Abstract: High temperature and intense light are the most common abiotic stresses in summer, seriously threatening crop yield and fruit quality. To provide a theoretical basis for stress tolerant variety breeding and high yield cultivation of sugar beet, we explored the photosynthetic performance of different varieties under compound environmental constraints of high temperature and intense light, to screen out resistance indicators. The leaf photosynthetic performance of eight sugar beet varieties under the compound stress of strong light and high temperature was evaluated at the tuber expansion stage. A total of 26 indicators, including gas exchange and chlorophyll fluorescence kinetic parameters of sugar beet leaves, were measured. The stress tolerance of sugar beet under combined environmental constraints was comprehensively evaluated. The results showed that different sugar beet varieties responded differently to combined stresses of high temperature and intense light. The 26 indicators were transformed into four independent composite indicators by principal component analysis, covering 95% of the total data information. The eight sugar beet varieties were classified into three categories by using the membership function and cluster analysis. An optimal evaluation mathematical model for sugar beet tolerance to combined stress of high temperature and intense light was established by using stepwise regression analysis: D=0.745φ_Eo-0.320 (D, the value of comprehensive evaluation of stress resistance; φ_Eo, the quantum yield of absorbed energy used for electron transfer). φ_Eo can be used as an auxiliary index to identify the strength of sugar beet varieties to tolerate combined stress of high temperature and intense light. Sugar beet varieties with strong tolerance to combined stress of high temperature and intense light have higher energy connectivity between photosystem II (PSII) units, higher photosynthetic electron transfer capacity between PSII and photosystem I (PSI), less damage to the activity of the oxygen release complex, and higher stability of PSII.

Key words: sugar beet, gas exchange, photochemical parameter, identification index, multivariate analysis.