Objectives We investigated the effects of different N application rates on photosynthetic physiology and yield of maize under high temperature stress at ear stage.

Methods Artificial high temperature stress experiment was carried out in 2020–2021. Three N application rates were low nitrogen (N 90 kg/hm², N90), medium N (N 180 kg/hm², N180) and high N (N 270 kg/hm², N270), while the maize cultivars were heat resistant cultivar Zhengdan 958 (ZD 958) and heat susceptive cultivar Xianyu 335 (XY 335). High temperature (HT) treatment lasted for 12 days (2020) and 9 days (2021) from the 11th leaf development stage to tasseling stage, and the naturally growing plants were used as the control (CK). The mean daily maximum temperatures of high temperature and control during the treatment period were 41.9℃, 35.9℃ (2020) and 40.8℃, 37.7℃ (2021), and the mean temperature difference between day and night were 19.3℃, 13.0℃ (2020) and 18.1℃, 14.8℃ (2021). The photosynthetic pigment content, photosynthetic parameters, chlorophyll fluorescence parameters, photosynthetic enzyme activity, grain yield and yield components in ear leaves of the two varieties were investigated, and the interaction among temperature, cultivars and N application rate was analyzed.

Results 1) From the 11th leaf development stage to tasseling stage, high temperature stress increased the activities of phosphoenolpyruvate carboxylase (PEPCase) and ribulose-1,5-diphosphate carboxylase/oxygenase (Rubisco), and decreased the content of photosynthetic pigment, net photosynthetic rate (P_n), chlorophyll fluorescence parameters of ear leaves, and the maize yield of two cultivars. The effect of high temperature on heat susceptive cultivar XY 335 was greater than heat resistant cultivar ZD 958. 2) Under the control condition, photosynthetic pigment content, P_n, maximum photochemical efficiency (F_v/F_m), PEPCase activity, Rubisco activity, grain yield and yield components of the two cultivars increased with the increase of N application rate. Under the high temperature condition, they showed a trend of rising first and then falling with the increase of N application rate, and N 180 treatment was the highest. 3) Cultivars temperature, N application rates and the interaction between temperature and N application rates had significant effects on photosynthetic performance indexes and yield (P<0.01). Compared with the CK, under the high temperature condition, all indexes decreased the most in N270 treatment. This indicated that high nitrogen aggravated the photosynthetic performance of maize ear leaves and intensified the damage of high temperature, which was more obvious in XY 335. 4) Correlation analysis showed that the yield of the two cultivars was extremely significantly and positively correlated with ear diameter, kernels per ear, photosynthetic pigment content, P_n, F_v/F_m, actual photochemical efficiency (YⅡ), apparent electron transfer efficiency (ETR) and photochemical quenching coefficient (qP) (P<0.01). Ear diameter and kernels per ear were significantly correlated with photosynthetic physiological indexes (P<0.01). Therefore, the results showed that the decrease of photosynthetic performance led to decrease in grain number per spike, which led to decrease in yield.

Conclusions High temperature stress from the 11th leaf development stage to tasseling stage can significantly inhibit photosynthetic physiology and decrease yield of maize. Medium N application (180 kg/hm²) can alleviate heat stress, improve photosynthetic physiological activity and increase yield, while high nitrogen application (270 kg/hm²) increase the yield loss caused by high temperature.