| Frost is one of the agricultural meteorological disasters occurred frequently in Hetao irrigation district. We used Kehe No.8 maize, a mid-late maturing variety widely planted in Hetao irrigation area, as the test material. 5℃ to -5℃ low temperature environment was set up, allowing the treatments of maize seedlings for 1 h, 3 h, and 5 h. Comparisons between the physiological index changes before and after treatments were analyzed. Physiology research under stress could result in insightful understandings of the physiological and biochemical changes in plants under various low temperature treatments, which could be used to determine the meteorological indexes of spring frost. The results showed that with the decrease of temperature and increase of time under low temperature treatment, chlorophyll content of maize seedlings showed a steady downward trend. While, soluble sugar content, SOD activity and MDA content became increased first and then went decreased, and the highest value appeared at -2℃ and -3℃. The analyses of these three physiological indexes showed that the cold resistance capability of the 4-leaf maize seedling was stronger than that of the 3-leaf one. Soluble protein content was decreased first and then became increased, and the lowest value appeared at -2℃. All the low temperature treatments resulted in decreasing of net photosynthetic rate, stomatal conductance and leaf water use rate of maize seedling. According to the results and the supercooling points of maize, the minimum temperature indexes of spring frost in Hetao irrigation district were as follows: light frost, 0℃～3℃; moderate frost, -2.5℃～0℃ and heavy frost, <-2.5℃. |