Abstract

Climate change imposes higher temperatures, especially at night. Its negative impact affects both yield components of wheat, grain number and average grain weight (TGW). Conversely, very low seeding rates may enhance TGW. However, the mechanistic basis of grain weight determination remains incompletely understood. Our study proposes a conceptual graphic-mathematical model based on the cell wall mechanical properties, distinguishing between plastic (irreversible) and elastic (reversible) growth to explain the contrasting effects of night warming and low seeding rate on individual grain weight (IGW). Two wheat cultivars (Bacanora and Kambara) were evaluated over two seasons in Valdivia, Chile. Treatments included night warming increase of 4°C applied at pre- or post-anthesis and contrasting seeding rates (370 vs. 44 seeds m-2). Grain volume, width, and height dynamics of the most contrasting grain positions within the spike were fitted using a tetra-linear broken-stick model to identify growth rates and durations of plastic and elastic phases. Under control conditions, Kambara exhibited up to 30% greater IGW than Bacanora. Within spikelets, grain position strongly influenced IGW (P > 0.05), with G2 consistently larger than other positions, whereas night warming reduced IGW up to 15% (P > 0.05). On the other hand, low seed rate increased (P > 0.05) IGW (5- 24%), particularly in distal grains. Similar effects were observed for grain volume and dimensions. Plastic growth was identified as the primary determinant of final grain weight and size, showing a strong positive association with IGW (R2 = 0.88). In contrast, elastic growth contributed minimally to final dimensions due to shrinkage near physiological maturity. Night warming shortened the grain-filling period, and low seed rate enhanced IGW (P > 0.05) by increasing the rate and duration of the plastic growth. This study demonstrates the negative impact of night temperature on TGW and IGW, even at preanthesis, showing different sensitivity among grain positions within the spike, and the improvement of these traits by the very low seed rate. In addition, plastic growth, defined as a primary and irreversible structural expansion, is associated with dry matter deposition, whereas elastic growth represents reversible fluctuations primarily driven by water dynamics.