Screening for cold tolerance during germination within sweet and fiber sorghums [sorghum bicolor (L.) moench] for energy biomass

Within the project “BIOSEA” funded by the Italian Ministry of Agriculture and Forestry, a preliminary laboratory test was conducted to assess the variability for cold tolerance during germination in 30 cultivars of biomass sorghum, among fiber and sweet types. Seed germination (%) and mean germination time (MGT) were examined at seven constant temperatures (from 8◦C to 35◦C) and base temperature (Tb) and thermal time (θT) for 50% germination were calculated. A wide genetic diversity in the germination response of sorghum was ascertained at 8◦C (CV 45%) and 10◦C (CV 25.4%). At 8◦C, in cultivars of ‘Padana 4’, ‘PR811F’, ‘PSE24213’, ‘PR849’ and ‘Zerberus’, seed germination exceeded 80%. Seeds of ‘Zerberus’ were also the fastest, requiring less than 13 days for final germination at this low temperature. Great differences were found in Tb and θT among cultivars. Tb varied between 7.44◦C (‘PR811F’) and 13.48◦C (‘Nectar’). Thermal time (θT) was, on average, 24.09◦Cd−1, and ranged between 16.62 (‘Nectar’) and 33.42◦Cd−1 (‘PSE24213’). The best combination of the two germination parameters (i.e., low Tb and θT) corresponded to ‘Zerberus’, ‘Sucrosorgo 506’, ‘Jumbo’ and ‘PR811F’. Accordingly, these cultivars are more tolerant to cold stress during germination and, thus, more adapt to early spring sowings in Mediterranean areas (March-April). Cultivars ‘PR811F’ (fiber type) and ‘Sucrosorgo 506’ (sweet type) also combine high cold tolerance with good productivity in terms of final dry biomass, as assessed in open-field conditions (late spring sowing). The genetic variation in the germination response to a low temperature is useful for the identification of genotypes of sorghum suitable to early sowings in semi-arid areas. Selection within existing cultivars for cold tolerance during germination may also contribute to the expansion of biomass sorghum into cooler cultivation areas, such as those of Northern Europe, which are less suitable to this warm season crop.