However, unlike the vast variety of commercial fertilizers available for traditional soil-based planting, relatively limited commercial nutrient supplements are currently available for application in hydroponic planting, especially those using PH derived from enzymatic-digested processing-wastes. In this paper, we aim to determine the effects of protein hydrolysate  on hydroponic planted lettuce. We would like to test if PH could help to improve the hydroponic crop’s yield, as well as the phytochemical and biochemical contents. Here, PH was firstly prepared from soy-based processing waste via enzymatic digestion method and then mixed into the hydroponic grow solution at different concentrations. Following the PH-treatments, the harvested lettuce samples were assessed for their selected phytochemical contents . Additionally, the harvested lettuces samples were also tested for their selected biochemical parameters . Lastly, the harvested lettuce weight and other physical properties that may affect consumer purchasing desires  were also assessed, along with factors that may affect the plant’s nutrient uptakes . Through this study, we are hoping to contribute toward the potential PH application as nutrient supplement for hydroponic plants, with accompanied improvements in harvest yields and nutritional contents. The hydroponic plant that had been chosen as testing model was Green Coral Lettuce . Firstly, germinated lettuce samples in mesh pots were placed into covered hydroponic containers following the Kratky method, with exposure to natural photo-period and ambient daily temperatures . The lettuce roots were submerged into a commercial hydroponic grow medium solution , whcih was prepared by diluting 5 mL of Hydroponic Solution A  and Solution B  with 2 L of distilled water, and the lettuce roots of the hydroponic plants were suspended into this prepared hydroponic medium solution. These young lettuce plant samples were then divided into treatment groups  and a control group . In the three treatment groups, the preparared soy PH was added into the hydroponic grow media, at concentrations of 0.001, 0.01, and 0.1 mg/ mL, respectively. These hydroponic-planted lettuce samples were then allowed to grow for the next nine weeks, with pH maintained at 6.0 and fresh grow media changed at three weeks interval.

After harvested, hydroponic nft system the fresh lettuce weights and other physical properties that may affect consumer’s purchasing desires  were measured and recorded, along with factors that may affect the plant’s nutrient uptakes . Phytochemical contents in the harvested lettuce samples were determined using previously published conditions . Briefly, Total phenolic content  was determined using Folin-Ciocalteu reagent and reported as mg gallic acid equivalents /g dry matter . Total flavonoid content  was determined using aluminium chloride reagent and reported as mg quercetin equivalents /g dry matter . Total hydroxycinnamic acid content  was determined using Arnow’s reagent and reported as mg caffeic acid equivalents /g dry matter . The absorbances were monitored at 765, 510 and 490 nm, respectively . Lastly, ascorbic acid content in the harvested lettuce was determined using 2, 6-dichlorophenol indophenol  method, with ascorbic acid as standard and reported as mg/g. After harvested, 1 g of hydroponic-grown lettuce samples were homogenized in an iced mortar and pestle using 10 mL of extraction buffer containing 50 mM phosphate buffer , 0.5 mM ascorbate and 1 mM EDTA. The mixture was then centrifuged at 10,000 rpm for 15 min, and the supernatant was collected and used for further analysis. The superoxide dismutase  activity was determined as previously reported, by measuring the ability of lettuce extract to inhibit the photochemical reduction of nitroblue tetrazolium , using a 3 mL reaction mixture containing 100 mM potassium phosphate buffer , 0.1 mM EDTA, 13 mM methionine, 2.25 mM NBT , 60 µM riboflavin , and leaf extract sample . After 15 min of fluorescent light exposure, the 560 nm absorbance was determined . At the same time, the catalase  activity of lettuce extract was determined using a 3 mL reaction mixture containing 50 mM potassium phosphate buffer , 0.25 mL leaf extract sample and 60 mM hydrogen peroxide . Next, the decrease in absorbance at 240 nm was monitored to calculate the H2O2 decomposition  . Chlorophyll and carotenoids contents were determined according to published conditions, by incubating 0.5 g of stripped fresh lettuce into 10 mL of 98% acetone.

After overnight incubation, the absorbances at 661.6, 644.8 and 470.0 nm were recorded to calculate the concentrations of chlorophyll a , chlorophyll b  and the sum of leaf carotenoid  . Mineral content analysis using atomic absorption spectroscopy  was performed using published conditions with some modifications . Briefly, 0.1 g of the powdered lettuce sample was added into 10 mL 65% nitric acid  and boiled for 15 min. After cooling at room temperatue and filtered, the filtrate was then topped up to the final volume of 50 mL with distilled water. After harvested, the hydroponic-grown lettuces were assessed for their selected physical parameters. Lettuce length and weight were measured using the aerial parts of the harvests, and reported as centimeter  and gram , respectively. For the determination of lettuce leaf surface areas, twenty six healthy lettuce leaves were selected from each treatment group, and a digital camera was used to capture the outlined tracing of each leaf sample. The captured images were then analyzed by ImageJ software  to determine the leave surface areas and reported as square centimeter . Lastly, the lettuce root length and weight were measured and reported as cm and g, respectively. Numerous studies have previously reported on how plant phytochencal contents are directly linked to their bioactivities and pharmacological potentials . In order to determine the effect of protein hydrolysate  treatments, the harvested hydroponic lettuce samples were firstly tested for the phytochemical contents in three categories, namely total phenolic content , total flavonoid content , and total hydroxycinnamic acid content . Here, when treated using PH concentrations ranged from 0 to 0.01 mg/mL, increasing concentrations of TPC, TFC, and THC were detected in the PH-treated lettuce . However, no further increase in phytochemical content was detected, in lettuce samples treated with higher PH concentration . Similar trend was observed for ascorbic acid contents. The overall increase ranged from 1.10 to 1.13 folds, compared to non-treatment group  . Previously, several studies had also reported on increases in plant’s phenolic content and ascorbic acid, following the PH applications in both soil-based and hydroponic-based plantings . After protein hydrolysate  treatments, the harvested hydroponic lettuces were also characterized for their biochemical profiles , as well as mineral contents.

These selected parameters were tested, as they may affect the harvested lettuce’s antioxidant and stress tolerance, photosyntesis and growth rates, as well as nutritional contents . For antioxidant enzymes contents, we focused on studying catalase  and superoxide dismutase . Based on our results , higher CAT and SOD contents were detected in PH-treated lettuces, in the PH concentration ranges of 0.001–0.1 and 0.01–0.1 mg/mL, respectively. In these mentioned PH concentration ranges, higher CAT  and SOD  were detected, compared to the control group  . However, we could not rule out the possbility that the elevated CAT and SOD levels were caused by increased stress levels in PH-treated lettuces. On the other hand, when tested with PH concentrations ranged from 0 to 0.1 mg/mL, both chlorophyll and caratenoid concentrations peaked in lettuce sample treated with 0.01 mg/mL of PH. No further increase in chlorophyll and caratenoid was detected at higher PH concentration . Compared to the non-treatment groups, the increase in chlorophyll and caratenoid ranged from 1.71 to 1.88 folds . Our observation is agreeable with previous studies which reported on increased chlorophyll contents in PH-treated peppermint, maize and patchouli plants, using both soil-based and hydroponic-based plantings . In future studies, it would be interesting to determine the exact mechanism that leads to the increase in chlorophyll contents, following PH treatment. In addition, to test how the PH will affect the mineral absorption and bio-accumulation of minerals in hydroponic-grown lettuces, we applied flame atomic absorption spectrometer to detect for the presence of eight selected minerals in our harvested lettuce samples. Here, aluminium, cadmium and lead were not detected in any of our lettuce sample . Whereas for the other five mineral elements , their presences were detected in all lettuce samples . Previous studies reported on enhanced levels of selected minerals in Diplotaxis tenuifolia and maize plants, following PH treatments . In our study using lettuce, we did not detected any significant difference of mineral contents in our PH-treated lettuces, compared to those in the control group. One interesting exception was the higher magnesium  content detected in lettuce samples grown at the highest PH concentration . Here, the Mg content is 1.2 fold higher, compared to the control group. The exact mechanism for this observed spike in Mg level remains to be determined. Lettuce , an important vegetable crop that is one of the Asteraceae species and is predominantly consumed fresh. Lettuce is typically low in calories, nft channel packed with fiber, and the source of many valuable bioactive components like vitamins, minerals, and phytochemicals such as phenolic compounds that act as antioxidants. Lettuce play important roles in the human diet. The consumption of lettuce may contribute to lowering of cholesterol, reducing the risks of coronary heart diseases, preventing some types of cancer, slowing down aging, and improving the overall vitality. In addition, lettuce is preferably grown in a hydroponic system because of the short growth cycle.

Also, hydroponic system uses less water due to constant reuse of nutrient solution and also reduces the risk of soil-borne diseases. Therefore, due to their advantages, the scale of hydroponic lettuce production is increasing in recent years. However, lettuce has a short shelf life and is easy to damage and rot. When lettuce losses quality or freshness, it affects its commodity value. Therefore, maintaining the freshness and quality characteristics of hydroponic lettuce are the main challenges. Various studies have suggested that vegetable roots can affect the freshness of vegetables during storage. For cabbage, Cui et al. indicated that the shorter root length causes lower weight loss rate. Weight loss rate of the fruit and vegetable is one of the important characteristics in determining their freshness level. For hydroponic lettuce, Yang proposed that selling lettuce with roots could retain its hydroponic characteristics and it can still be cultivate in water even after comsumer purchase. Quality of fresh lettuces at post harvest stages could be maintained by some treatments. The storage environment and packaging are important post harvest factors that determine the quality of lettuce during storage. Leafy vegetables like lettuce are highly perishable. Especially harvesting has a major impact on subsequent storage and retaining freshness. Therefore, lettuce is recommended to be stored in a low temperature with high humidity environment . Some studies have shown that polymeric fifilm packaging and packaging method can largely inhibit the deterioration processes of fresh vegetable quality during storage . In addition, changing the lettuce characteristics through some treatments such as light and allicin treatment can extend its shelf life. Most studies focus on the storage and postharvest quality changes of hydroponic lettuce and very few studies suggest whether its roots should be retained for longer shelf life. However, no research has been reported on the effect of hydroponic lettuce root length on retaining its freshness. Therefore, studying the effect of hydroponic lettuce root length on its freshness could help to set standards in determining its shelf life. It has potential in decision making for robotic harvesting or determining potential storage ways based on their root length that could lead to longer shelf life. Due to this reason, this work aims to find the optimal root length of hydroponic lettuce and to provide a basis for retaining freshness with hydroponic lettuce roots. For the aims, quality factors such as changes in weight loss rate, color, chlorophyll contents were measured and discussed.The chlorophyll contents of all samples decreased gradually during storage. The relative content of chlorophyll is shown in Table 1. In these 15 days, the group of 9 cm had the largest decrease in chlorophyll relative content which was 5.2 Spad. And the group of 0 cm had the smallest decrease in relative chlorophyll relative content which was 4.4 Spad.