High temperature (temperature over 35 °C) is an extremely important environmental factor that affects the maize grain quality in Southern China. The effects of heat stress after pollination on grain protein and starch deposition and activities of involved enzymes were studied in a pot trail in 2014 and 2015. Results showed that grain dry weight reductions at maturity were 19.8% and 19.1%, whereas starch contents (mg g−1) were reduced by 3.0% and 3.3%, and starch accumulation (mg grain−1) were reduced 22.2% and 21.8% in 2014 and 2015, respectively. Protein content was decreased by heat stress before 15 DAP and increased thereafter. At maturity, protein contents (mg g−1) were increased by 24.5% and 25.3% in 2014 and 2015, while protein accumulation (mg grain−1) were not affected by heat stress. In response to heat stress, glutamate synthase activity was enhanced by 29.1–82.9% in 2014 and 2.0–141.8% in 2015, whereas glutamine synthetase activity was reduced by 1.9–43.5% in 2014 and 0.1–27.4% in 2015 throughout the grain filling. The activities of sucrose phosphate synthase were decreased by heat stress at 10–25DAP (12.7–32.0%) in 2014 and 15–20 DAP (23.2–27.5%) in 2015, and activities of sucrose synthase were decreased by heat stress at 5–15 DAP (20.0–45.0%) in 2014 and 15 DAP (22.0%) in 2015, repectively. The activities of enyzmes that involved in starch synthessis were all suppressed by heat stress during grain filling, and the reduction of adenosine diphosphate-glucose pyrophosphorylase, soluble starch synthase, and starch branching enzyme were decreased by 21.3–43.1%, 19.1–29.2%, and 7.0–45.6% in 2014 and 1.8–78.5%, 21.4–51.2%, and 11.0–48.0% in 2015, respectively. Conclusively, grain weight and starch deposition were suppressed by heat stress due to the decreased activities of enzymes involved in starch synthesis, and the increased protein content was due to the enhanced activity of glutamate synthase.

The effects of heat stress during early (1-15 d after pollination, DAP) and late (16-30 DAP) grain filling stages on the starch physicochemical properties of four waxy maize varieties were evaluated. Crystallinity only increased in Suyunuo5 after exposure to high temperature at late grain filling stage. The effects of heat stress on digestibility and swelling power were dependent on varieties and stages. Generally, swelling power was increased by heat stress at early grain development stage and digestibility was increased by high temperature at late grain filling stage, respectively.The results of correlation analysis indicated the starch with large granule size could swell well and easy digest. Peak, trough, final, and breakdown viscosities in response to heat stress were dependent on stages and varieties. In general, peak, trough and final viscosities were decreased and increased by heat stress at early grain formation and late grain filling stages, respectively; whereas the breakdown and setback viscosities were similar among the three treatments.Heat stress increased the gelatinization temperatures and retrogradation percentage. Gelatinization range decreased under heat stress at 1-15 DAP but remained constant under heat stress at 16-30 DAP in all varieties. The starch exposed to high temperature at 16-30 DAP presented higher digestibility and peak viscosity and lower retrogradation percentage than those at 1-15 DAP. Therefore, heat stress at early grain formation stage severely affects the physicochemical properties of starch.

Rainfed maize in Southern China and frequently suffer water deficit at later plant growth periods. A pot trial in 2014–2015 was conducted to study the effects of drought stress (the relative soil moisture contents are 70–80% and 50–60% under control and water deficit conditions, respectively) after pollination on grain filling and starch accumulation, activities of starch synthetic enzymes, and contents of indole-3-acetic acid (IAA) and abscisic acid (ABA), with Suyunuo5 as test material. The grain fresh weight, volume, and dry weight were not affected by drought before 10 days after pollination but were restricted thereafter. The reduction at maturity was reduced by 33.3%, 40.0%, and 32.3% in 2014 and by 21.7%, 24.3%, and 18.3% in 2015. The grain filling rate was suppressed by water deficit, whereas grain moisture and starch content were slightly affected. The starch accumulation was decreased by 33.5% and 20.0% at maturity in 2014 and 2015, respectively. The activities of starch synthetic enzymes (sucrose phosphate synthase, sucrose synthase, ADP-glucose pyrophosphorylase, soluble starch synthase, and starch branching enzyme) were downregulated by post-silking drought. The ABA content was increased, whereas IAA content was decreased when plants suffered water deficit during grain filling. In conclusion, post-silking water deficit increased ABA content, decreased IAA content, and weakened the activities of starch synthetic enzymes, which suppressed grain development and ultimately reduced grain weight.

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Core Ideas Heat stress decreased the translocation of dry matter from vegetative organs after pollination. Heat stress during grain filling accelerated leaf senescence of waxy maize. This study provides leaf senescence and dry matter translocation data for waxy maize production. Waxy maize (Zea mays L. sinensis Kulesh) is frequently exposed to high temperatures (higher than 35.0°C) during grain filling in southern China. This pot trial was conducted to study the effect of heat stress during grain filling on grain yield, dry matter (DM) accumulation and translocation, and leaf senescence of plants. Post‐pollination plants of Suyunuo 5 and Yunuo 7 (waxy maize varieties) were planted under high temperature (35.0°C) and ambient temperature conditions (daytime temperatures during grain filling were 29.0 and 28.2°C in 2014 and 2015, respectively).The decrease rate of grain yields for Suyunuo 5 and Yunuo 7 were 31.3 and 21.9% due to the reduced grain number and weight, respectively, indicating that Suyunuo 5 was more sensitive to heat stress. Heat stress decreased dry matter accumulation after pollination (DMA) . The photosynthate of vegetative organ post‐pollination translocated to grain decreased by 36.1 and 55.9% for Yunuo 7 and Suyunuo 5, respectively. However, the translocation amount and rate of assimilates pre‐pollination increased, with greater increase observed in Suyunuo 5. The malondialdehyde (MDA) content was increased by heat stress 25 and 20 d after pollination in Yunuo 7 and Suyunuo 5, respectively. Leaf chlorophyll (Chl), carotenoid (Car), and soluble protein contents were decreased, and the activities of antioxidant enzymes (superoxide dismutase [SOD], catalase [CAT], and peroxidase [POD]) were suppressed by heat stress in both varieties. The findings suggested that reduced DMA and accelerated leaf senescence under heat stress post‐pollination result in the grain yield loss.

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