Integrating Chlorophyll Fluorescence with Anatomical and Physiological Analyses Reveals Interspecific Variation in Heat Tolerance Among Eight Rhododendron Taxa

To investigate interspecific variation in heat tolerance and underlying adaptation mechanisms in <i>Rhododendron</i>, three-year-old potted seedlings of eight taxa, representing four subgenera within the genus <i>Rhododendron</i>, were subjected to 40 °C high-temperature stress. Heat tolerance was comprehensively assessed using phenotypic observation, chlorophyll fluorescence imaging, microscopic examination, and physiological measurements. Results revealed that leaf damage in <i>Rhododendron oldhamii</i> and <i>Rhododendron × pulchrum</i> reached grade III, whereas <i>Rhododendron latoucheae</i> exhibited only grade II injury with rapid recovery. Chlorophyll fluorescence analysis showed a significant decrease in the maximum quantum efficiency of PSII (F_v/F_m) in <i>R. liliiflorum</i> and <i>R. × pulchrum</i>, followed by rapid recovery, while <i>R. latoucheae</i> maintained stable F_v/F_m values. Stomatal closure occurred in all taxa post-stress; stomatal characteristics of <i>R. liliiflorum</i> and <i>R. simiarum</i> remained stable, and leaf tissue structure was least affected in <i>R. kiangsiense</i>. <i>R. × pulchrum</i> demonstrated the most pronounced structural recovery. Physiologically, <i>R. oldhamii</i> exhibited the greatest increases in electrolyte leakage (EL) and malondialdehyde (MDA) content. <i>R. simiarum</i> accumulated the highest proline content under stress, while <i>R. latoucheae</i> showed the most significant proline reduction during recovery. By integrating multiple indicators through principal component analysis (PCA) and a membership function, and assigning weights based on variance contribution, the heat tolerance was comprehensively evaluated and ranked as follows: <i>R. latoucheae</i> > <i>R. simiarum</i> > <i>R. oldhamii</i> > <i>R. ovatum</i> > <i>R. fortunei</i> > <i>R. liliiflorum</i> > <i>R. kiangsiense</i> > <i>R. × pulchrum</i>. These findings demonstrate significant differences in heat tolerance among <i>Rhododendron</i> taxa at the subgenus level, with the subgenus <i>Azaleastrum</i> generally possessing stronger short-term heat tolerance compared to the subgenus <i>Tsutsusi</i>. This study provides a theoretical basis for heat-tolerant cultivar breeding and landscape application of <i>Rhododendron</i>.