Elevated Respiration-to-Photosynthesis Ratio Under High Nighttime Temperature Constrains Mungbean Yield and Identifies Genotypic Targets for Climate Adaptation in Subtropical Farming Systems

Mungbean is a nitrogen-fixing break crop in subtropical broadacre cereal rotation, yet how high nighttime temperatures (HNTs) threaten its agronomic contributions remain poorly characterized. A controlled environment (CE) experiment (low nighttime temperature, LNT: 20 °C vs. high nighttime temperature, HNT: 25 °C) and a two-year staggered-sowing field trial at Rockhampton, Queensland, evaluated HNT impacts on two genotypes, Jade AU and Green Dragon. In the CE experiment, HNT produced a strong, time-dependent increase in the respiration-to-photosynthesis (Rn/A) ratio, evident within two days of treatment imposition, compressed the pod-filling duration by 15%, and reduced grain yield by 21% through fewer mature pods without impairing individual grain filling. The sowing dates generated a contrasting combination of nighttime heat, daytime heat, solar radiation, and seasonal conditions. The harvest index was lower across the 2024 environments than the 2025 environments, but these year-level contrasts reflect co-varying thermal and radiation conditions rather than isolated HNT and High daytime temperature (HDT) environments. Although unadjusted Rn/A–grain yield associations differed between years, split-plot mixed models showed that Rn/A did not explain additional grain yield variation after accounting for sowing date, genotype, and experimental blocking structure. Therefore, Rn/A remains a candidate physiological response indicator, requiring further validation across broader germplasms and independently replicated environments. Green Dragon maintained a larger pod pool and showed less observed Rn/A than Jade AU in selected field environments. These traits identify Green Dragon as a promising source of reproductive resilience for further multi-environment evaluation. To align the reproductive phase with fewer warmer nights, sowing date management is the most accessible strategy for sustaining mungbean yield in subtropical broadacre cereal rotation systems under asymmetric warming.