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

Boron (B) toxicity is a common issue in Citrus production regions worldwide. Zinc (Zn) has been reported to mitigate B toxicity in plants, but the underlying regulatory mechanisms remain poorly elucidated in Citrus. In this study, ’Sweet orange’ (Citrus sinensis) and ’Sour pomelo’ (C. grandis) seedlings differing in B tolerance were irrigated with nutrient solutions containing different levels of boric acid (BA; 0, 200, and 400 μM) and ZnSO4 (0, 0.05, 0.1, 0.2, and 0.25 mM) for 15 weeks. Thereafter, leaf photosynthesis, antioxidant enzyme activity, plant biomass and mineral composition in leaf and root tissues were determined to elucidate the possible Zn-detoxication mechanisms. 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. Appropriate Zn application significantly alleviated B toxicity symptoms in B-sensitive C. grandis, and vice versa. Severe B toxicity caused a significant reduction of Fe, N, P and Ca contents and a significant increase of Cu and Mn contents in C. grandis roots, yet it only led to a significant reduction of Mn, N, P and Ca contents in C. grandis leaves. Unlike this, B toxic treatments in C. sinensis resulted in a significant reduction of merely Fe content and a significant increase of Cu, Mn, N and K in roots, and a significant reduction of Mn and Ca contents in the leaves. Under normal BA condition, supplementation with proper amount of Zn increased root Zn, Cu and Mn contents as well as leaf Zn and Cu contents in both Citrus species in nearly a dose-dependent manner. However, Zn supplement decreased root Fe content and leaf Fe and Ca contents in both species, in addition to leaf N, P, K and Mg contents in C. grandis, in the same dose-dependent manner. B toxicity C. grandis leaves had lower CAT activity and higher APX and GPX activities when compared with the control treatment; Under toxic BA stress, 0.2 mM Zn supplement significantly increased CAT activity, but decreased APX and GPX activities. Our study confirms the potential use of Zn fertilization to mitigate B toxicity in citrus cultivation, although subsequent field studies are still needed to evaluate its long-term effects.