Application of moderate nitrogen levels alleviates yield loss and grain quality deterioration caused by post-silking heat stress in fresh waxy maize

High temperature (HT) during grain filling is one of the most important environmental factors limiting maize yield and grain quality. Nitrogen (N) fertilizer is essential for maintaining normal plant growth and defense against environmental stresses. The effects of three N rates and two temperature regimes on the grain yield and quality of fresh waxy maize were studied using the hybrids Suyunuo 5 (SYN5) and Yunuo 7 (YN7) as materials. N application rates were 1.5, 4.5, and 7.5 g plant−1, representing low, moderate, and high N levels (LN, MN, and HN, respectively). Mean day/night temperatures during the grain filling of spring- and summer-sown plants were 27.6/21.0 °C and 28.6/20.0 °C for ambient temperature (AT) and 35/21.0 °C and 35/20.0 °C for HT, respectively. On average, HT reduced kernel number, weight, yield, and moisture content by 29.8%, 17.9%, 38.7%, and 3.3%, respectively. Kernel number, weight, yield, moisture, and starch contents were highest under MN among the three N rates under both temperature regimes. HT reduced grain starch content at all N levels. HT increased grain protein content, which gradually increased with N rate. Mean starch granule size under MN was larger (10.9 μm) than that under LN and HN (both 10.4 μm) at AT. However, the mean size of starch granules was higher under LN (11.7 μm) and lower under MN (11.2 μm) at HT. Iodine binding capacity (IBC) was lowest under MN and highest under HN among the three N levels under both temperature regimes. In general, IBC at all N rates was increased by HT. Peak viscosity (PV) was gradually reduced with increasing N rate at AT. In comparison with LN, PV was increased by MN and decreased by HN at HT. Retrogradation percentage gradually increased with N rate at AT, but was lowest under MN among the three N rates at HT. LN + AT and MN + HT produced grain with high pasting viscosity and low retrogradation tendency. MN application could alleviate the negative effects of HT on the grain yield and quality of fresh waxy maize.