Heat stress during the milky stage reshapes phenology, assimilate partitioning, and yield formation in rice cultivars with contrasting heat tolerance

Heat stress during the grain-filling period of rice cultivars leads to a reduction in both yield quality and quantity. This research examined the effect of milky-stage heat stress (milky HT; 41/28°C day/night, for 7 days) on phenology, partitioning, and yield in 12 rice cultivars compared to standard heat-tolerant (N22 and Dular) and heat-sensitive (IR64) cultivars. Milky HT caused a shortened phenology from milky to physiological maturity in RD41, N22, and Dular, whereas others exhibited prolonged phenology. Milky HT influenced partitioning, with RD41 increasing dry weight in all parts, and with PSL2 rapidly assimilating translocation to grains. However, CN1 and RD49 experienced rapid dry weight loss across all parts. Additionally, milky HT caused a significant reduction in all yield components and overall yield in RD63, while N22 maintained its parameters. Phenology shift (from milky to physiological maturity and from dough to physiological maturity) and seed set were considered key indicators associated with heat tolerance. Based on these traits, rice cultivars were classified into four groups: heat-tolerant (N22, Dular, and SPT1), moderate heat-tolerant (PSL2, RD61, Rice berry, and RD41), moderate heat-sensitive (CN1), and heat-sensitive (IR64, PTT1, RD29, RD31, RD49, RD57, and RD63). These findings revealed that heat-tolerant rice under milky HT is associated with different phenological and assimilate allocation strategies, in addition to clarifying useful traits and rice seed resources for a heat-tolerance breeding program.