Effects of fruit bagging on the accumulation of volatile compounds in ‘Majiayou’ pumelo

Abstract: 【Objective】Bagging, as a common cultivation technique in fruit industry, is widely used in many types of fruit trees except citrus，such as apple, peach, grape, etc. Volatile compounds can directly affect fruit flavor s and are an important indicator of fruit quality. Till now, little research has conducted on the physiological and molecular changes of volatile compounds accumulation in response to environ- mental stimuli in citrus. The present study was mainly to explore the effect of bagging on the accumula- tion of volatile compounds in the peel and pulp of unbagged and bagged‘Majiayou’pumelo fruits at five developmental stages, for purpose of providing some guidance in the application of bagging and new clues for understanding the molecular mechanism to volatile compound regulation in citrus.【Meth- ods】The determination of volatile compounds can be divided into three steps: The first step was the ex- traction of volatile compounds. 0.5 g peel or 1.0 g pulp was weighed, put into a 2 mL centrifuge tube, 0.5 mL ultrapure water was added, and mixed evenly in vortex. Then 0.5 mL MTBE was added contain- ing methyl nonanoate (Vmethyl nonanoat∶e VVMTBE=1∶80), and vortex was performed with the mixture again. The mixture was then subjected to ultrasonic extraction at 4 °C for 40 min. Next, the extract was centri- fuged at 12 000 g for 10 min below 4 °C, and then filtered through a 0.22 μm membrane prior to GC- MS analysis. The second step was determination. GC-MS was performed by tr-5ms gas chromatogra- phy column (30 m × 0.25 mm × 0.25 μm, Thermo Scientific, Bellefonte, PA, USA), the specific temper- ature of sample inlet and ion source were 280 °C and 260 °C, respectively. The programmed tempera- ture rise conditions were as follows: treating at 40 °C for 3 min, then raising the temperature to 160 °Cat2°C·min 1andholdingfor1minute,andthenraisingthetemperatureto200°Cat5°C·min-1,next raisingthetemperatureto240°Cat8°C·min-1 andholdingfor3min.Thelaststepwasdataanalysis. The analysis of volatile components was carried out by Xcalibur and AMDIS. In this study, the quantita- tive calculation of volatile components was performed by semi quantitative analysis. First, the area nor- malization method was used to get the area percentage of each component. Then, the concentration of each component was calculated by the concentration of internal standard compounds in the sample.【Re- sults】Esters and terpenes were the main volatile compounds in‘Majiayou’pumelo fruit. A total of 69 and 40 kinds of volatile compounds were identified in peel and pulp, respectively. The alcohol, ester, al- dehyde, terpene and other compounds in the pulp were less than those in the pericarp, and nootkatone and some monoene compounds such as β-myrcene and β-phellandrene were found to be specifically ac- cumulated in the peel. Bagging affected the content of volatile compounds but not the composition. During maturation stage, the total content of volatile compounds in peel with bagged fruit (1 294.20±14.67)μg·g-1 was significantly lower than that with unbagged fruit (4 869.44±30.78) μg·g-1. Especially, com- pared to unbagged fruits, the content of sesquiterpeneketone and monoterpene decreased dramatically in peel of bagged fruit. With regard to the pulp, there was no significant difference in the total content of volatile compounds between bagged fruits (257.06 ± 29.90) μg · g-1 and unbagged fruits (355.30 ± 36.54) μg·g-1, which indicated that bagging had little effect on the volatile compounds in the pulp. In ad- dition, the contents of volatile compounds in‘Majiayou’pumelo fruit at different developmental stages (105, 135, 150, 180 and 210 days after flower, DAF) were analyzed in this study. In the peel, we found that oxides, alcohols and ketones accumulated more in bagging than unbagging treatments at the middle stage of development, but alkenes, acids and aldehydes accumulated more and faster in unbagging than bagging treatments, and no significant difference were found in the content of esters between bagging and unbagging treatments. The content of volatile compounds in the peel of bagged and unbagged‘Ma- jiayou’pumelo fruits showed a similar trend of increasing at first and then decreasing during developmental stages. In the pulp of both bagging and unbagging treatments, the content of oxides, alkenes and ketones decreased gradually with the development of fruit, and the accumulation of acids, aldehydes and alcohols increased at first and then decreased. However, no significant differences were detected in the content of each substance in the pulp between bagged and unbagged‘Majiayou’pumelo fruits, es- pecially for the mature stage.【Conclusion】In this study, we found that bagging treatment had a great ef- fect on the volatile compounds in the peel of‘Majiayou’pumelo fruits, among which the content of ke- tones and terpenes changed significantly. However, the composition and content of volatile compounds in the pulp of‘Majiayou’pumelo fruits were not affected by bagging treatment, suggesting bagging treatment had little effect on the aroma and the overall flavor of the pulp, the main edible part of‘Majiayou’pumelo fruits. Therefore, bagging can be widely used in‘Majiayou’pumelo cultivation in terms of our volatile compounds analysis. Previous studies have shown that the accumulation of volatile com- pounds is related to light quality and intensity. The peel is normally exposed to the sunlight, while the wrapped pulp is always under dark condition due to the thick peel either for bagging or unbagging treat- ment. This may explain the different behavior of peel and pulp with bagging treatment. These results can provide a basis for improving the fruit quality in the field practice with bagging treatment, and for studying the mechanism of light signal to regulate the formation of aroma quality in citrus.