 AD, Scientific Equipment F.E.L.I.S.A. S.A. de C.V., Ciudad de Mexico, Mexico) at 70 \({}^{\circ}\)C for 72 hrs. Subsequently, the dry mass was measured using a digital scale (model GY20002, Goyojo, Guangzhou, China).