Adaptability and production potential of sorghum in cool regions: heterosis analysis of parameters of light response and CO2 response curves.

Improving photosynthetic performance may contribute to the productivity and adaptation of hybrid sorghum in cool regions. This study aimed to characterize stage-specific photosynthetic performance, heterosis, combining ability, genetic effects, and yield performance in sorghum hybrids and their parental lines. Six maternal lines, six restorer lines, and 36 F1 hybrids generated using a North Carolina II mating design were evaluated in field experiments conducted in 2021 and 2022. Leaf light- and CO2-response curves were measured at the jointing, flowering, and maturity stages. Photosynthetic parameters, heterosis, general combining ability (GCA), specific combining ability (SCA), variance components, heritability, heterotic grouping, and grain yield were analyzed. Photosynthetic performance of the F1 hybrids varied among developmental stages. At the jointing and flowering stages, the hybrids generally exhibited higher light- or CO2-saturated photosynthetic capacity, Rubisco carboxylation capacity, and electron-transport capacity than one or both parental groups, together with relatively low respiratory or photorespiratory carbon loss. However, apparent quantum efficiency (AQE) was lower in the hybrids at jointing, and several photosynthetic advantages were not maintained at maturity. Carboxylation efficiency (CE), maximum Rubisco carboxylation rate (Vcmax), and maximum electron-transport rate (Jmax) showed positive average mid-parent and high-parent heterosis, whereas photorespiration rate (Rp) and CO2 compensation point (CCP) showed negative average heterosis. TAM428A, I15A, L407A, JiR107, T40, and Ji318R showed complementary GCA advantages, while Ji2055A × T40, TAM428A × T40, TAM428A × 0-30, and I15A × 0-30 exhibited favorable SCA effects for multiple traits. Except for dark respiration rate (Rd), SCA accounted for more than 70% of the total combining-ability variance for all response parameters. Broad-sense heritability exceeded 89%, whereas narrow-sense heritability was generally low, indicating a major contribution of non-additive genetic effects. The mean grain yield of the F1 hybrids was 50.54% and 54.10% higher than that of the restorer lines in 2021 and 2022, respectively. The results demonstrate that photosynthetic advantages in hybrid sorghum are strongly dependent on developmental stage and are largely influenced by non-additive genetic effects. Integrating stage-specific photosynthetic evaluation, parental GCA, cross-specific SCA, and multi-environment yield testing may improve the identification and selection of promising sorghum parents and hybrids for cool-region production.