Assessment of cold tolerance in early field emergence in sweet, fiber, and grain sorghums [Sorghum bicolor (L.) Moench].

INTRODUCTION: Constraints to the adoption of early sowings in biomass sorghum were found in its sensitivity to cold stress during germination. METHODS: A preliminary test was conducted in the laboratory on seven sorghums among sweet, fiber, and grain types, to assess their cold tolerance during germination. Sorghums were germinated at 8 °C and 25 °C (control). At the end of the test, final germination percentage (FG), mean germination time (MGT), and germination index (GI) were calculated. Thereafter, a first-year (2009) field study was conducted under the semi-arid climate of Southern Italy, using 10 cultivars of sorghum, including those from the laboratory test, to assess their response to cold during germination and subsequent seedling emergence. Five sowings were considered: 15 and 30 March (S1 and S2, respectively), 15 and 30 April (S3 and S4, respectively), and 15 May (S5, control). Field emergence was recorded from sowing up to approximately 5 weeks after sowing. At harvest, final plant density, number of leaves per plant, plant height and weight, and biomass yield were measured. A second-year (2010) field experiment was conducted in the same experimental site, using four sorghums among those that performed best in the previous year. Sorghums were sown on 16 March (S1) and 15 May (S5). RESULTS: FG as assessed in the laboratory was >90% while, at 8 °C, it dropped to 54.9%. At 8 °C, the most tolerant cultivar, in terms of FG, MGT, and GI, was "H128". Results from the first-year experiment revealed a low seedling emergence (<17%) in all cultivars in S1 (mid-March). Field emergence was positively correlated with maximum air temperature recorded during the 10-day period following sowing, in all cultivars except "Kaoliang" (control). Very low field emergence in early sowing (S1) led to poor plant density in all cultivars except "NK180", where it matched the predicted value (11 plants m-2). Very low temperatures in S1 decreased plant productivity, which was maintained at <8 t ha-1 in most cultivars. Biomass yield in S2 (late March) matched that measured in S5 (mid-May). Warmer temperatures (+4-5 °C) in S1 in the second year induced greater field emergence and dry biomass than those measured in the first year for the same sowing times. DISCUSSION: The availability of cold-tolerant cultivars of sorghum during the early stage of germination and seedling growth may help breeders in their genetic studies for the development of new cultivars that combine cold resilience and high productivity, and help farmers to predict sowing rate based on the expected temperature conditions. In this regard, the sorghum "NK180", resilient to cold, although low-yielding even in regular sowings, may represent a valuable genetic source for breeding programs to enhance chilling tolerance in the field during the very early stages of growth, among the existing cultivars of sorghum. The cultivar "H128", also resistant to cold, showed more stable production over sowing times, thus becoming agronomically more suitable. Although seed germination in the laboratory may not be considered a reliable predictor of successful field emergence under very limiting environmental conditions, it can help in a first screening within the available germplasm of sorghum for cold tolerance.