Air conditions are usually used to control the environmental parameters in plant factories with artificial light. Among them, the air velocity is generally lower than the optimum range of 0.1-1 m/s that is required for plant growth. The occurrence of tipburn behavior is often observed on the inner and newly developed leaves of lettuce plants, especially during the last days of transplanting. It is very necessary to properly design the airflow in the indoor cultivation systems, thus promoting the growth for less occurrence of tipburn in the lettuce plants. The perforated air tubes can be used to improve the airflow. However, this ventilation scheme is required by the installation of pipelines in the plant factory, inevitably leading to an increase in the installation process and the complexity of the project. In this study, a kind of air-conducting cultivation tank was designed to integrate with the cultivation bed (CBT). The airflow disturbance increased inside the crop canopy in the multi-layer cultivation mode of the plant factory, indicating the simple construction of the ventilation pipe. A model of CBT was also constructed using three-dimensional computational fluid dynamic software (CFD). The airflow velocity was simulated and measured inside the plant canopy with the inlet velocity of 5 m/s. The root mean square error was 0.22 m/s between the calculated and measured airflow velocity. There was a consistent distribution of the simulated and measured geometric center section in the cultivation area, indicating the accurate simulation of the airflow velocity. The validated model was used to simulate the effects of different air inlet velocities on the airflow direction and distribution inside the crop canopy. The simulation results showed that the airflow entered the crop canopy area via the holes of the cultivation pipe, thus forming a regular airflow beam profile. There was a larger airflow area of high velocity in the crop canopy with the increase of the inlet airflow velocity. Once the inlet velocity was 6 m/s, the percentage of volumes was 56.3% inside the plant canopy with the air velocity between 0.1 and 1 m/s, and the volume-weighted average air velocity was 0.15 m/s in the canopy interior. Correspondingly, 6 m/s was selected as the entrance speed of CBT, where the mature lettuce was used as the test material. The microenvironment was compared in the inner part of the lettuce canopy under CBT and traditional ventilation control mode (TVC) under the same environmental conditions. A systematic investigation was carried out to test the effect of ventilation and temperature regulation on the air flow velocity. The results showed that the air temperature was 22.4 ℃ inside the canopy under CBT treatment during the light period, which was lower than the TVC treatment of 23.7 ℃. There was a relatively less significant difference in air temperature in the dark period. The average air relative humidity in the light and dark periods inside the canopy under the CBT treatment was 65.8% and 71.6%, respectively, which were 11.4 and 3.0 percentage points lower than those under the TVC treatment. As such, the CBT greatly contributed to the regulation and control of various micro-environmental parameters inside the crop canopy, compared with the traditional ventilation mode. The finding can provide the promotion value to reduce the requirements of environmental control for the high efficiency of air conditioning temperature.
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A stagnant airflow area can often occur in the increasing cultivation layers in a plant factory with artificial light, leading to the low growth of the plant, and even physiologic diseases, like tipburn. Furthermore, stagnant airflow can also lead to uneven distribution of environmental factors in the plant canopy, resulting in uneven growth of plants. The optimal air velocity is in the range of 0.3-1.0 m/s in plant canopy. A ventilation system can effectively solve these challenges. Previous studies have focused on the airflow to the plant cultivation spaces. However, these ventilation systems can only provide a single direction of airflow with low penetration, which is heavily obstructed by leaves, resulting in stagnant airflow zones within the canopy. Besides, the devices can also increase the equipment complexity to decrease operational efficiency. In this study, an assembled system of dual channel aeration cultivation was designed to increase the airflow within the plant canopy using simple equipment with operation convenience. A cultivation experiment was conducted in a plant factory. There was one control group (common aeration ventilation of a plant factory generated an airflow velocity of 0.2 m/s within the plant canopy) and three experiment groups (the dual channel aeration ventilation mode generated the airflow velocity of 0.6 (T1), 0.9 (T2) and 1.2 (T3) m/s, respectively). A systematic investigation was made to explore the impact of different ventilation modes on plant growth, tipburn occurrence, heat exchange with the surrounding environment, and the canopy microenvironment. The results showed that the dual channel aeration ventilation outperformed the conventional one, in terms of the lettuce canopy environment, lettuce growth, and heat exchange capacity. Specifically, the best growth for lettuce plants was observed at a canopy airflow velocity of 0.9 m/s, with a shoot fresh weight of 56.7 g. The optimal canopy environment and heat exchange capacity for lettuce were achieved at a canopy airflow velocity of 1.2 m/s. There was a decrease of 8.8% and 2.8 ℃ in average canopy relative humidity and average air temperature, compared with the control group. The airflow regime was first transitioned from the laminar to a transitional flow and then changed to a turbulent flow at an airflow velocity of 0.9 m/s. The convective heat transfer coefficient was also significantly improved with the increasing airflow velocity. The sensible heat flux in the light and dark periods increased by 48.5% and 52.3%, respectively, while the latent heat flux rose by 52.9% and 37.9%, respectively, with the airflow velocity within the plant canopy increased from 0.2 m/s to 1.2 m/s. Besides, there was no tipburn occurred in the experiment groups, while the tipburn occurrence of the control group was 20.9%. It infers that the dual channel aeration ventilation can be expected to effectively alleviate tipburn. In conclusion, compared with the conventional ventilation mode, dual channel aeration ventilation can effectively enhance the plant canopy environment, plant yield, and quality, as well as heat exchange between plants and their surroundings. And the assembled structure can improve transportation and installation efficiency. Furthermore, the integrated structure of ventilation ducts and cultivation tanks also reduced equipment complexity. This mode can provide technical support to precise microenvironment control in plant factories.
In the recent years, the plant factories have been developing very fast in the world due to the incomparable advantages, such as labor-saving, pesticide-free, clean, safe and annual planned leafy vegetables production with vertical multi-layer cultivation systems. However, because of the faster growth rate and the weaker transpiration rate of the newly developed leaves, the vegetables in plant factories are very susceptible to tipburn caused by calcium deficiency, and therefore the appearance and nutritional qualities of leafy vegetable are significantly reduced.
In this study, the effects of different relative humidity levels on the tipburn injury, growth and nutritional qualities of hydroponically grown lettuce were investigated. The objective of this study was to find out the optimum relative humidity levels for preventing or alleviating tipburn occurrence of vegetable in plant factories. Furthermore, this study also could provide the technical and theoretical support to prevent the tipburn occurrence, and improve vegetable quality and economic benefits of plant factories.
‘Tiberius’ lettuce, which was susceptible to calcium deficiency, was used in this experiment. Three different air relative humidity levels of 50% (RH50), 70% (RH70) and 90% (RH90) were set up during the light period (16 h), while the relative humidity was maintained at 70% during the dark period (8 h). The other environmental factors, such as light environment, air temperature and carbon dioxide concentration, were kept constant during this study. The tipburn occurrence, yield, photosynthetic parameters and nutritional quality of the lettuce plants were investigated.
Compared with RH70 and RH90, the calcium content in the newly developed lettuce leaves plants under RH50 was increased significantly, the percentage of lettuce plants with tipburn injury was decreased while harvesting, and the initial tipburn occurrence time was postponed significantly. The stomatal conductance and transpiration rate of lettuce plants increased significantly, which in turn enhanced transpiration and accelerated the absorption and transportation of Ca2+, under the lower relative humidity. The photosynthesis rate and fresh weight of lettuce plants were not significantly affected by different relative humidity levels. Moreover, the lower relative humidity also improved the nutritional quality of lettuce plants, including improving the contents of starch, ascorbic acid, and soluble protein.
The high humidity environment in a plant factory was also one of the reasons for the frequent tipburn occurrence of lettuce plants. Based on the above analysis, it could be concluded that the tipburn occurrence of lettuce plants could be alleviated, the nutritional and commercial qualities of lettuce plants could be improved without affecting its yield and photosynthetic capacity by appropriately controlling the air relative humidity in a plant factory.
This study aimed to investigate the effects of different ratios of green-blue light (G/B) with constant red light (R, 660 nm) on the growth, stomatal characteristics, photosynthetic capacity, and energy utilization efficiency of basil plants (Ocimum basilicum Linn.). The ultimate goal of this study was to provide the technical support for efficient and energy-saving production of basil plants in plant factory.
Basil seedlings were subjected to seven different light quality treatments of RB (R:G:B=8:0:8, set as control), RG1B7 (R:G:B=8:1:7), RG1B3 (R:G:B=8:2:6), RG1B1 (R:G:B=8:4:4), RG3B1 (R:G:B=8:6:2), RG7B1 (R:G:B=8:7:1), and RG (R:G:B=8:8:0) for a duration of 19 days after transplanting. The total light intensity was 160 μmol∙m-2∙s-1, with red light accounting for 50% of the total light intensity. The photo/dark period was 16/8 hours per day. The air temperature during the light/dark period was set at 25 ℃/23 ℃, and the relative humidity was set at 65%. Additionally, the CO2 concentration during the light period was 1 000 μmol∙mol-1. The photosynthesis parameters of basil plants were determined after 13 days of experimental treatment, while the growth parameters, stomatal characteristics, and energy use efficiency of basil plants were determined after 19 days of experimental treatment.
Except for treatment RG1B3, the addition of green light significantly enhanced height, leaf area, and dry and fresh weight of basil plants. The shoot dry and fresh weights of basil plants under different G/B treatments showed an increase of 19%-51% and 26%-64%, respectively, compared with those under the control. However, no significant differences in leaf area and dry/fresh weights were observed among the G/B treatments, except those under treatment RG. The treatment with low G/B (RG1B7) only caused a reduction in the stomatal density on the abaxial surface of basil lower leaves. As G/B increased, the stomatal density decreased on the adaxial surface of the lower leaves and both surfaces of the upper leaves. In general, the leaf nitrogen content decreased with an increase in the G/B. This decrease in leaf nitrogen content and stomatal density resulted in a decline in CO2 assimilation capacity and light utilization capacity of basil plants. The reduction in these capacities generally increased with the increasing G/B. Due to the lower light-emitting efficiency of green LEDs compared with blue light, the total power consumption increased with an increase in the G/B. Only the treatment RG1B7 significantly improved electric use efficiency by 25% than that under the control. With the exception of RG1B3, all other G/B treatments improved light utilization efficiency by 30%-57% than that under the control, and no significant differences were found among different G/B treatments, except for RG. RG3B1 and RG improved water use efficiency of both upper and lower basil leaves, with RG3B1 showing a 58% and 74% increase, and RG showing a 67% and 90% increase, respectively compared with that under the control.
By comprehensively analyzing, basil plants growth and energy utilization efficiency, the RG1B7 could be considered as a more suitable light quality combination for basil plants production in plant factories. The findings of this study could serve as a technical support for understanding the impact of different G/B ratios on basil growth, photosynthetic characteristics, and energy utilization efficiency. This research provided the valuable insights for developing optimized light formulations that enable efficient and energy-saving production of basil plants in plant factory environments.
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