Low Temperature at the Booting Stage Reduced Wheat ( <scp> <i>Triticum aestivum</i> </scp> L.) Grain Yield by Affecting Material Transport and Starch Synthesis Initiation

ABSTRACT As spring cold events become more prevalent, low temperature has become a major abiotic stress factor limiting wheat yields in China. Previous studies have shown that spring cold spells lead to shrivelled grains and poor grain filling. To explore the physiological mechanisms underlying these effects, two wheat cultivars, ‘Yannong 19’ (low temperature‐tolerant) and ‘Wanmai 52’ (low temperature‐sensitive), were used. Controlled low‐temperature treatments were applied in artificial climate chambers using three temperature levels (2°C, 0°C, and −2°C) and two treatment durations (24 h and 48 h). This study evaluated the impact of low‐temperature stress at the booting stage on grain yield, sucrose and starch accumulation, the activity and gene expression of starch‐synthesizing enzymes, and the dynamics of dry matter accumulation and translocation. The results showed that low‐temperature stress significantly decreased the number of grains per spike, thousand‐grain weight, and grain starch content. Notably, these reductions were exacerbated with lower temperatures and longer exposure times. The activities of key starch‐synthesizing enzymes, including invertase, starch phosphorylase 1, disproportionating enzyme 1, and ADP‐glucose pyrophosphorylase, decreased progressively with decreasing treatment temperature and increasing stress duration. Meanwhile, the relative expression levels of starch synthesis–related genes in grains ( AGPase , GBSSI , SSSI , SSSII , and Pho1 ) were significantly downregulated. After low‐temperature stress, non‐structural carbohydrate content in the stem sheaths decreased at heading but increased at maturity, indicating that the non‐structural carbohydrate translocation amount, translocation rate, and its contribution to grain weight were reduced. Similarly, dry matter allocation and proportion in grains at maturity were significantly decreased. In summary, low‐temperature stress during the booting stage suppresses starch synthesis initiation, starch accumulation and the translocation of assimilates to the grains, thereby significantly reducing wheat grain yield.