Concentration-dependent effects of exogenous phytomelatonin on flowering and physiological changes in azaleas (<i>Rhododendron maculiferum</i>)

The effects of exogenous phytomelatonin (PMT) on the flowering and physiology of azaleas were investigated in the cultivar <italic>Rhododendron maculiferum</italic> 'Pink Round Cluster'. Experimental plants in the budding stage were treated with PMT at concentrations of 100, 300, 500 μmol·L⁻¹, 1, 3, and 5 mmol·L⁻¹, with water serving as the control group. The results showed a concentration-dependent effect of exogenous PMT, with low concentrations promoting flowering and high concentrations delaying flowering. The sequence of flowering was as follows: PMT2 (300 µmol∙L⁻¹) &gt; PMT1 (100 µmol∙L⁻¹) &gt; PMT3 (500 µmol∙L⁻¹) = CK &gt; PMT4 (1 mmol∙L⁻¹) &gt; PMT5 (3 mmol∙L⁻¹) &gt; PMT6 (5 mmol∙L⁻¹). All treatments extended the flowering duration, with the longest duration observed under PMT2 (32 d). PMT treatment significantly enhanced endogenous nutrient levels. Compared to the control, low-concentration PMT treatments increased the endogenous gibberellic acid (GA₃), indole-3-acetic acid (IAA), and zeatin riboside (ZR) levels of flower buds in the early treatment phase (15 d after treatment) while significantly decreasing abscisic acid (ABA) levels. High-concentration PMT treatments significantly increased GA₃, IAA, and ZR levels in the later treatment phase (45 d after treatment). High GA₃, IAA, ZR, and low ABA levels promoted flowering during the floral bud differentiation phase. Exogenous PMT spraying induced physiological changes in <italic>R. maculiferum</italic> 'Pink Round Cluster' while altering its flowering time and duration. These findings provide theoretical support for regulating the flowering duration in azaleas by exogenous PMT and its application in the floriculture industry.