Effects of foliar iron fertilizers on iron-deficient leaves and fruit of ‘Ougan' (Citrus reticulata) planted in the tideland in Zhejiang

Abstract:【Objective】Iron is one of the essential microelements for plant growth and development. As a main component of electron carriers or catalysts, it plays an important role in many physiological processes. However, iron deficiency is a common problem in most areas of China. Leaf chlorosis is the main symptom of iron deficiency because iron is an essential element for chlorophyll biosynthesis. Young leaves usually show chlorosis first since iron is immobile. However, when iron deficiency continues, the entire leaves turn yellow, develop necrotic spots and gradually die. Leaf chlorosis in fruit trees causes slow growth, yield reduction and quality decline. However, plants have developed a complicated mechanism of iron-deficiency tolerance during long-term evolution. Morphologically, roots become shorter with increased apical diameter and root hairs and formation of transfer cells in root epidermis and cortex under iron deficiency stress. Physiologically, rhizosphere acidification, synthesis of organic acids and phytosiderophore areenhanced. These changes contribute to iron absorption under iron deficiency condition. Malus xiaojinensis is resistant to chlorosis associated with iron deficiency, and its special root structures decrease the p H of the rhizosphere, which increases the affinity to and thus the absorption of iron. In orange, a large of number lateral roots are formed to increase the absorption area under iron deficiency stress.‘Ougan'is a major citrus cultivar in Wenzhou, where there is a tendency of developing the crop in tidelands. However, the high soil p H results in leaf chlorosis and impacts growth and development and fruit quality of‘Ougan', because the ferric ion in the soil is less available for the plant. In this study, foliar-spraying of iron fertilizers was conducted to choose an appropriated iron fertilizer to solve the problem of iron deficiency, which is an urgent research task.【Methods】Foliar-sprays of the same concentration of iron fertilizers including Fe SO4, self-prepared EDTA-Fe, Vc-Fe and organic-Fe were conducted on 8-year-old‘Ougan'trees.The contents of iron and chlorophylls in leaves were measured at fruit maturing period, and fruit quality was evaluated.【Results】The available iron content in‘Ougan'leaves and fruit was significantly increased by 53.9%-91.6% and 28.7%-83.2%, respectively, after foliar-spraying of iron fertilizers. Vc-Fe was much more effective than the other iron fertilizers, increasing the iron content in leaves (113.91 mg·kg^-1) and fruit (11.38 mg·kg^-1) by 91.6% and 83.2% compared with the control, respectively. After EDTA-Fe treatment, the ion contents in leaves and fruit reached 101.51 mg·kg^-1and 10.21 mg·kg^-1, respectively, which fell within the appropriate levels based on the diagnostic criteria. The content of chlorophylls increased after iron spraying, and chlorophyll a, b and total chlorophylls were increased by 4.1%-24.7%, 6.8%-32.2% and 4.7%-26.5%, respectively. However, compared with the control, chlorophyll a, b and total chlorophylls in Vc-Fe treatment were significantly increased by 24.7%, 32.2% and 26.5%, respectively. Fruit quality was also improved by iron sprays, and the total soluble solids and soluble sugars in fruit were increased by 4.6%-11.6% and 6.2%-13.4%, respectively. In organic-Fe treatment, the total soluble solids and soluble sugars were 10.57% and 7.85%, respectively, which were 1.10 and 0.93 folds higher than that of control, respectively.【Conclusion】Ferrous sulfate fertilizer had no obvious effect in preventing leaf chlorosis due to its high susceptibility to oxidation. Although chelated iron was superior to ferrous sulfate, it is more costly. Vc-Fe was most effective in increasing the iron content both in the leaves and the fruit, and its effectiveness might be relate to the inhibition of ferrous oxidation by vitamin C. Organic Fe was obviously effective in increasing the total soluble solids and the soluble sugars and the treatment also induced re-greening in yellowed leaves. The effect might be related to abundant organic matter in addition to effective ferrous iron. The results provide a reference for the preparation of special iron fertilizer used for citrus planted in the tidelands in Zhejiang.