High temperature is one of the main limiting factors for the introduction and landscape application of Hydrangea in summer. In order to construct the heat resistance evaluation system of Hydrangea cultivars and select heat resistant cultivars, eight Hydrangea cultivars were used to observe the high temperature semi-lethal temperature (HLT50) and the heat injury indexes (HII) under high temperature stress, and 14 physiological and biochemical indexes were measured after six days of high temperature stress at 44 ℃/33 ℃, after which heat resistance was evaluated by multivariate statistical analysis method. The original 14 indexes were transformed into four independent comprehensive indexes by the principal component analysis, and the comprehensive evaluation value (D value) of each variety was obtained by the subordinate function method. According to D value, the eight Hydrangea cultivars were divided into three groups. The moderate heat resistance cultivars included ‘Essar’ ‘Elbtal’ ‘Komachi’ and ‘Shakespeare’. The heat sensitive cultivars included ‘Tricolor’ and ‘Inspiration’. The heat resistance cultivars included ‘Stockings’ and ‘Corsage’. On the basis of D value, the stepwise regression method was used to set up the variable heat resistance evaluation model with the content of proline (Pro), chlorophyll (Chla+Chlb), carotenoid (Car), chlorophyll a/b value (Chla/Chlb), superoxide dismutase (SOD) activity, relative water content (RWC) and soluble sugar (SS) contents as indicators. In addition, HLT50 and HII of high temperature could also be used as a reliable method to identify the heat resistance of Hydrangea cultivars. This study would provide a theoretical basis for the application and popularization of Hydrangea cultivars in southern urban and rural gardens as well as the breeding of heat-resistant hydrangea cultivars.
