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Compared to other potential feedstocks such as sugarcane, sugar beet, maize, and watermelon, sweet sorghum possesses higher levels of directly fermentable reducing sugars within the culm and the ability to accumulate high biomass under low-input production systems. In addition, it is tolerant to drought and has more efficient utilization of solar radiation and nitrogen-based fertilizers than maize and sugar cane on marginal lands that are not optimal for food production. These collectively make sweet sorghum to be considered with huge potential as a biofuel crop. Novel phenotypes generated during plant domestication and continued crop improvements via artificial selection constitute the domestication syndrome ( Am. J. Bot., 101, 2014, 1711). Here, we draw an analogy and introduce the term the biofuel syndrome to refer to a suite of sweet sorghum traits, such as plant architecture (root, leave, and stem), flowering time and maturity as well as biomass bioconversion efficiency, that are associated with biofuel production and distinguish it from grain and forage sorghum traits. We discuss the biofuel syndrome amenable for targeted genetic modulation and what is currently known about the genetics and genomics of these traits as a potential route to optimize sweet sorghum for biofuel production. Continuous availability of sweet sorghum, transport and storing much mass and minimizing the postharvest loss of fermentable sugars are fundamental to exploiting sweet sorghum as a bioenergy crop. Due to the relatively short history of sweet sorghum breeding, we consider the development of ideotypes adapting to various phenological requirements to maximize the rapid deployment of sweet sorghum for biofuel production. [ABSTRACT FROM AUTHOR]

Key message: Eight morphological, biomass and biofuel traits were found with high broad-sense heritability and 18 significant QTLs discovered including one locus controlling the stem juice trait for sorghum grown in Denmark and China. Sweet sorghum with tall plant, fast maturation and high stem Brix content can be bred as a biofuel crop for Northern Europe. Abstract: Sweet sorghum ( Sorghum bicolour), a native tropical C crop, has attracted interest as a bioenergy crop in northern countries due to its juice-rich stem and high biomass production. Little is known about the traits important for its adaptation to high altitude climatic conditions and their genetic controls. Recombinant inbred lines derived from a cross between a sweet and a grain kaoliang sorghum were used in five field trials in Denmark and in China to identify the stability and genetic controls of morphological, biomass and biofuel traits during three consecutive summers with short duration, cool temperatures and long days. Eight out of 15 traits were found with high broad-sense heritability. Strong positive correlations between plant height and biomass traits were observed, while Brix and juice content were under different genetic controls. Using newly developed PAV (presence and absence variant) markers, 53 QTLs were detected, of which 18 were common for both countries, including a locus controlling stem juice (LOD score = 20.5, r = 37.5 %). In Denmark, the heading stage correlated significantly with biomass and morphology traits, and two significant maturity QTLs detected on chromosomes SBI01 and SBI02 co-localised with QTLs previously associated with early-stage chilling tolerance, suggesting that accelerating maturation might be a means of coping with low-temperature stress. Our results suggest that selection for tall and fast maturating sorghum plants combined with high Brix content represents a high potential for breeding bioenergy crop for Northern Europe. [ABSTRACT FROM AUTHOR]

Stress tolerance is a prerequisite for the success of biofuel production, which normally requires the use of marginal lands and nonfood biofuel feedstocks. Sorghum is known for its ability to withstand stress conditions, however, terminal stresses threaten its growth and development negatively impacting yield and sugar accumulation. It is crucial, therefore, that research aimed at developing sorghum resistance to stress factors should be pursued to expand the range of its growth to marginal and barren soils to meet the needs of a growing population, changing diets, and biofuel production. In this context, the leaf architectural trait of stay-green drought tolerance, in addition to salinity, cold, and aluminium toxicity and biotic stress tolerance and their genetic basis discussed in this review are expected to be available in future sweet sorghum ideotypes. Also highlighted is the key role of efficient management of farming systems, in particular the use of herbicides to control weeds, to ensure the sustainability of the sweet sorghum biomass productions. [ABSTRACT FROM AUTHOR]

In view of the effects of global climate changes on plant growth conditions in Europe, specific traits of sorghum make it a promising candidate for a future bioenergy crop in this region. The main limiting factor seems to be low tolerance of sorghum seedlings to chilling temperature and possible damage caused by cold spells in April and May. Seedlings of six sorghum genotypes (M71/SS79/Etian/Keller/Ji2731/Btx623) at third-leaf stage were exposed to five-day chilling (13/10 °C, day/night) in a growth chamber and then recovered for five days at control temperature (25/20 °C). Before and during cold treatment as well as during recovery, photosynthetic efficiency (PE), stomata status, and osmotic potential (OP) were measured for the first three leaves. As early as after 4 h of chilling there was a significant drop in PE measured as chlorophyll a fluorescence parameters, which decreased further in the course of chilling with significant genotypic differences. These were particularly pronounced after five- day recovery – M71 and Ji2731 recovered almost fully while SS79, Keller, and Btx623 – to a very limited degree. In these susceptible lines a significantly bigger drop in PE was observed for the first (oldest) leaf compared to the third (youngest) one, whereas in the tolerant lines these differences were not significant. All genotypes exhibited stomata closure as late as the fifth chilling or first recovery day and a drop in leaf OP after 52 h of chilling. The results show that the fast and significant drop in PE of sorghum seedlings under chilling conditions is caused by metabolic (non-stomatal) limitations during exposure to low temperature and by stomatal limitations after its cessation. The most pronounced genotypic differences in the reaction of sorghum seedlings to chilling stress were in the extent of osmotic adjustment and the capability for PE recovery after chilling treatment. Research supported by NCBR, Poland, ERA-NET-BIOENERGY-3/2013. [ABSTRACT FROM AUTHOR]