Boron-deficiency and aluminum toxicity activate antioxidant defense and disorganize the cell wall composition and architecture in trifoliate orange leaf

Abstract

Boron (B)-deficiency and aluminum (Al) phytotoxicity are two main factors that influence the yield and quality of citrus. In this study, we investigated the effects of B-deficiency, Al stress, and double stress of B-deficiency and Al toxicity on antioxidant enzyme system and cell wall components in leaf of trifoliate orange seedlings. Boron deprivation and Al toxicity severely induced morphological and anatomical changes in leaves in terms of higher lipid peroxidation, inhibited photosynthesis and impeded seedling growth, and the damages was more pronounced by double stress. Boron-deficiency disturbed the distribution pattern of pectin in leaves, suppressing the content of alkali-soluble pectin and two types of 2-keto-3-deoxyoctanoic acid (KDO), thus reduced the binding of B to the leaf cell wall. Meanwhile, exposure of Al stress alone resulted in increased content of chelator-soluble pectin and alkali-soluble pectin, whilst an opposite trend was detected in the degree of methylation of the above two pectins. Transmission electron microscope (TEM) micrographs of –B and –B +Al-treated leaves showed destruction of chloroplasts and a distinct thickening of cell wall. Fourier-transform infrared spectroscopy (FTIR) and 13C-nuclear magnetic resonance spectroscopy (13C NMR) in conjunction with X-ray diffraction (XRD) exhibited cellulose accumulation while its crystallinity was reduced by Al toxicity and double stress. Taken together, our results reveal that B-deficiency and Al toxicity caused oxidative damage to plants, changed the pectin content and characteristics, and altered the cell wall components and architecture, which could be a major cause of deleterious seedling growth.