Responses of photosynthetic characteristics and yield of summer maize to high temperature stress during flowering

【Objective】To explore the effects of high temperature stress during flowering on the temperature and humidity conditions in summer maize fields, photosynthetic characteristics of leaves, water use efficiency and yield composition,which could provide theoretical basis for high-temperature resistant cultivation of summer maize. 【Method】This study used heat resistant variety Xundan 20（XD20）and heat sensitive variety Nonghua 101（NH101）as materials,and a polyolefin（PO） film greenhouse was used to simulate high temperature during flowering, with field normal temperature treatment as the control（CK）. A continuous 10 d simulated high temperature stress treatment（HT） was set at the tasseling stage to determine the field air temperature and humidity conditions, leaf photosynthetic characteristics, water use efficiency, and yield of summer maize under different treatments. 【Result】The daily changes of air temperature and water vapor pressure difference in the maize field showed a single peak curve of low and stable in the morning and evening,and high and fluctuating during the day,and the canopy was higher than the ear layer during the day. The daily variation of relative humidity showed a single valley curve with high and stable morning and evening,and low and fluctuating daytime,and the canopy was lower than the panicle during the daytime period. The daily variation trends of actual water vapor pressure and enthalpy values in the fields of the 2 treatments were inconsistent. Under CK treatment,the daily variation trend was not obvious,and the difference in morning,noon and evening was small. However,under HT treatment,the daily variation showed a single peak curve with low and stable morning and evening,and high and fluctuating day,which was significantly higher during the day than at night;HT treatment enhanced the correlation between actual field water vapor pressure and enthalpy values with temperature and relative humidity. During the high-temperature treatment period,compared with CK,HT treatment increased daytime air temperature and increased the water vapor pressure difference in the morning,leading to increased evapotranspiration. This in turn led to an increase in field relative humidity,actual water vapor pressure and enthalpy values,and the increase in canopy was greater than that in spike layer,affecting the photosynthetic performance of the leaves of the 2 varieties. During high-temperature treatment,HT treatment increased the stomatal conductance,intercellular CO₂ concentration and transpiration rate of XD20 and NH101 ear leaves by 36.60%,157.20%,19.72%,and 41.67%,143.90%, 27.08% respectively. The leaf carboxylation efficiency,stomatal limit value,instantaneous water use efficiency and intrinsic water use efficiency decreased by 59.61%,29.94%,25.08%,31.65%,and 63.09%,31.00%,33.54%,33.45% on average,while the net photosynthetic rate fluctuated with air temperature,causing a decrease in the overall photosynthetic performance of the plant. After the end of high temperature stress,as the growth process of maize progresses,the photosynthetic parameters of the ear leaves treated with HT could gradually recover to the CK level. HT treatment resulted in a significant decrease in the dry matter accumulation in the late growth stage of maize plants（P＜0.05, the same below） and the number of grains per spike, a slight increase in 100-grain weight,but a significant decrease in grain yield. Compared with CK,under HT treatment,the number of grains per ear and yield of XD20 decreased by 33.55% and 35.49% respectively,while NH101 decreased by 36.89% and 37.63% respectively. The decrease in NH101 was greater than that of XD20. 【Conclusion】High temperature stress leads to faster air heating,higher temperature and larger vapor pressure deficit. This enhances farmland evapotranspiration,leading to an increase in relative humidity,actual water vapor pressure and enthalpy in the field. These conditions increase stomatal conductance,enhance transpiration heat dissipation,decrease water use efficiency,lower stomatal limit values,and raise intercellular CO₂ concentration. Additionally,leaf carboxylation efficiency declines,and net photosynthetic rate fluctuates with air temperature. Collectively,these changes reduce the overall photosynthetic performance of the plants,significantly decrease dry matter accumulation,and consequently reduce the number of grains per ear and cause a significant reduction in grain yield. The yield reduction of the heat sensitive variety NH101 is greater than that of the heat tolerant variety XD20.