Evaluating root yield and growth indices of sugar beet under transplanting and direct sowing in cold semi-arid regions

Introduction
Sugar beet (Beta vulgaris L.) is one of the most important field crops worldwide and in Iran, playing a crucial role not only in sugar production but also in maintaining sustainable crop rotations and improving soil properties. In recent decades, increasing water scarcity and climate change have intensified the need for improving crop management strategies in sugar beet cultivation systems. One of the promising agronomic approaches to improve crop establishment, shorten the time required to achieve full canopy cover, and enhance water productivity is transplanting instead of direct-seeded. From a physiological perspective, growth analysis is a powerful tool for studying biomass accumulation and plant development dynamics during the growing season. Growth indices such as total dry weight (TDW) and leaf area index (LAI) are widely used indicators for evaluating crop growth and physiological status. Therefore, the objective of the present study was to evaluate root yield and growth indices of sugar beet under transplanting and direct-seeded systems in three cold semi‑arid regions of Iran.
 
Materials and Methods
The current research aimed to evaluate the effects of two transplanting ages (35 and 45 days) and direct-seeded sowing on growth indices and root yield of sugar beet across three cold semi-arid regions: Ardebil, Tabriz, and Urmia. Crop growth was simulated using the SUCROS model based on long-term climatic data (1980-2020) and soil physical-hydrological characteristics. Growth indices including TDW, LAI, RGR, and CGR were derived using functional growth models and compared across planting systems. To determine the initial physiological conditions of seedlings, the model was first run independently under potential growth conditions to estimate the initial dry weight of different plant organs (leaf blades, petioles, roots, and storage organs) as well as initial LAI values. These simulated values were then used as initial parameters in the main model runs. For comparison, direct-seeded was also simulated as the conventional planting method. All three planting systems (direct-seeded, 35‑day transplanting, and 45‑day transplanting) were simulated using the common planting dates of each region. Irrigation scheduling was applied when soil moisture declined to approximately 70% of field capacity according to FAO irrigation management recommendations. Other agronomic parameters such as plant density and row spacing were kept constant across all simulations. To analyze growth indices, TDW and LAI were recorded throughout the growing season. The logarithm of TDW was fitted using a third‑order polynomial model, from which RGR was calculated as the derivative of ln (TDW). CGR was then calculated as the product of RGR and TDW. LAI dynamics were described using a logistic sigmoid model. Model fitting was performed using least‑squares regression in the R software environment. Root yield was analyzed as a long‑term time series for each location and planting system. Linear regression was applied to evaluate annual trends in root yield and to determine statistical significance of changes over time.
Results
Transplanting markedly improved early crop establishment. In transplant treatments, LAI peaked around 120 days after planting at 7.4 in Ardebil, 6.6 in Tabriz, and 7.05 in Urmia, whereas corresponding values under direct-seeded sowing were approximately 6.1, 3.3, and 3.8, respectively. Maximum CGR in the 45-day transplanting treatment reached 19.2 g m^-2 day^-1 in Ardebil, 21 g m^-2 day^-1 in Tabriz, and 22.3 g m^-2 day^-1 in Urmia. In contrast, RGR was naturally lower in transplant treatments due to higher initial dry weight and elevated respiratory costs; nevertheless, this reduction did not hinder the superior final dry matter accumulation of transplants. TDW was also higher under 45-day transplanting, reaching 1902 g m^-2 in Ardabil, 2001 g m^-2 in Tabriz, and about 2471 g m^-2 in Urmia by day 170, all substantially exceeding the values under direct sowing. Root yield followed the same trend: the highest root yield (108 t ha^-1) occurred in the 45-day transplanting treatment in Urmia, whereas the lowest (68 t ha^-1) was observed under direct sowing in Tabriz. The long‑term trend analysis showed that annual yield fluctuations in Ardebil were not statistically significant (p> 0.14), with a maximum annual slope of 0.15 t ha^-1. In contrast, all cropping systems in Tabriz and Urmia exhibited significantly declining yield trends (p< 0.001). The estimated annual rate of decline ranged from 0.20 to 0.27 t ha^-1 in Tabriz and approximately 0.55 to 0.60 t ha^-1 in Urmia. These findings are consistent with previous studies indicating that transplanting improves canopy development, radiation interception, and biomass accumulation in sugar beet and other crops. Similarly, it was reported that transplanting 35–40‑day‑old seedlings resulted in the highest root yields in sugar beet production systems. Furthermore, transplanting has been shown to improve water productivity and reduce irrigation requirements compared with direct.
Conclusion
Overall, the results of this study demonstrate that transplanting provides clear advantages for sugar beet growth and productivity in cold semi‑arid regions. Transplanted plants exhibited earlier canopy development, higher LAI, greater crop growth rate, and increased total dry matter accumulation compared with direct-seeded. Although relative growth rate was slightly lower in transplanted plants due to higher initial biomass, this did not negatively affect final root yield. Among the studied regions, Urmia showed the highest yield potential due to more favorable climatic conditions, whereas Tabriz showed lower productivity. Overall, transplanting 35–45‑day‑old seedlings increased root yield by approximately 8–11% compared with direct-seeded. Therefore, transplanting can be considered an effective agronomic strategy for improving sugar beet establishment, growth performance, and yield stability, particularly in environments where early‑season conditions limit successful crop establishment. Future studies should evaluate transplanting performance under a wider range of climatic conditions to further improve sugar beet production systems.