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Four maize inbred lines with different photosyn-thetic rates and their two hybrids were used as test materials,and the diual variations of their photosynthesis parameters in the silking stage were measured to study the heterosis of photosynthetic performance.Results showed that net photo-synthetic rate (In),transpiration rate (Tr) and stomatal conductance (Gs) all presented an obvious single-peaked curve in a day,with the peak values occurring at 10:00-12:00,12:00,10:00-12:00 a.m.,respectively,while water use efficiency (WUE) had a"V"type variant trend,with the lowest value appearing at 12:00.The diual variation of Pn and Tr was correlated markedly with Gs,suggesting that Gs played an important role in regulating the diual variation of Pn and Tr,and Pn,Tr and Gs had a higher heterosis in the afteoon than in the moing,while the WUE was in reverse,indicating that maize hybrid had higher resistance to the high temperature and dehydration in the afteoon,which provided a new path to select varieties with a high net photosynthetic rate.