Cytological and Physiological Analyses Reveal the Mechanism of Wheat (<i>Triticum aestivum</i> L.) Superior and Inferior Grains Weight Reduction Under Spring Low‐Temperature Stress

ABSTRACT Global climate change has resulted in an increase in frequency and intensity of low‐temperature events, which severely affect wheat growth and yield. In the Huang‐Huai wheat growing area of China, spring low‐temperature stress (SLTS) at the jointing‐booting stage results in significant yield losses of wheat. In this study, the effects of SLTS on caryopsis development, starch synthesis and grain filling of superior and inferior grains were examined in the wheat cultivars ‘Yannong 19’ (YN19, cold tolerant) and ‘Xinmai 26’ (XM26, cold sensitive) for two consecutive growing seasons (2021/22 and 2022/23). Treatment with 2°C and −2°C was performed in the anther differentiation period, and the control group were treated with 10°C. The grain external morphology and endosperm cell microstructure were observed, sucrose‐starch enzyme activities and starch content were measured in superior and inferior grains, a logistic equation for grain filling was fitted, and a structural equation model was constructed. SLTS resulted in slow proliferation of endosperm cells in superior and inferior grains, and limited growth in grain length, width and height. The YN19 and XM26 superior grain volume were reduced by 3.96%–12.65% and 14.92%–28.27%, respectively, and that of inferior grains decreased by 6.72%–23.99% and 13.61%–30.75%, respectively. During grain filling, SLTS decreased the sucrose content, and sucrose synthase and ADP‐glucose pyrophosphorylase activities in grains, resulting in decreased starch accumulation, reduced average and maximum grain‐filling rates and ultimately decreased grain weight. The actual 1000‐grain weight of YN19 and XM26 superior grains decreased by 5.38%–14.97% and 3.98%–20.41%, respectively, and that of inferior grains decreased by 6.02%–12.40% and 9.61%–15.20%, respectively. Thus, inferior grains are more sensitive to SLTS compared with superior grains. The research results provide an important theoretical basis for understanding and addressing the effect of SLTS on wheat yield.