Abstract: 【Objective】With the global warming, summer high temperature weather occurs more frequently and more intensely, and the duration of high temperature is more longer, which is the main abiotic stress limiting the improvement of summer maize yield in Huang-Huai-Hai region. In this study, the effects of high temperature stress on dry matter production performance and grain yield of summer maize were investigated by analyzing the differences of photosynthetic characteristics, carbon assimilate accumulation and distribution and grain yield of two different heat sensitive summer maize varieties under high temperature stress at V12 stage and VT stage, respectively. 【Method】In this study, heat resistant maize variety Zhengdan 958 (ZD958) and heat sensitive maize variety Xianyu 335 (XY335) were used as materials. The high temperature stress treatments (day 38℃12 h/night 28℃12 h) were set at V12 and VT stage, respectively, at the same time, thenormal temperature treatment (day 32℃12 h /night 22℃12 h) was set as the control. High temperature greenhouse equipped with automatic temperature and humidity control facilities was used to simulate the effect of natural field high temperature. The dynamic characteristics of leaf area index (LAI), carbon metabolism enzyme activities, photosynthetic rate and carbon assimilate accumulation and allocation were compared after high temperature stress, aimed to determine the response mechanism of dry matter production performance and grain yield to high temperature stress. 【Result】After high temperature stress, LAI, carbon metabolism enzyme activities, net photosynthetic rate and dry matter accumulation of two cultivars were significantly decreased. LAI, RuBP carboxylase activity, PEP carboxylase activity, net photosynthetic rate and dry matter accumulation of ZD958 and XY335 decreased by 2.98%-4.21%, 40.38%-54.46%, 16.88%-30.60%, 18.14%-25.49%, 12.83%-19.38% and 3.80%-5.07%, 56.56%-76.16%, 26.33%-33.66%, 22.37%-34.62%, 22.07%-26.72%, respectively. The decrease range of high temperature stress in VT stage was larger than that in V12 stage. After high temperature stress, transpiration rate of summer maize leaves increased, while leaf water use efficiency decreased significantly. Under high temperature stress, ¹³C assimilation of ZD958 and XY335 decreased by 18.48% and 22.82%, respectively, and the proportion of ¹³C assimilation in grains decreased. The high temperature stress significantly decreased grain number per spike and grain yield, although 1000 grain weight increased slightly. After V12 high temperature stress, 1 000 grain weight of ZD958 and XY335 increased by 2.36% and 3.17%,respectively, while thegrain number per spike and yield decreased by 62.53% and 70.50%, 45.87% and 62.87%%, respectively. After VT high temperature stress, grain number per spike and yield decreased by 70.53% and 85.41%, 66.89% and 80.61%%, respectively. The decrease range of high temperature stress in VT stage was larger than that in V12 stage, andXY335 decreased more than ZD958. 【Conclusion】The high temperature stress reduced LAI, RuBP carboxylase and PEP carboxylase activities, and significantly reduced photosynthetic rate and dry matter production performance of summer maize. Under high temperature stress, thegrain number per spike decreased significantly, which inhibited the transportation of carbohydrate from leaf and stem to grain, resulting in lower grain yield. The effects of high temperature stress on dry matter performance and grain yield of summer maize in VT stage was significantly greater than that in V12 stage. The decrease of heat sensitive variety XY335 was significantly greater than that of heat resistant variety ZD958 in two periods. 

Key words: Summer summermaize, High hightemperature stress, Carbon carbonmetabolism enzyme activity, Carbon carbonassimilate accumulation and distribution, Yieldyield