Environmental effects of root zone ventilation on canopy and rhizosphere of lettuce in plant factory

The plant factory has been developed for many years, a series of theory and approach of environmental control in such facilities have been established. However, the environmental parameters closed to the plants were significantly affected by the physiological activity of leaves and boundary layer resistance of the canopy, leading to suboptimal microclimate deviated from the set values, poor ventilation uniformity and inefficiency of air conditioning. In this study, a new environmental control technic was proposed to introduce the regulated ambient air into the interlayer between cultivation plates and nutrient solution surface in a deep flow hydroponic system, then flow out upward into the internal canopy through the reserved vent hole units on cultivation plates, which was called root zone ventilation(RV). By adjusting the fan speeds and their running modes, low rotation rate continuous root zone ventilation (LCRV) and high rotation rate intermittent root zone ventilation (HIRV) treatments were proposed. The effects of these treatments on microclimate change of mature butterhead lettuce (Lactuca sativa var. capitata) were examined, and the performances on environmental regulation were evaluated by comparison with conventional environment control (CEC) under identical ambient environment conditions at the same time. RV treatments were carefully isolated by black films to prevent airflow impact from ambient. The CEC cultivation area was in an open state, under the air distribution of the plant factory, air flowed over the canopy from one side to another, formed the traditional side ventilation mode. The results showed that the interlayer air temperature of all treatments were lower than ambient value, while higher in relative humidity (RH). Specifically, LCRV earned the lowest temperature, which was 2.67°C lower than ambient. RH of CEC was the highest, which was 100%. Identical trend was also observed in the internal canopy, where LCRV temperature was 3.47°C lower than ambient, and the RH of CEC was 27.56% higher. The CO2 concentration in such area under the action of LCRV is 139×10-6 higher than CEC. With the increasing of height and canopy occlusion, the influence of RV on the environmental parameters of canopy was gradually weakened. Although LCRV was still the lowest in temperature, the gap from ambient was narrowed down to 0.75°C, while CEC temperature grew from below to 0.84°C above ambient. Reduction was observed in RH differences as well. LCRV, the highest in RH, was only 15.8% higher than ambient. In nutrient solution, its temperature in LCRV was 4.03°C lower than CEC. Meanwhile, HIRV slowed down the dissolved oxygen (DO) depletion to a big extent. At the end of the experiment, DO in HIRV was still maintained at 3.8 mg/L, while the other treatments were only 2.8 mg/L. In conclusion, RV technique overcomes the airflow occlusion and boundary layer resistance of canopy effectively. Benefit from it, uniformity, efficiency of ventilation and environment control were improved. In addition, there were obvious differences between the microclimate around canopy in CEC and plant factory ambient, showed the importance of using microenvironment parameters around the plants as the temperature control targets, instead of the ambient. RV played a positive role in lowering internal canopy temperature. It was feasible to cut down energy consumption by increasing ambient setting temperature while reducing refrigeration capacity and start-up frequency of air condition system. For CEC, further refrigeration capacity and energy consumption were inquired to achieve approximate environmental parameters around canopy as RV did. In addition, RV influenced the rhizosphere parameters positively. The decreasing of nutrient solution temperature and DO descent rate may have great impact on root development and plant growth. In future technological renewal of plant factories, this kind of precise microclimate control technology for each cultivation units will replace existing whole space ventilation mode and applied in medium and large plant factories.