Effects of boron toxicity on root and leaf anatomy in two Citrus species differing in boron tolerance

KEY MESSAGE : Typical toxic symptom only occurred in B-toxic C. grandis leaves. B-toxicity induced PCD of C. grandis leaf phloem tissue. The lower leaf free B might contribute to the higher B-tolerance of C. sinensis. Seedlings of ‘Xuegan’ (Citrus sinensis) and ‘Sour pummelo’ (Citrus grandis) differing in boron (B)-tolerance were irrigated with nutrient solution containing 10 (control) or 400 (B-toxic) μM H₃BO₃ for 15 weeks. Thereafter, the effects of B-toxicity on leaf photosynthesis, chlorophyll, plant B absorption and distribution, root and leaf anatomy were investigated to elucidate the possible B-tolerant mechanisms of Citrus plants. Typical toxic symptom only occurred in B-toxic C. grandis leaves. Similarly, B-toxicity only affected C. grandis photosynthesis and chlorophyll. Although total B concentration in B-toxic roots and leaves was similar between the two species, leaves from B-toxic C. grandis plant middle had higher free B and lower bound B as compared with those from C. sinensis. Effects of B-toxicity on leaf structure were mainly limited to the mesophyll cells and the phloem of leaf veins. Although irregular cell wall thickening was observed in leaf cortex cells and phloem tissue of B-toxic C. grandis and C. sinensis leaves, exocytosis only occurred in the companion cells and the parenchyma cells of B-toxic C. sinensis leaf phloem. Also, B-toxicity induced cell death of phloem tissue through autophagy in C. grandis leaf veins. B-toxicity caused death of root epidermal cells of the two Citrus species. B-toxicity restrained degradation of middle lamella, but did not alter ultrastructure of Golgi apparatus and mitochondria in root elongating zone cells. In conclusion, C. sinensis was more tolerant to B-toxicity than C. grandis. The lower leaf free B and higher bound B might contribute to the higher B-tolerance of C. sinensis.