Soil salinity is a major environmental hurdle for agricultural production, inhibiting crop growth reducing yield, and resulting in huge economic costs every year. Boron (B) is an essential micronutrient for plants that play a critical role in cell wall composition, membrane structure, and a broad range of metabolic activities. A few studies reported the beneficial effects of B on salinity tolerance in plants; however, the mechanistic basis of this process remains highly elusive. This study aimed to explore the mechanistic basis of the role of B for salt-grown plants including pea (Pisum sativum), Arabidopsis (Arabidopsis thaliana), and barley (Hordeum vulgare) with contrasting salinity stress tolerance, using a range of electrophysiological, molecular, and phenotyping techniques.

In the salt-sensitive pea plants (Chapters 3-4), the detrimental effects of salinity on pea seedlings were reduced by the presence of appropriate B due to stimulation of H+-ATPase and the prevention of NaCl-induced K+ loss in the root, the higher shoot K retention, and Ca2+ influx functions as the signal response. Subsequently, we found B also exhibited an effect on halotropism besides root growth stimulation in model plant Arabidopsis (Chapter 5). The ameliorative effect of B on the Arabidopsis under salt stress exhibits a time- and dosedependent pattern based on H+-ATPase stimulation and a subsequent K+ retention, involving auxin- and RALF-FERONIA- pathways. In Chapter 6, B availability resulted in better salt tolerance in barley, which was achieved by improved photosynthetic performance, a more optimal K+/Na+ ratio and reduced Na+ and Cl− accumulation, higher K+ retention resulting from stimulation of H+-ATPase and a consequent plasma membrane (PM) hyperpolarization. The better performance of salt-tolerant TX9425 compared with salt-sensitive Gairdner response to salinity could be attributed to its superior capacity in the sequestration shoot Na+ and root K+ retention as well as desensitization of K+ loss response to ROS.

Overall, this work demonstrates that ameliorating effects of B on plant adapt to salinity include (amongst others): the stimulation of root H+-ATPase and a consequent PM hyperpolarization, the prevention of NaCl-induced root K+ loss, and the reduced Na+ and Cl− accumulation. Thus, B-treated plants maintained a more optimal K/Na ratio and showed improved photosynthetic performance and growth under saline conditions. Also, stressinduced Ca2+ signatures and the desensitization of K+ loss response to ROS affected by B. All reported effects of B exhibited strong time- and dose-dependency. These traits may be considered new targets in crop breeding programs aimed at enhancing salt tolerance.