Observations on Morphological Abnormalities of the Vessel Elements of Veins and Fruit of Citrus under Boron Deficiency

Boron deficiency resulted in thick and brittle leaves, suberification, and "contract fruit" in citrus, due to damaged vascular tissue, which usually functions for transportation. Vessels are an important component of vascular tissue. Morphological abnormalities of the vessel elements were observed using the leaves of ‘Newhall’ (Citrus sinensis Osbeck) Navel orange and HB Pummelo Citrus maxima (Burm.) Merr. by separating the vessels by means of segregation. Results indicated that: (1) The number of pitted vessels in ‘Newhall’ Navel oranges with boron deficiency increased; (2) A larger number of pitted vessels in the nervure and mesocarp of HB Pummelo with boron deficiency were proliferated, while scalariform vessels and reticulate vessels decreased; (3) Normal vessels were round and full, while vessels under boron deficiency were transubstantiated shriveled; (4) Compared with the control vessels, vessels under treatment became short and narrow, cell walls turned thin and the percentage of no-tail under boron deficiency condition was increased, which finally led the vascular tissue to lose its function; (5) The headstream of the suberification and "contract fruit" was that the vascular bundles with boron deficiency were damaged and transportation efficiency was decreased.