Abstract: Background, aim, and scope Enhancement of carbon dioxide (CO2) in the atmosphere has received great attention due to its potential repercussion on global warming and direct effects on the vegetation, especially with a potential increase in atmospheric CO2 level from 400 μmol∙mol-1 to 1000 μmol∙mol-1 by the end of 21st century according with currents environmental studies. Therefore, development of new technologies on controlled environment conditions are needed to investigate plant response to CO2 enhancements and its possible repercussion on world food security. Among the controlled environment facilities such as the Free Air CO2 Enrichment (FACE), Soil-Plant-Atmosphere research chambers (SPAR), and CO2-Temperature Gradient Chambers (CTGC), the open-top chambers (OTC) are commonly used to control elevated CO2 concentration for plant science research. In the present study, we aimed to evaluate a modified OTC, designed and constructed based on previous OTC experiences, which provides a precise control of CO2 under different concentrations, with excellent control of air temperature and humidity. Materials and methods Three parts of OTC chamber structure, control system and monitoring system are improved and upgraded. (1) The modified OTC has a regular octagonal prism structure made of plastic steel 4 mm thick high transmittance glass material, an improvement over the previous system. The structure dimensions are 1.08 m length (diagonal), of 2.78 m diameter, and the inner and outer height (top) of 2.55 m and 2.10 m, respectively. (2) The monitoring system consisted of CO2 analyzers, temperature and humidity sensors, and a data acquisition system. (3) The control system is also composed by other features like programmable logic controller, GPRS communication module, a touch screen, a micro-relay, CO2 pressure reducing valves, solenoid valves, perforated windpipes, and CO2 cylinders. The OTC control system automatically collects and uploads data every six minutes using a system control coupled to the PI regulation mode（Proportional integral controller）. A linear controller generates deviation monitoring according to a given and an actual output value. The system is also equipped with a GSM communication module connected with the PLC through the Protocol PPI(Point to point interface) to upload all the data to a web server. The OTC control, monitoring system and the data can be accessed in real time through web browser or mobile App, reducing operation costs and allowing environmental variables monitoring. (4) To test the functionality of the modified OTC, Goji berry (Lycium barbarum L.) plants were grown from May to October on 2017 inside the chambers. Real-time data of CO2 concentration, temperature, and air relative humidity of the chambers were collected. Results As a result, the average CO2 levels obtained in the chamber during the study period was 369.33 μmol∙mol-1 for ambient conditions, while elevated group 1 and group 2 showed concentrations of 558.35 μmol∙mol-1 and 772.71 μmol∙mol-1 respectively. The fluctuation for elevated group 1 ranged from 551.82 to 572.40 μmol∙mol-1 with a variation amplitude of 20.58 μmol∙mol-1. In the elevated group 2, the range of CO2 concentration was from 756.71 to 779.79 μmol∙mol-1 with variation amplitude of 23.08 μmol∙mol-1. In addition, no significant differences were found in temperature and air relative humidity among the chambers treatments (P > 0.05). Discussion The improved OTC simulation control system can well control the CO2 concentration to meet the preset concentration requirements. In actual control, it needs to further debug and improve to achieve stable operation. Hysteresis usually occurs in the actual control, mainly because in the actual control, CO2 sensor from the monitoring of indoor air CO2 concentration to the control system takes a certain time, the control system automatically adjusts the solenoid valve opening and closing frequency through real-time monitoring data, keeping the CO2 concentration in the air chamber always close to the preset value, the CO2 concentration in the air chamber will be higher than the preset value due to the time difference. In addition, the CO2 concentration is also affected by the outside wind speed and the photosynthetic respiration of the plant, and the CO2 concentration in the air chamber changes greatly. The main purpose of using OTC simulation system in this study is to long-term study the effects of different CO2 concentration treatments on the physiological process of plants. The influencing factors are the same except for the CO2 concentration. This system monitors the temperature and humidity of OTC chambers in real time through different treatment. The results show that the difference in temperature and humidity in different treatment chambers is small, the trend of change is consistent, and the test control effect is good, and the expected purpose of the test is achieved. In addition, if it is necessary to increase temperature simulation in the study, a temperature increase control system may be correspondingly increased to achieve different CO2 concentration and temperature interactive processing simulation test. Conclusions These results demonstrated precise control of CO2 concentration, temperature, and humidity inside the modified OTC chambers, showing an excellent development of CO2 effect improvement on Goji berry, and it can be used to test climate change response in other plant species. Recommendations and perspectives The improved OTC control system realizes automatic unsupervised operation 24 hours a day. All data systems are automatically uploaded to the Web server. The system operation status can be monitored in real time through a Web browser/mobile phone APP, and the system can be controlled and the solenoid valve can be opened and closed. Download data, no geographical restrictions, no manual on-site supervision and save operating costs, improve work efficiency. The system can be used to study the simulation test of plants responding to climate change, and it can also provide reference for other studies related to the simulation of climate change.
Keywords: Open-top chamber; CO2 concentration; Air temperature; Air relative humidity