Abstract: In order to explore the heat resistance of different varieties of Rhododendron and the physiological response mechanism of Da Yuanyang Jin Rhododendron, the semi-lethal temperature and high temperature leaf browning index of five oriental Rhododendron varieties including Da Yuanyang Jin, Yanzhi Mi, Shanhu Fen Mudan, Liehuo and Edinburgh were calculated, and cluster analysis was carried out. Da Yuanyang Jin Rhododendron was treated with different temperature stress to study the changes of physiological indexes under high temperature stress. The results showed that five Rhododendron cultivars could be divided into three groups: in the first group, Da Yuanyang Jin was a strong heat-resistant cultivar; the second group included Shanhu Fen Mudan, Yanzhi Mi and Edinburgh, which were semi-heat-resistant varieties. The third group was more heat-tolerant variety Liehuo. Da Yuanyang Jin Rhododendron showed different physiological responses under different high temperature stress conditions. With the increase of high-temperature degree stress, leaf MDA content increased under mild and moderate stress, but declined rapidly after 7 d under severe stress. Soluble sugar content showed a sustained rise under mild stress, initial decrease followed by increase under moderate stress, and an initial rise followed by decline under severe stress. Proline content significantly increased under moderate stress, while soluble protein content initially rose then fell under stress. Chlorophyll content decreased in controls and mild stress but slightly increased late under moderate/severe stress. POD activity generally increased first and then decreased under moderate and severe stress; SOD activity remained stable, whereas CAT activity declined more significantly under mild stress.

Key words: Rhododendron simsii Planch., evaluation of heat tolerance, high temperature stress, physiological response