Boron mitigates citrus root injuries by regulating intracellular pH and reactive oxygen species to resist H+-toxicity.

Boron (B)-deficiency and H+-toxicity are important limiting factors for plants growth in acid soils. High B supply may reduce H+-toxicity-induced inhibition of growth in citrus. Trifoliate orange rootstock seedlings were irrigated with nutrient solution containing either 0 μM or 10 μM H3BO3 at two pH levels (pH4 (H+-toxicity) and pH6 (normal)). The results showed that H+-toxicity without B severely hampered main root elongation. Simultaneously, oxidative damage caused by H+-toxicity led to severe damage to the apical structure of root such as root crown abscission. However, B application promoted the root length, root cell viability and reduced cell wall (CW) thickness of root tips under H+-toxicity. Additionally, B application reduced the H+-toxicity-induced reactive oxygen species (ROS) accumulation in roots as characterized by lower fluorescence intensity of H2O2 and O2- staining. Moreover, 31P-NMR (31P nuclear magnetic resonance) spectra revealed B application regulated the pH of vacuoles and cytoplasm in root tips by reducing phosphoenolpyruvate carboxykinase (PEPCase) activity while enhancing NADP malic enzyme (NADP-ME) activity during H+-toxicity. Collectively, our results demonstrate that B supply alleviates H+-toxicity and promotes root growth by reducing ROS accumulation, attenuating intracellular acidic microenvironment to ensure normal chemical reactions in root tip cells.