| Boron (B) is an essential micronutrient element required for growth and development of higher plants. Ganzhou city in Jiangxi province in south China is a predominant region of navel orange production, where the hot water soluble B concentration is extremely low. In recent years, leaf chloresis a symptom of B deficiency occurs in the main local cultivar Newhall navel orange (Citrus sinensis Osb.), meanwhile the tree vigor declines rapidly after fruit set, and therefore affects fruit yield and quality in the coming years. Rootstock can greatly affect the growth and development of navel orange. Therefore, it is significant to carry out research works on the mechanism of B efficiency in navel orange grafted on different rootstocks and on the response mechanism of B deficiency on the cell wall structure and metabolism in navel orange plants. Furthermore, the study could provide a scientific basis for the study on perennial citrus rootstock-scion interaction of B nutrition and the development of citrus industry. In the present study, we compared the capacity of boron utilization, boron absorption and distribution, and boron mobility in Newhall navel orange plants grafted on citrange (Citrus sinensis (L.) Osb.×Poncirus trifoliate (L.) Raf.) and trifoliate orange (Poncirus trifoliate (L.) Raf), investigated the changes in cell wall composition and the structure of leaf, cell, and cell wall using the chemical extraction and biophysical methods (SEM, TEM, XPS, XRD, and FTIR) under B limitation, and analyzed the changes in metabolism profiling of leaves in Newhall navel orange plants due to B deficiency using metabolomics method (GC-MS) by hydroponic experiment. The main results were summarized as following:1. The relatively high B concentration observed in plants grafted on trifoliate orange suggested higher demand for B than those grafted on citrange, suggesting that the utilization efficiency of B in citrange-grafted plants was greater than that in trifoliate orange-grafted plants at whole plant level. There existed a balance for the B forms within plants to maintain normal growth. R value (R=semi-bound B/free B) was much higher in citrange-grafted plants than in trifoliate orange-grafted plants under same B supply level, suggesting that B transmembrane transport is easier in citrange than in trifoliate orange. This explained why citrange-grafted plants had less B but produced relatively more dry mass at cellular level.2. The newly absorbed B in the new leaves was much higher than that in the lower-old leaves and the upper-old leaves in both grafted plants. Citrange-grafted plants showed higher newly acquired total B content and B concentration in both lower-old and upper-old leaves than those in trifoliate-orange-grafted plants. These results suggest that different rootstocks affect the pattern of B distribution in the old leaves between citrange and trifoliate orange, resulting in greater proportion of B partitioning into the old leaves of citrange than trifoliate orange. This difference of B distribution between the two grafted plants may result in the good performance in the old leaves in citrange-grafted plants with limited-B conditions.3. Foliar application of10B to the lower-old leaves resulted in B translocation to the upper-old leaves and the new leaves with preference mainly to the new leaves in both citrange and trifoliate orange when root B supply was relatively low. However,10B sprayed to the lower-old leaves not only did not increase the abundance percentage of10B in the root, but also reduced B concentration and the total B content in the root. At least15.8%and17.6%of the absorbed10B was translocated to other plant parts in citrange-and trifoliate-orange-grafted plants, respectively. There was no significant difference for the B mobility between citrange and trifoliate orange.4. After35days under B deprivation, shoot dry mass in trifoliate orange decreased by28%, but shoot dry mass of citrange was not significantly affected. Root growth of both types of rootstock seedlings was inhibited, but the trifoliate orange was affected more than the citrange. In comparison with B concentrations in plants prior to the commencement of B treatments, B deprivation for35days decreased B concentration in various parts of citrange plants, and the reduction was much greater in trifoliate orange plants. Trifoliate orange seedlings contained higher B concentration and total B in cell wall on a dry leaf basis than citrange subject to5μM B treatment. However, the proportion of leaf B allocated in cell wall was higher in citrange than trifoliate orange when B supply was deficient in the nutrient. The changes in pectin composition in cell wall due to B deprivation differed between citrange and trifoliate orange. The decreased uronic acid (UA) content in the Na2CO3-soluble pectin was observed in both rootstock, but the increased UA content in CDTA-soluble pectin was observed only in citrange. These results demonstrated that a combination of greater B uptake ability, greater B accumulation in cell walls, as well as the increased CDTA-soluble pectin, under limited external B supply, contribute to the integrity of cell walls in citrange and therefore increased tolerance to B deficiency.5. The extent of B deficiency in the upper leaves was more serious than that in the lower leaves. Therefore, the symptoms of curling of the leaves and leaf chlorosis were observed only in the upper leaves of B-deficient plants. Boron deficiency increased cell wall content and changed the cell wall composition. Boron deficiency had no significant effect on the lignin concentration in both upper leaves and lower leaves, and hardly affected the ionically bound pectin. However, boron deficiency significantly increased the relative hemicellulose and cellulose concentrations, and decreased covalently bound pectin in both upper and lower leaves. These results suggested that B deficiency probably enhanced the synthesis of secondary cell wall. Boron deficiency significantly decreased the B concentration in ionically bound pectin in the upper leaves but not in the lower leaves. The B concentrations in covalently bound pectin and hemicellulose was both decreased in the upper-and lower-leaves due to B deficiency, while there was no significant difference in the cellulose B concentration between the control and deficient-B plants. The results indicated that B seemed to be preferentially translocated to the ionically bound pectin of cell walls under B limitation.6. Seriously B deficiency destroyed the epidermis structure of navel orange leaves, and decreased the content of cutin and wax. Boron deficiency reduced the content of cell wall proteins and pectin but increased the content of cellulose and hemicellulose. The crystallinity of the cellulose and the anomeric configuration of carbohydrates in cell walls were largely destroyed by B deficiency. Furthermore, the mode of hydrogen bonding and the linkage pattern among cell wall components were also destroyed by B deficiency. Therefore, we speculated that the amount of wall components is not decisive for B deficiency symptoms, but that rather structural changes within these fractions are important.7. The metabolite profiling of the upper and lower leaves in orange plants showed remarkable differences under adequate B conditions. Boron deficiency increased the contents of glucose and fructose in upper leaves, but did not change the concentration of sucrose. In contrast, there were no significant differences in sucrose and hexose (glucose and fructose) contents of lower leaves between the B-deficient and control plants. The starch content was higher in both upper and lower leaves in B-deficient than in control plants. In addition, the contents of soluble sugars, sucrose, and starch in roots on a dry weight basis were higher in B-deficient than in control plants. Citrate and succinate decreased very strongly in both the upper and lower leaves under B deficiency. The increased level of malate occurred only in the lower leaves of B-deficient plants. The concentrations of inositol and quinate, which were highly associated with sugar metabolism, increased due to B deficiency. All these results suggested that boron deficiency inhibited sugar usage and altered TCA cyclic flux modes in citrus plants. |
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