Beneficial Effect of Exogenously Applied Calcium Chloride on the Anatomy and Fast Chlorophyll Fluorescence in Rhododendron × pulchrum Leaves Following Short-Term Heat Stress Treatment

The heat tolerance of plants can be improved by using exogenous calcium chloride (CaCl2) to cope with temperature fluctuations. Since global climates continue to warm, it is important to further explore the way in which plants respond to heat stress with the use of CaCl2. We aimed to explore the effect of exogenous CaCl2 on the leaf microstructure, leaf epidermal ultrastructure, and chlorophyll a fluorescence of Rhododendron × pulchrum (R. × pulchrum) under heat stress. In the leaves of R. × pulchrum treated with exogenous CaCl2, compared to the control, the thickness of the epidermis, spongy tissues, and stomatal aperture increased, whereas the stomata density and ratio of closed/open stomata decreased. In the leaves of R. × pulchrum under heat stress conditions, compared to the control, the values of the maximal photochemical efficiency of photosystem II (Fv/Fm), the performance index on an absorption basis (PIABS), the quantum yield for the reduction of terminal electron acceptors on the acceptor side of PSI (φRo), and the energy absorbed per unit cross-section of a photosynthesizing object at the moment of achieving the fluorescence maximum (ABS/CSM) all decreased, whereas the quantum yield of the energy dissipation (φDo) increased significantly. However, these differences disappeared when R. × pulchrum was treated with exogenous CaCl2. This suggests that exogenous CaCl2 can improve the heat tolerance in R. × pulchrum by regulating the leaf anatomical structure and the behavior of epidermal cells and stomata in leaves, protecting the stability of photosystems I and II and improving the electron transfer from QA to QB. Our study could provide a theoretical basis for the breeding, further research, and utilization of Rhododendron in the context of global warming.