Effects of heat stress on changes in physiology and anatomy in two cultivars of Rhododendron

Rhododendron × hybridum, an ornamental plant, is usually heat sensitive. High temperatures cause an array of morphological, anatomical, physiological, biochemical changes in plants, which affect plant growth and development. Evaluating difference between two cultivars in R. may facilitate future understand the heat tolerance mechanisms. We conducted a comparative study on growing plants of R. 'Lan Yin' and R. 'Liu Qiu Hong' at 38/30 °C (day/night) and 25/17 °C (day/night) to elucidate the heat tolerance mechanism of Rhododendron in terms of morphology, physiology, anatomy and biology. After the plants were subjected to 38/30 °C (day/night) incubation for 6 d, the chlorophyll and carotenoid contents decreased, malondialdehyde (MDA) and hydrogen peroxide (H2O2) accompanying lesser increase in R. 'Lan Yin' revealed that the heat resistant cultivar was easier to adjust physiologically in heat disappearing. Morphologically, the xylem vessels, leaf ultrastructure and chloroplast of R. 'Lan Yin' is superior to R. 'Liu Qiu Hong', and R. 'Lan Yin' had a decreased stomatal opening and stomatal aperture under heat stress, confirming stomatal factors play a crucial role in heat tolerance in R. cultivars. The electrolyte leakage increased, superoxide dismutase (SOD) and peroxidase (POD) activities, osmotic adjustment solute contents (proline, total soluble protein, and sugar) decreased to scavenge reactive oxygenspecies (ROS) and maintain cell membrane stability in two cultivars, but R. 'Lan Yin' had a higher level relatively, which may suggests that CMT may use a reliable index for selecting heat-tolerant cultivars of R. In addition, there was evidence of plasmolysis and chloroplast damage in R. 'Liu Qiu Hong'.