Application of Electron Probe to the Observation of in situ Calcium Distribution in Fruit Tissues

Abstract

【Objective】 Distribution of mineral nutrition in plant tissues reflects the patterns of nutrient transportation and utilization. Using calcium analysis in fruit tissues as an example, this paper demonstrates a method that combines electron probe microanalysis and image processing technique, which generates a direct view of in situ mineral distribution in plant tissues. 【Method】 Pedicels and pericarps of mature fruits of litchi ‘Huaizhi' and citrus ‘Shatangju' and pedicels of persimmon ‘Cilang' were used as materials for this study. Tissue slices coated with platinum were observed under scanning electron microscope equipped with an electron probe(wave dispersive spectroscopy, WDS). Secondary electron images and Ca-specific X-ray signal mapping images(Ca mapping image) were separately collected. The map of bright dots representing abundance of Ca was extracted using image processing Photoshop and combined with the secondary electron image. 【Result】 The combined image provided a direct view of detailed in situ Ca distribution in fruit tissues. In the pedicels of all the three fruit speciess, Ca abundance in the phloem is significantly greater than in the xylem. Within the phloem, fiber cells have the lowest Ca content, while the parenchyma cells contain higher calcium. Many of these cells have Ca-rich bodies(calcium oxalate crystals). The epiderm of litchi pericarp has high calcium and also contains Ca-rich bodies. Pericarp of citrus has a relatively even Ca distribution. 【Conclusion】 Electron probe combined with image processing technique provides an effective method for observing in situ nutrition distribution in plant tissues. According to the distribution pattern of Ca in fruit pedicel, it is suggested that Ca transport to fruit is likely through the phloem instead of xylem.