Late Harvest: Freezing Temperatures Reduce the Root Yield and Sugar Content of Beta vulgaris L.

Sugar beet (Beta vulgaris L.) is the main source of white sugar in northern China, and an optimal harvesting time is key for maximising its yield and sugar content. The goal of this study was to investigate the effect of gradually extending harvest times on the growth, physiological characteristics, yield and sucrose accumulation in sugar beet and identify the optimal harvesting time. We conducted a 3-year experiment across different harvesting times from 23 Sep to 28 Oct (harvest every 7 days) to examine the effects of different magnitudes of temperature reduction. The yield and sugar content were the highest in time III (the daily mean/minimum temperature: 9.5°C/5°C) in 2020, time III (the daily mean/minimum temperature: 8.2°C/2°C) and IV (the daily mean/minimum temperature: 10.1°C/2°C) in 2021 and time IV (the daily mean/minimum temperature: 10.5°C/5°C) and V (the daily mean/minimum temperature: 7°C/−1°C) in 2022. The yield and sugar content were low at an early harvest, as the biomass and sucrose accumulation process was not complete. However, the decrease in temperature (daily minimum temperature below 0°C) during late harvest leads to a decrease in the yield and sugar content because of the decreased sucrose accumulation of source leaves, increased sucrose decomposition and the poor transport capacity of phloem in the stem. Therefore, the optimal harvesting time for sugar beet in northern China depends on temperature conditions. It is optimal when (1) the daily minimum temperature gradually decreases to 0°C but not lower than 0°C and when (2) the daily mean temperature is approximately 10°C. This work will help sugar beet producers harvest high-quality crops and reduce unnecessary losses in northern China.