Zinc Fertilization Mitigates Boron Toxicity in Citrus: Plant Growth, Tissue Mineral Composition and Antioxidant Enzyme Activity

Abstract

Boron (B) toxicity severely restricts Citrus growth and productivity in high-B soil and irrigation water environments. Although exogenous Zinc (Zn) has been reported to mitigate B toxicity in plants, the underlying physiological regulation mechanisms—particularly in Citrus—remain incompletely understood. In this study, seedlings of two Citrus species differing markedly in B tolerance—Citrus sinensis (sweet orange, B-tolerant) and C. grandis (sour pomelo, B-sensitive)—were subjected to three boric acid treatments (10, 200, and 400 μM) and five ZnSO4 treatments (0.002, 0.05, 0.1, 0.2, and 0.25 mM) in a fully factorial design over 15 weeks. Comprehensive physiological assessments—including growth parameters, photosynthetic performance, mineral-nutrient profiling (in leaves and roots), and antioxidant enzyme activity—were conducted to decipher the mechanistic basis of Zn-mediated B detoxification. Results showed that 400 μM BA alone triggered severe B toxicity symptoms in the older leaves of C. grandis, whereas 0.25 mM Zn independently led to chlorosis symptoms similar to Fe deficiency in the apical leaves of both Citrus species. Either B or Zn toxicity affected photosynthesis in matured leaves. Exogenous Zn at 0.05–0.20 mM alleviated B toxicity symptoms in B-sensitive C. grandis, yet consistent alleviation of Zn toxicity by excess B was not observed across all measured variables. Severe B toxicity caused a significant reduction in Fe, N, P and Ca contents and a significant increase in Cu and Mn contents in C. grandis roots, yet it only led to a significant reduction in Mn, N, P and Ca contents in C. grandis leaves. Unlike this, B toxic treatments in C. sinensis resulted in a significant reduction in merely Fe content and a significant increase in Cu, Mn, N and K contents in roots, and a significant reduction in Mn and Ca contents in the leaves. Under basal B supply, 0.05–0.20 mM Zn supplementation dose-dependently increased root Zn, Cu and Mn contents as well as leaf Zn and Cu contents in both Citrus species, but concurrently decreased root Fe content and leaf Fe and Ca contents—additionally, in C. grandis, leaf N, P, K and Mg contents were further reduced. Regarding antioxidant responses in C. grandis, B toxicity significantly suppressed leaf CAT activity while enhancing APX and GPX activities. Under B toxicity conditions, 0.2 mM Zn supplement significantly restored CAT activity and suppressed APX and GPX activities. These antioxidant modulations were highly dependent on species identity and treatment combination, and were markedly more pronounced in B-sensitive C. grandis. Collectively, this work clarifies the physiological interplay between Zn and B in Citrus, uncovers divergent adaptive strategies employed by contrasting citrus species under combined B–Zn stress, and provides a mechanistic foundation for optimizing exogenous Zn application to mitigate B toxicity in citrus production.