Role of boron and its detoxification system in trifoliate seedlings (Poncirus trifoliate (L) Raf.) response to H+-toxicity: Antioxidant responses, stress physiological indexes, and essential element contents

Boron (B) deficiency and H+toxicity are two prominent factors that severly restrict trifoliate seedlings growth in acid soil. Exogenous B can support to mitigate the harm of H+toxicity, however, research on the protective mechanism and the impact of physiological processes in citrus is limited. To fill the above gaps, the current study was performed on trifoliate rootstock using two pH levels (pH 4 and pH 6) and various B treatments (B0, B1, B2, B3, B4) to identify how B confers tolerance to H+ toxicity in trifoliate rootstock. Our results illustrate that H+toxicity suppressed plant growth, specifically the root elongation (27.22%). Boron capitalized the positive effect on plant growth as soon as it is applied to culture medium. The supply of B increased the activity of antioxidant enzymes and the content of antioxidant substances in root of trifoliate seedlings under H+ toxicity to scavenging the massively accumulated reactive oxygen species (ROS), reducing the malondialdehyde (MDA) content (12.07%-53.45%) and promoting root development. Meanwhile, the higher levels of osmolytes after B supply in root were better for maintaining water in cell and resisting H+ toxicity. Furthermore, B supplementation improved nutrients content including macro and microelements in root, especially the elements constituting superoxide dismutase (SOD), which in turn activated the antioxidant enzymes system. In conclusion, the ameliorative effects of B to H+toxicity in trifoliate seedlings was based on B homeostasis, improved antioxidant defense system, and then nutrient content. These findings provide a new insight into the regulation of H+toxicity tolerance of citrus by B fertilizer application in acid soils.