5-Aminolevulinic Acid and Soil Fertility Enhance the Resistance of Rosemary to Alternaria dauci and Rhizoctonia solani and Modulate Plant Biochemistry

Fungal infection of horticultural and cereal crops by Alternaria dauci and Rhizoctonia solani represents an important biotic stress that could be alleviated by application of 5-aminolevulinic acid (ALA) to fertile and poor soils. Therefore, in this study, the morphological, physiological, biochemical, and genetic effects of ALA application (eight weekly applications at 3–10 ppm) to A. dauci- and R. solani-infected Salvia rosmarinus (rosemary) in fertile and poor soils were investigated. ALA-treated plants produced the longest and highest number of branches and had higher fresh and dry weights. There were increases in the major essential oil constituents (1,8-cineole, linalool, camphor, and borneol), as shown by Gas chromatography–mass spectrometry (GC-MS); higher antioxidant activities in DPPH and β-carotene-bleaching assays; upregulated superoxide dismutase (SOD) and catalase (CAT) antioxidant enzyme activities; increased total phenolics, chlorophyll, soluble sugars, and proline; increased gas exchange parameters; enhanced leaf water potential and relative water content (RWC); and upregulated expression of DREB2 and ERF3 (stress-related genes) and FeSOD, Cu/ZnSOD, and MnSOD (antioxidant genes). Several mechanisms were involved, including stress tolerance, antioxidative, and transcription regulation mechanisms. Furthermore, ALA performance was increased in higher-quality soils with higher nutrient content. This study demonstrated the novel application of ALA as a biotic stress ameliorant with enhanced performance in fertile soils.