High temperature limits the development of young ears of waxy maize by altering photosynthesis distribution and sugar metabolism

• HT during the V12-R1 stage damaged ear and silk morphology, causing yield loss. • Heat-tolerant genotype maintained peduncle vascular integrity and transport capacity. • Heat-tolerant ear exhibited stronger antioxidant capacity and redox homeostasis. • Heat-tolerant ear showed stable sugar uptake and conversion capacity. High temperature (HT)-induced yield loss has become an important factor limiting maize production. As the primary site of grain development, the ear critically influences yield formation; thus, understanding the impacts of HT on ear development is essential. This study aimed to elucidate the effects of HT during the V12-R1 stage (V12, 12-leaf stage; R1, silking) on ear development and its underlying physiological mechanisms in two waxy maize inbred lines. Results showed that the W27 (heat-sensitive line) suffered severe damage to the peduncle and silk, accompanied by reduced ear volume, fresh weight, stigmatic hair density, and vascular bundle integrity, which collectively led to a 25.6% yield decrease. Conversely, the W13 (heat-tolerant line) maintained structural integrity, with only a 9.4% yield reduction. Both genotypes exhibited a decline in photosynthetic efficiency and disordered photosynthetic enzyme activities under HT. Peduncle transport capacity was impaired in W27 (reduced area, increased sugar accumulation) but remained intact in W13. The ears of W27 were severely damaged under HT, showing excessive peroxide accumulation and increased levels of IAA, ABA, and ETH, whereas these changes were minimal in W13. Transcriptomics revealed that HT activated starch and sucrose metabolism pathway and response to heat term in ears. This activation suppressed starch synthase activity in W27, thereby inhibiting starch synthesis. Conversely, it enhanced sucrose phosphate synthase, sucrose synthase, and cell wall invertase activities in W13, consequently promoting sucrose conversion. Comprehensive analysis demonstrated that the thermotolerance of W13 was attributed to maintained redox homeostasis and enhanced sugar conversion and utilization in the young ear.