## 5 Conclusions  
The summer cultivation of lettuce in greenhouses frequently encounters heat stress challenges. In hydroponic systems, cooling the nutrient solution to reduce root zone temperature is an effective strategy to alleviate heat stress. To address the issue of temperature control instability in hydroponic nutrient solutions under high-temperature conditions, this study developed a nutrient solution temperature control system based on an adaptive DBO-fuzzy PID controller. Firstly, the system integrates high-precision sensor networks and air-source heat pump units, forming the hardware foundation of the system. Simultaneously, a fuzzy PID controller optimized by the DBO algorithm was designed for this system, enabling real-time adjustment of quantization and scaling factors in the fuzzy controller.  
Comparative simulation experiments of the adaptive DBO-optimized fuzzy PID controller demonstrated superior performance: the DBO-Fuzzy PID achieved a settling time of 35.2 s, representing reductions of 165.1 s and 123.2 s compared to conventional PID and fuzzy PID controllers, respectively. The overshoot was 2.2%, lower than conventional PID and fuzzy PID by 22.3% and 6.9%, respectively. When subjected to disturbance signals, the DBO-Fuzzy PID controller exhibited faster and more stable disturbance rejection, demonstrating enhanced stability and robustness.  
System performance tests in the summer greenhouse demonstrated that the DBO-Fuzzy PID optimized nutrient solution temperature control system exhibited better adaptability to high-temperature environments compared to traditional PID control. Cultivation experiments revealed that lettuce grown with traditional PID and DBO-Fuzzy PID temperature control showed fresh root weight increases of 24.7% and 31.7%, respectively, and fresh leaf weight increases of 33.3% and 36.1%, respectively, compared to non-cooled conditions. These results confirm the system's effectiveness in mitigating heat stress and maintaining optimal nutrient solution temperatures for lettuce growth. The results can provide a theoretical basis and practical reference for precise and stable temperature control in hydroponic nutrient solutions.  
Conceptualization, W.G., H.X., Y.T. and L.W.; methodology, W.G. and Y.C.; software, X.H. and Q.Z.; validation, Y.C., W.G. and Z.Z.; formal analysis, Y.C.; investigation, W.G. and H.X.; resources, W.G. and Q.Z.; data curation, Y.C. and Z.Z.; writing--original draft preparation, Y.C.; writing--review and editing, W.G.; visualization, Y.C.; supervision, W.G., Z.Z. and X.H.; project administration, W.G. and Z.Z.; funding acquisition, W.G. and H.X. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the Special Program for International Science and Technology Cooperation, Ningxia Academy of Agriculture and Forestry Sciences (DW-X-2023001) and the research and demonstration on the integration of modern technologies and equipment between China and the Netherlands in facility vegetables in Pinggu (Z221100006422008).  
Not applicable.  
The original contributions presented in the study are included in the article; further inquiries can be directed to the corresponding author.  
We thank all the authors for their support. The authors are also very grateful for the kind suggestions and valuable comments by the anonymous reviewers.  
The authors declare no conflicts of interest.  
The following abbreviations are used in this manuscript:  
\begin{tabular}{l l} PID & Proportional-Integral-Derivative \\ DBO & Dung Beetle Optimizer \\ NSTCS & nutrient solution temperature control system \\ NFT & Nutrient Film Technique \\ \end{tabular}