The tension meter was suggested to be placed vertically because the results from the tensile experiment were affected by the inclined angle of the tension meter. The tension meter was pull up slowly at a constant speed until the root of hydroponic lettuce was separated from the planting board. Then the maximum pulling force in this experiment was recorded. Thirty samples were randomly selected for the experiment. The sketch map and experimental field of the measurement method were shown in Fig. 3.Shear experiment was carried out to determine the necessary cutting force of a product with different cutting speeds and cutting positions when shearing the hydroponic lettuce root. The cutting force plays an essential role in designing cutting devices of harvesting machinery. The experiments were performed by using an electronic universal testing machine controlled by a computer, as shown in Fig. 4. The cutting angle was selected as 20 according to the actual cutting operation requirements. The root tip of hydroponic lettuce was connected with a tension meter. Then the root of hydroponic lettuce was slowly straightened. The tension value of tension meter is recorded as the specified tension when the root was just in a flat state, the sketch map and experimental field were shown in Fig. 5a and b, respectively. A single bevel blade with knife edge angle of 25 and made of carbon steel 0.5 mm thick was used in this study because of its strength and durability.
The hydroponic lettuce root could not be cut off when the cutting speed was less than 500 mm/min, but the cutting force tends to be stable with little change when the cutting speed was more than 900 mm/min. Therefore, the loading speeds of the cutting blade were selected as 500 mm/min, 600 mm/ min, 700 mm/min, 800 mm/min, 900 mm/min, respectively. The average MRL of hydroponic lettuce is 95 mm, the cutting positions were selected as 0 mm, 30 mm, 60 mm, and 90 mm away from the planting board, respectively. These position corresponds to MED0, MED3, MED6, and MED9, as shown in Fig. 1. The hydroponic lettuce root was fixed on a shearing fixture and it was cut with different cutting speeds. The root was moved to the next position after completing shear experiment of one position. Every experiment was repeated 3 times in different cutting speeds and different cutting positions combination, then average results were compared statistically.One-Way analysis of variance with Microsoft Office Excel 2016 software and Multi-Way analysis of variance with Design Expert 8.0 software were used for the analysis of variance of experimental data. In addition, a root cutting force model was built by a multivariate regressive method. The model was checked by an F-test in Design Expert 8.0 software.The moisture content was determined by the direct drying method.
The main distribution range of CD of hydroponic lettuce was from 300 to 359 mm, which accounts for 87% of the total samples; the main distribution range of OL was from 50 to 89 mm, 4×8 flood tray which accounts for 90% of the total samples; the main distribution range of pH was from 170 to 210 mm, which accounts for 83% of the total samples; the main distribution range of TH was from 450 to 569 mm, which accounts for 77% of the total samples; the main distribution range of TRL was from 240 to 359 mm, which accounts for 83% of the total samples; the main distribution range of MRL was from 80 to 109 mm, which accounts for 90% of the total samples; the main distribution range of SDi was from 11.0 to 14.9 mm, which accounts for 90% of the total samples; the main distribution range of MED3 was from 25.00 to 34.99 mm, which accounts for 94% of the total samples; the main distribution range of MED6 was from 45.00 to 59.99 mm, which accounts for 80% of the total samples; the main distribution range of MED9 was from 40.00 to 54.99 mm, which accounts for 83% of the total samples; the main distribution range of the total weight was from 130 to 179 g, which accounts for 80% of the total samples; the main distribution range of the net weight was from 120 to 169 g, which accounts for 77% of the total samples. The histograms of these data obtained from physical properties experiments on hydroponic lettuce were shown in Fig. 6, and the geometric parameters of hydroponic lettuce were shown in Table 1. The longest MRL was 114 mm. Therefore, the depth of the nutrient tank should be greater than 114 mm to avoid the influence on the growth of lettuce. The varying of PH and CD were significant. The maximum values of PH and CD were suggested to be referenced when designing harvesting machinery to avoid the failure of harvesting. The MED can provide support for the design of the cutting devices. The MED with 63.21 mm is required to be acceptable when the cutting device is fully open. In addition, planting density and root extension of hydroponic lettuce will also affect the growth of the crop. Therefore, OL,TRL, and TH can be referenced when determining the plant spacing and cultivation mode. To select a planting board with suitable strength, the weight should be used as a reference standard.
And the size of the planting hole in the planting board could be determined according to the maximum value of SDi of hydroponic lettuce. Meanwhile, it can be seen that the dispersion of physical parameters of hydroponic lettuce was small from the Coefficient of Variation less than 0.15. Therefore, harvesting machinery can be designed according to the main distribution range of physical parameters of hydroponic lettuce.The moisture content of hydroponic lettuce was high, but the value of different parts were quite different. The moisture content of leaf was considerably higher than root and stem, the average moisture contents were 95.73%, 91.71%, 91.85% for leaf, stem, and root, respectively, as shown in Table 2. The CV of moisture content of stem, root, and leaf were less than the reference value of 0.15, which means that the data were reliable in this study. Xiang et al. concluded that the moisture content of fresh-cut lettuce leaves was 96.8% during storage, which was similar to the result of this study. In addition, leafy vegetables with high moisture content were more likely to wither and rot. Therefore, the automatic equipment should finish harvesting and packaging as soon as possible to avoid the impact on the quality of the product. Besides, although the moisture content of the stem was the lowest, it was easier to break because of its small toughness. Therefore, more protection should be given to this part in harvesting and packaging.The maximum force and the minimum force of pulling hydroponic lettuce out of the planting board were 16.5 N and 9.6 N, respectively. The value of the average pulling force was 13.03 N with standard deviations of 1.85 N. The experimental data were within the normal range with CV was less than 0.15 specified in the evaluation data dispersion. The main distribution range of the pulling force of hydroponic lettuce was from 11.0 to 15.9 N, which accounts for 83% of the total samples, as shown in Fig. 7.
Shandong Agricultural University Hu reported the pulling force of spinach was 14 N with the soil moisture content was 12.3% during harvest. The harvesting style of lettuce was comparable with spinach. Therefore, the harvesting machinery of spinach can be referred to when designing the harvesting machinery of lettuce. In addition, the planting board may be pulled up together with the hydroponic lettuce due to the light weight of the planting board. Therefore, the planting board was suggested to be fixed during the experiment and the actual harvest to avoid harvest failure.Thirty results were obtained through the shear experiment, as shown in Table 3. The scatter distribution was shown in Fig. 8. The maximum and minimum root cutting force were 27.43 N and 1.41 N, respectively. The maximum root cutting force appeared under the combination of cutting position of 0 mm and cutting speed of 500 mm/min. The minimum root cutting force appeared under the combination of cutting position of 90 mm and cutting speed of 900 mm/min. Generally, the root cutting force of hydroponic lettuce increased with the increase of cutting speed, and also increased with the distance from the planting board decreased.The variance analysis of the regression equation for the root cutting force was shown in Table 4. The F-value of the model of the regression equation was 86.95, and the F-value as a reference can be obtained from the standard F distribution table, which was 18.51. This result means that the model was significant . Meanwhile, the effect of cutting position and cutting speed on root cutting force were significant with P-value less than 0.05. In addition, the F-value of cutting position and cutting speed were 148.85 and 25.04, respectively. The influence of cutting position on the root cutting force was more significant than cutting speed . The response surface method in Design Expert 8.0 was used to summarize the regulation of root cutting force. The response surface figure between the cutting speed, cutting position, and root cutting force was shown in Fig. 9. It can be concluded: for the same cutting speed, the farther the cutting position from the planting board, the smaller the root cutting force, ebb flow tray and for the same cutting position, the higher the cutting speed, the smaller the root cutting force. The minimum root cutting force was 1.41 N with the 900 mm/min of cutting speed and 90 mm of the distance between the cutting position and the planting board. Du et al. and Li et al.have reported the trend of root cutting force of cabbage with cutting position: root cutting force decreases with the cutting position close to the outer most leaf of cabbage, which consistent with the results of this study.
Meanwhile, the trend of response surface was unanimous with the result of variance analysis.The value of lettuce characteristic research can be used in the harvesting stage and packaging stage of hydroponic lettuce processing. The ideal result used minimum force to finish harvesting without damaging the hydroponic lettuce. The average value of the moisture content of hydroponic lettuce was more than 90%. To ensure the quality of lettuce, the harvesting and packaging processing should be shortened as much as possible. The minimum value of the pulling force of 16.5 N can be applied effectively to ensure the hydroponic lettuce can be pulled out successfully. The planting board should be fixed to avoid pulling out together with hydroponic lettuce during harvest. The values of the root cutting force indicated the necessary forces to cut off the root with different cutting positions and different cutting speeds. The F-value of cutting position was 148.85, but the F-value of cutting speed was 25.04. The influence of cutting position on the root cutting force was more significant . To save energy, a further away from the planting board position should be selected to cut.Hydroponic growth system is a versatile platform to study plant behavior under controlled conditions. It is a convenient system for not only controlling the plant nutrition but also provides an excellent model for researchers to study root system and its responses under different environmental conditions. Certain hydroponic systems have been developed for Arabidopsis and other crop species to study their behavior under various environmental stress conditions. To our best knowledge, only few studies were available on trees species which were grown hydroponically under controlled environment. Based on existing knowledge, we have modified/ customized hydroponic system for Pongamia seedlings. Pongamia is known to possess elongated tap root and lateral root system. To accommodate Pongamia tap root system and without facing the root inhibiting effect, we have designed new experimental system. Our method was uniquely designed to perform tree species stress studies with minimal resources. Pods of P. pinnata accession TOIL 12 were obtained from Tree Oil India Limited , Zaheerabad, Hyderabad, Telangana.