Sorghum (Sorghum bicolor L. Moench) is a promising novel crop for Central Europe. However, enhancements in cold tolerance and early maturity are essential for a successful adaptation to cooler climates. We scored a broad sorghum diversity set (n = 338) for early chilling tolerance, high-latitude adaptation, and bioenergy related agronomical traits in multi-environment trials. Our results show a high phenotypic variation and medium to high heritabilities for most traits, indicating that a robust breeding progress is feasible. Several public accessions with a good adaptation to cooler climates were identified, which can serve as valuable base material for sorghum breeding in temperate areas. Genome-wide association studies reveal a polygenic (quantitative) character for most of the traits, confirming previous studies. Hence, for practical breeding, it will be difficult to conduct efficient marker-assisted selection for temperate-adaptation traits in genetically diverse material.

The sensitivity of sorghum to pre-flowering cold stress, resulting in reduced pollen viability and poor seed set, is a major constraint for expanding growing areas into higher altitudes and latitudes. Nevertheless, compared to juvenile cold tolerance, reproductive cold tolerance in sorghum has received much less attention so far, and very little is known about its inheritance in F₁-hybrids. We have composed a representative factorial (n = 49 experimental F₁-hybrids) for a comprehensive study on heterosis and combining ability for crucial tolerance traits as spikelet fertility (panicle harvest index), seed yield and pollen viability, using field trials in stress- and control environments in Germany and Mexico as well as climate chamber experiments. Our results indicate a heterotic and rather dominant inheritance of reproductive cold tolerance in sorghum, with strong effects of female general combining ability (GCA) on F₁-hybrid performance in our material. These findings, together with the comparatively low contribution of specific combining ability (SCA) effects and high heritability estimates, suggest that robust and efficient enhancement of reproductive cold tolerance is feasible via hybrid breeding.

Enhancements in reproductive cold tolerance of sorghum are essential to expand growing areas into both high-latitude temperate areas and tropical high-altitude environments. Here we present first insights into the genetic architecture of this trait via genome-wide association studies in a broad genetic diversity set (n = 330) phenotyped in multi-location field trials including high-altitude tropical (Mexico) and high-latitude temperate (Germany) environments. We observed a high degree of phenotypic variation and identified several novel, temperate-adapted accessions with superior and environmentally stable cold tolerance. Good heritability indicates strong potential for implementation of reproductive cold tolerance in breeding. Although the trait was found to be strongly quantitative, promising genomic regions with multiple-trait associations were found, including hotspots on chromosomes 3 and 10 which contain candidate genes implicated in different developmental and survival processes under abiotic stress conditions.

Understanding sorghum response to water stress at different developmental stages is important for developing sorghum varieties with improved tolerance to drought. This study set out to finely characterize key agro-physiological responses and water use of sorghum subjected to different drought scenarios. A greenhouse experiment was conducted using a DroughtSpotter facility that enables real-time quantification of water use by gravimetric tracking. Four different water treatments were assessed: (i) early vegetative drought stress (DS1), (ii) DS1 plus recovery (DS1R), (iii) late vegetative drought stress (DS2), and (iv) well-watered (WW). Plant pheno-morphology and yield data were recorded. Leaf fluorescence and photosynthesis were continuously recorded throughout the experiment. Our results showed that green leaf number and plant leaf area were more affected by DS2 than DS1 and delayed plant flowering. Nevertheless, plants in DS2 were taller and produced higher stem and leaf biomass compared to DS1. No significant difference was recorded in grain yield between DS1 and DS2 but were outperformed by DS1R and WW. The transpiration and photosynthetic rates were shown to decrease at the same time as the stomatal conductance. This can therefore be assimilated to a stomatal down-regulation limiting CO₂ uptake. However, the increase in intercellular CO₂ concentration is likely to indicate the presence of CO₂ in the substomatal cavity that was not conveyed to the carboxylation sites. This suggests a non-stomatal limitation of the photosynthesis. Moreover, the plants recovered quite well from DS1, and this was more prominent for physiological parameters than morphological ones. Globally, water use efficiency (WUE) for DS2 was higher compared to WW and DS1 treatments, confirming the growing point differentiation as a critical stage where drought stress should be avoided to ensure yield and better WUE. Adaptation responses were related to the reduction of transpiration through plant leaf area reduction, the reduction of stomatal conductance, and the increase of intercellular CO₂ limiting photosynthesis. Further studies focusing on the biomarkers of stress and transcriptomic analyses are needed to provide further insight into the drought adaptation mechanisms of this line.

Background: The ability to withstand periods of light frost during juvenile development is an important adaptive trait for sorghum (Sorghum bicolor L. Moench) production in high-latitude temperate areas. Frost tolerance would allow for earlier sowing and prolongation of growing season and maturity, thereby increasing yield potential. Using a broad diversity set (n = 285), a field cold emergence trial and two semi-controlled frost survival experiments were conducted to investigate the phenotypic diversity and genetic architecture of frost survival traits. Results: The results show high phenotypic diversity for frost survival, including superior genotypes combining high frost tolerance and cold emergence levels via quantitative trait inheritance. Tolerance to temperatures above and below 0 °C was not correlated, suggesting separate genetic control. The found QTL do not overlap for the traits of frost survival and plant emergence under cold sowing conditions. Genome-wide association studies revealed four significant associations for frost survival, where the QTLs are located on chromosomes Sb02, Sb07, and Sb08. For the trait of emergence under cold sowing conditions, a total of 12 QTLs were found on chromosomes Sb01, Sb03, Sb04, Sb05, and Sb06. Conclusion: A good overview regarding the suitability of the tested genotypes for early sowing was obtained due to the large diversity investigated in this study, with surprisingly high survival rates of numerous genotypes after frost stress. These results will contribute to the breeding of cultivars with improved cold and frost tolerance, facilitating better adaptation for production in high-latitude temperate areas, as well as earlier sowing to avoid summer drought.

Abundance and diversity of pollinating insects are decreasing. Intensification of agricultural bioenergy production is presumed to accelerate the decline of pollinators. Sorghum (Sorghum bicolor L. Moench) is a promising bioenergy crop. Enhanced dual-purpose type cultivars have been developed and tested for suitability for bioenergy cropping in Germany. Sorghum is assumed to be a nutritional resource for pollen-collecting insects. To evaluate this assumption, we studied the foraging strategy of A. mellifera colonies, which were migrated to sorghum fields in Germany. The bee hives were equipped with bottom fixed pollen traps. The pollen loads of the colonies contained variable shares of sorghum pollen ranging between approx. 10% and 50% (weight/weight). Sorghum pollen occurred frequently in more than 50% of all pollen samples. Experimental mini colonies were placed in plots which were grown with two varieties of sorghum, phacelia (Phacelia tanacetifolia), maize (Zea mays) and a control plot without any vegetation. All plots were encased with flight tents. Significant effects of the crop were found for the productivity parameters brood rearing and pollen collection. The sorghum and maize variants performed significantly better than the controls but significantly poorer than phacelia (p < 0.05). The parameters number of dead bees and colony sizes were not affected by the crops (p < 0.05). Pollen of sorghum is a valuable food for bees which supports nursing of bee brood, but its availability proved to be inferior to phacelia as the pollen shedding of sorghum lasted a considerably shorter time. Pollen collection by honeybees did not negatively affect seed yield of sorghum in any case. Under unfavourable weather conditions, flower visiting bees enhanced seed yield of sorghum.