Source-sink foliar boron (10B) mobility modulated by carbohydrate synthesis and confers antioxidative defense in sugar beet (Beta vulgaris L.) under water deficit conditions

• Deficiency of B augmented oxidative stress and circumvented foliar boron ( 10 B) transport failure from source to sink tissue. • Sufficient B facilitated the mobility of foliar-applied 10 B by increasing carbohydrate synthesis from source to sink leaves and roots under water deficit conditions. • Thereby enhancing sink strength and promoting antioxidative defense against ROS under water deficit conditions. Drought is a global abiotic stressor which affects boron (B) availability due to reduced mobility. This is particularly affecting B-demanding crops such as sugar beet. However, linking B translocation dynamics and drought-mitigating factors is largely unknown. Therefore, we hypothesize that foliar B application facilitates B long-distance transport, triggering, e.g., osmoregulation and the antioxidant defense system. Plants were grown under sufficient (2.5 mg kg −1 ) and deficient (0.25 mg kg −1 ) conditions of 11 B-boric acid for 28 days after sowing, then 24 days of ample and limited water, with or without foliar 10 B-boric acid of 300 mg L −1 . In this study, it was observed that foliar-applied 10 B translocates from source to sink tissue and increases mobility at sufficient 11 B supply under water-deficit conditions. Conversely, B-induced accumulation of sucrose and osmolality ( ψ s) indicates lower foliar 10 B transport failure from source to sink tissue. Further, higher levels of H 2 O 2 and MDA under B deficiency and limited water supply cause a reduction of dry matter. However, foliar-applied 10 B at sufficient 11 B resulted in the upregulation of the antioxidant defense systems, as reflected by enzyme activities, e.g., CAT, and AsA-GSH cycle enzymes. In contrast, it leads to a decrease in SOD activity, indicating the cellular redox balance. Our findings highlight and provide unprecedented insights into understanding the dynamics of B translocation from source to sink using B tracers ( 10 B and 11 B), where B status delineates the distribution and mobility of foliar B. This suggests protection against ROS and coordination of carbohydrate metabolism while mitigating antioxidative stress.