To explore the effect of the high temperature stress on the chlorophyll content and the conductivity in different alfalfa varieties, a field trial was conducted. Used SPAD-502 type chlorophyll analyzer to measure the chlorophyll SPAD in leaves, meanwhile determined relative conductivity and semilethal temperature and analyze. Results show that high temperature led to the decrease of chlorophyll content in different alfalfa varieties. The chlorophyll content of Leafy King, GL0602, Ameri Graze and Algonquin was significantly higher than that of Victoria, WL621, Flourish Age and Affinity. Through a relative conductivity calculated in 16 kinds of alfalfa breeds, semilethal temperature difference was obtained among the varieties. The findings indicated that Derby, Algonquin, Victoria and Affinity had poor comprehensive performances while Sidel, Alfalfa King and CUF101 had a stronger comprehensive performances. The results showed that Ameri Graze, Alfalfa King, Sitel and GL0602 had a less effect in high temperature stress of physiological changes index, showed strong heat resistance; however Victoria, Affinity, WL612 and Sanditi had a poor heat resistance.
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