Effects of sowing date on bolting and frost damage to autumn-sown sugar beet (Beta vulgaris L.) in temperate regions

Sugar beet is mostly cultivated in relatively cool regions of the temperate zones by sowing in spring, but in some Mediterranean areas with mild winters, the crop is sown in autumn. Due to global warming, autumn cultivation of sugar beet is gradually extending towards new areas that are still characterized by relatively cold winters. Seedling loss and bolting are two factors limiting the adoption of autumn sugar beet cultivation in new areas. The objectives of this study were to determine the effects of sowing date on (i) duration and rate of field emergence, (ii) phenological stages of sugar beet during early growth, (iii) quantitative traits of seedlings and (iv) bolting occurrence as well as frost killing of autumn-sown sugar beet. Field experiments were conducted in a randomized complete block design to determine the appropriate sowing date for autumn-sown sugar beet in 2017/18 and 2018/19 in the Karaj and Mashhad regions of Iran, which are both characterized by relatively cold winters. The experiment was conducted with a bolting-resistant cultivar and six sowing date treatments. The results showed that to reach plant growth stages of cotyledon, 2, 4, 6, 8, 10, 12, 14 and 16 leaves, 163, 200, 321, 418, 500, 600, 639, 700 and 757 growing degree days (GDD), respectively, were required. The average duration and speed of seedling emergence increased and decreased, respectively, with delay in sowing. The results suggest adjusting the sowing date of winter sugar beet so that when temperatures effective for bolting (6-8 °C) occur, the plant has already received about 300 to 400 GDD. At this time, the growth stage and the largest root diameter of sugar beet are approximately 4-6 leaves and 0.11-0.27 cm, respectively. After 14- to 16-leaf stage (≥700-750 GDD), the percentage of killed plants due to low temperatures were negligible. Although, the risk for frost losses is higher at 4- to 6-leaf stage (300-400 GDD), accepting higher losses is justified by a lower probability of bolting.