Source-sink foliar boron (10B) mobility modulated by carbohydrate synthesis and confers antioxidative defense in sugar beet (Beta vulgaris L.) 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 ¹¹B-boric acid for 28 days after sowing, then 24 days of ample and limited water, with or without foliar ¹⁰B-boric acid of 300 mg L^-1. In this study, it was observed that foliar-applied ¹⁰B translocates from source to sink tissue and increases mobility at sufficient ¹¹B supply under water-deficit conditions. Conversely, B-induced accumulation of sucrose and osmolality (ψs) indicates lower foliar ¹⁰B transport failure from source to sink tissue. Further, higher levels of H₂O₂ and MDA under B deficiency and limited water supply cause a reduction of dry matter. However, foliar-applied ¹⁰B at sufficient ¹¹B 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 (¹⁰B and ¹¹B), 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.