Comparative Effects of GABA, 5-Aminolevulinic Acid, and Bacillus-Based Treatments on IBA-Pretreated Tea Chrysanthemum Cuttings Under Plateau Cultivation Conditions

Plateau environments are characterized by low temperatures, intensive radiation and drastic diurnal temperature fluctuations, all of which greatly suppress rooting and seedling establishment of tea chrysanthemum cuttings. To compare the regulatory effects of different rooting regulators on tea chrysanthemum cuttings under plateau cultivation conditions, this study conducted a 60-day plug-tray cultivation experiment from late July to late September with three biological replicates. All the cuttings, including the IBA-pre-treated control (CK), were uniformly pretreated with 500 mg·L−1 indole-3-butyric acid (IBA). On this basis, four treatments, including 5-aminolevulinic acid hydrochloride (5-ALA, T1), γ-aminobutyric acid (GABA, T2), Bacillus amyloliquefaciens (T3) and Bacillus velezensis (T4), were applied to determine root morphology, seedling growth, physiological stress metabolism, photosynthetic capacity and rhizosphere substrate characteristics. The different rooting promoters exerted distinct regulatory effects on cutting performance. The GABA treatment significantly improved leaf gas exchange, seedling growth, antioxidant status and rhizosphere nutrient conditions, with net photosynthetic rate, stomatal conductance and transpiration rate improved by 316.34%, 92.31% and 168.00%, respectively, and significantly increased seedling vigor index, plant height and stem diameter by 21.74%, 60.85% and 46.08%, respectively. It also elevated the soluble sugar content and the ascorbate peroxidase (APX) activity, reduced malondialdehyde (MDA) and hydrogen peroxide (H2O2) accumulation, and optimized rhizosphere available nitrogen and phosphorus levels, as well as related enzyme activities. Nevertheless, the 5-ALA treatment exhibited unique advantages in improving rooting rate and seedling survival. Mantel tests confirmed that the seedling vigor index closely correlated with the root architecture, the total chlorophyll and the transpiration rate. A principal component analysis (PCA) and a cluster heatmap both identified GABA as the treatment with relatively balanced overall performance. A comprehensive D-value evaluation ranked the treatments as GABA > B. amyloliquefaciens > 5-ALA > B. velezensis > control. The treatment with 5-ALA mainly improved the antioxidant capacity, the B. amyloliquefaciens treatment favored root elongation and total nutrients, and the B. velezensis treatment only produced mild improvements. This study indicates that the different exogenous regulators target divergent growth and physiological processes, and the GABA treatment could coordinately boost root development, photosynthetic performance, antioxidant defense and rhizosphere nutrient cycling, thus presenting great application potential for tea chrysanthemum cutting propagation under plateau cultivation conditions.