Evaluating the response of sugar beet hybrids to drought stress using stress tolerance index and hierarchical clustering analysis

Introduction
Sugar beet (Beta vulgaris L.) is the world’s second most important source of sucrose and plays a vital role in food security and agricultural economics. However, its production is constrained by several factors, among which drought stress caused by water deficit is one of the most important. Water scarcity induces physiological responses such as stomatal closure and reduced photosynthetic activity, ultimately leading to a substantial reductions in root yield. Given the increasing temperatures and declining water resources in sugar beet growing regions, effective management and breeding strategies are essential to mitigate the adverse effects of drought. One promising approach is the development and cultivation of drought-tolerant cultivars and hybrids that maintain high water-use efficiency and satisfactory economic performance under limited irrigation. Therefore, this study aimed to identify promising sugar beet hybrids using the Stress Tolerance Index (STI) and cluster analysis to select hybrids with superior sugar yield and water use efficiency under both normal and water-deficit c conditions.
Materials and Methods
The experiment was conducted over two growing seasons (2016 and 2017) at the Sugar Beet Research Station in Karaj, Iran, affiliated with the Sugar Beet Seed Institute. The experiment was arranged as a split-plot based on a randomized complete block design with four replications. Irrigation regime was assigned to the main plots at two levels: normal irrigation (after 90 mm evaporation from a class A pan) and restricted irrigation (after 180 mm evaporation). In the first year, the subplots consisted of 57 experimental test-cross hybrids and seven commercial check cultivars. Based on the highest STI values obtained in the first year, 16 superior hybrids were selected and evaluated in the second year together with four superior check cultivars. Measured traits included root yield, sugar content, sugar yield, root impurities (Na⁺, K⁺, and α-amino nitrogen), and irrigation water use efficiency (IWUE). In the second year, total chlorophyll and carotenoid contents were also determined. The Stress Tolerance Index (STI) was calculated according to the method proposed by Fernandez, and cluster analysis was performed to classify the genotypes according to drought tolerance. Data analysis was carried out using SAS and Minitab statistical software.
Results and Discussion
Water deficit stress significantly reduced root yield, with average reductions of 41% in the first year and 20% in the second year. The greater reduction observed during the first year was likely associated with the genetic diversity of the tested hybrids and the occurrence of unfavorable environmental conditions including pest and disease pressure. Among the evaluated hybrids, (7112KWS) S1-100 and (7112KWS) S1-70  showed competitive root yields of 70.84 t ha⁻¹ and 69.71 t ha⁻¹, respectively comparable to those of the check cultivars. The main effect of genotype was highly significant ( P < 0.01) for all major traits, including root yield, sugar content, and sugar yield. Sugar content increased under water-deficit conditions compared with normal irrigation by approximately 20% in the first year and 6% in the second year, although this increase was statistically significant only in the first year. This response may be attributed to reduced root diameter and enhanced sugar accumulation within storage tissues. A significant irrigation × genotype interaction for sugar content was detected only during the first year, indicating differences among genotypes in their responses to irrigation regimes. Sugar yield was significantly affected by both irrigation treatment and genotype (P < 0.01) in both years, whereas their interaction was not significant. Under restricted irrigation, sugar yield decreased by 29% in the first year and 15% in the second year. In the second year, the check cultivars Mandarin (10.88 t ha⁻¹) and IR7 (9.49 t ha⁻¹) produced the highest sugar yields, whereas hybrid (7112KWS) S1-03 recorded the lowest value (6.70 t ha⁻¹). Among the experimental hybrids, (7112KWS) S1-100 achieved the highest sugar yield (9.38 t ha⁻¹). The irrigation × genotype interaction was not significant for root yield and sugar yield, indicating relatively consistent genotype responses across irrigation regimes for these traits. However, significant interactions were observed for sodium and potassium concentrations in the first year. Under water-deficit conditions, root sodium concentration decreased by 27%, whereas potassium concentration increased by 3%, suggesting osmotic adjustment as an adaptive response to drought stress. Irrigation water use efficiency increased by an average of 22% under restricted irrigation across the two years, reflecting improved water productivity under limited water supply. During the second year, Mandarin (1.06 kg m⁻³) and Paya (0.93 kg m⁻³) exhibited the highest IWUE values, whereas hybrid (7112KWS) S1-03 showed the lowest value (0.65 kg m⁻³). Among the experimental hybrids, (7112KWS) S1-100 (0.90 kg m⁻³) and (7112KWS) S1-70 (0.82 kg m⁻³) exhibited high IWUE values comparable to those of  the superior check cultivars. The approximately 35% reduction in irrigation water applied under restricted irrigation contributed substantially to the observed increase in IWUE. Based on STI values in the second year, the check cultivars Paya (1.40) and Mandarin (1.01) exhibited the greatest drought tolerance, whereas Jolgeh (0.67) had the lowest STI among the checks. Among the experimental hybrids, (7112KWS) S1-100 (0.97) and (7112KWS) S1-70 (0.96) recorded the highest STI values. Cluster analysis based on root yield, sugar yield, and IWUE classified hybrids into drought-tolerant and susceptible groups. Notably, only hybrids (7112KWS) S1-100 and (7112KWS) S1-70 were consistently classified as drought tolerant in both years. Their STI values were also higher than the overall mean, demonstrating strong agreement between STI and cluster analysis as complementary two selection methods. Furthermore, these hybrids exhibited competitive root and sugar yields relative to the superior commercial cultivars Mandarin and Paya while maintaining high irrigation water use efficiency (0.90 and 0.82 kg m⁻³, respectively).
Conclusion
Results demonstrated that although water-deficit stress substantially reduced root yield, combining the Stress Tolerance Index with cluster analysis effectively identified drought-tolerant sugar beet hybrids capable of maintaining high irrigation water use efficiency and acceptable sugar yield under limited irrigation. Among the evaluated materials, hybrids (7112KWS) S1-100 and (7112KWS) S1-70 consistently exhibited superior agronomic performance, high drought tolerance, and competitive productivity compared with commercial cultivars. These hybrids represent promising genetic resources for future breeding programs and for cultivation in drought-prone environments. Therefore, the combined application of STI and cluster analysis is recommended as a reliable and effective strategy for selecting superior sugar beet genotypes under water-limited conditions.