Comprehensive Assessment of Aluminum Tolerance in Celery (Apium graveolens L.) Germplasm and Its Physiological Basis

Aluminum (Al) toxicity is an important factor limiting crop production in acidic soils; however, systematic evaluation of Al tolerance and its physiological basis in celery (Apium graveolens L.) remains limited. In this study, 400 μmol·L−1 AlCl3 was identified as the appropriate concentration for Al-tolerance screening through a concentration-gradient experiment. Based on this concentration, 43 celery germplasm accessions were evaluated using 14 morphological and physiological traits. A comprehensive evaluation framework for Al tolerance was established using principal component analysis, membership function analysis, and hierarchical cluster analysis. The comprehensive A-value index enabled quantitative evaluation and classification of Al tolerance, and the accessions were divided into five categories ranging from highly Al-tolerant to highly Al-sensitive. Furthermore, key indicators were identified through stepwise regression analysis, which simplified the evaluation system while maintaining its assessment reliability. Physiological analysis of contrasting accessions showed that Al tolerance in celery was closely associated with restricted Al accumulation, enhanced redox homeostasis, and maintenance of photosynthetic system stability. Among these processes, the coordinated regulation of antioxidant defense and light energy utilization efficiency may represent an important physiological basis for tolerance differentiation. Overall, this study established an integrated framework from screening-concentration optimization to comprehensive evaluation and physiological characterization, providing a technical reference for the screening, evaluation, and breeding utilization of Al-tolerant celery germplasm.