Objectives This study explored the effect of KH₂PO₄ foliar application on alleviating the damage of heat stress on weight formation of superior and inferior grains.

Methods A field experiment was conducted using a semi-winter wheat cultivar Annong 0711 as experimental material. Four spraying treatments were setup, including: water (SW), water+heat stress (SW+HT), 0.3% KH₂PO₄ (KDP) and 0.3% KH₂PO₄+heat stress (KDP+HT). Spraying treatment was conducted on the 3rd and 11th day after anthesis, and heat stress treatments were made by shelting wheat with plastic film between 11: 00 and 16: 00 during post-anthesis 20−24 days. The chlorophyll, soluble sugar, and sucrose contents in the flag leaves were measured at the 19th day (before heat stress), the 24th day (after heat stress), and during the maturing stage. Additionally, the starch accumulation and grain weight of both superior and inferior grains were measured.

Results Heat stress significantly reduced flag leaf chlorophyll content, while KH₂PO₄ foliar application mitigated the decline and maintained higher chlorophyll levels until maturity stage. Compared to SW, KDP treatment significantly increased the contents of soluble sugars and sucrose in flag leaves and stems + sheaths before high-temperature stress. Heat stress significantly reduced the contents of these two carbohydrates in vegetative organs at the maturity stage, with the order of SW+HT<SW<KDP+HT<KDP. Post-anthesis spraying of KH₂PO₄ notably elevated the amylose, amylopectin, and total grain starch contents in both superior and inferior grains under both pre- and post-heat stress. However, high stress significantly decreased amylopectin content, with a greater reduction magnitude observed in inferior grains than in superior grains. Foliar KH₂PO₄ application alleviated the decline in grain starch content caused by heat stress. Heat stress significantly decreased the average grain-filling rate, with a larger decline in inferior grains than in superior grains. Compared to SW, KDP significantly improved grain-filling rates in the late grain-filling stage, resulting in a notable increase in grain weight, with a greater enhancement in inferior grains compared to superior grains. Compared to SW+HT, SW, KDP and KDP+HT treatments increased grain yield at maturity by 9.46%, 11.06%, and 16.79%, respectively.

Conclusions Heat stress weakens wheat photosynthetic and biomass production capacity, inhibits grain-filling rate, especially that inferior grains, and ultimately reduces grain weight and yield. Post-anthesis foliar application of 0.3% KH₂PO₄ helps delay the senescence of flag leaf under heat stress, maintain the supply of photosynthetic assimilates, improve grain-filling capacities of both superior and inferior grains, and narrow the lag between superior and inferior grains weight, thereby increasing overall grain yield.