Analysis on the composition and content of organic acids and vitamin C in avocado flesh of twelve introduced cultivars

Astract:【Objective】Avocado (Persea americana Mill.) is an important evergreen fruit crop grown in several countries with tropical and subtropical climates. Total world production was more than 4 million tons in 2011, with 70% of the production coming from countries including Mexico, Chile, Dominican Republic, Indonesia, Colombia, Peru, USA and Guatemala. Avocado was introduced to China at the beginning of 20 thcentury, and is now widely planted in provinces in south China, such as Hainan, Guangxi, Guangdong, Guizhou and Yunnan. The demand for avocado has been increasing year after year, but there are limited germplasm resources and production in China. Therefore, it is important and urgent to evaluate and utilize introduced avocado cultivars to develop an avocado industry in our country. Organic acids are one of main factors affecting the quality of fruits. Organic acids are important fruit metabolites associated with photosynthesis, respiration, and the metabolisms of phenolic compounds, amino acids, esters and aromatic compounds. The objective of the study is to analyze the composition and contents of organic acids inthe flesh of twelve introduced avocado cultivars. The results will provide a reference for utilizing these cultivars and breeding new avocado varieties.【Methods】The experiment was carried out in National Germplasm Resources Nursery of Tropical Fruits in South Subtropical Crops Research Institute (Zhanjiang, Guangdong) . The area has a tropical oceanic monsoon climate, which is warm in winter and hot in summer, and generally humid and rainy, with a mean daily temperature of 22.8 ℃ to 23.2 ℃, an average daily relative humidity of 80%, and a total yearly rainfall of 1 100 mm to 1 800 mm. The avocado trees were grafted onto the seedling-plant rootstock, planted 5 m×5 m apart, drip irrigated, and fertilized as required. The twelve avocado cultivars introduced from USA included‘Beta'‘Dupuis' ‘Tonnie'‘Tonnage'‘Simmonds' ‘Miguel'‘Brooks late'‘Lula'‘Monroe'‘Loretta'‘Wilson seedless' and‘Peterson'. Three healthy trees of each variety were randomly selected, and five mature fruit collected uniformly from the canopy of each tree were studied. Extraction of organic acids was conducted using the method of Nispems-Carriedo et al. (1992) and Hu et al. (2005) . HPLC analyses were carried out using LC-20 A (Shimadzu) with VWD ultraviolet detector. Chromatographic separation was carried out using a C18 column (250 mm×4.6 mm) . The mobile phase was 2 g·L^-1 metaphosphoric acid at a flow rate of 1.0 m L·min^-1.【Results】 (1) Six organic acids, including tartaric acid, malic acid, maleic acid, citric acid and fumaric acid and ascorbic acid were detected in the flesh of the twelve avocado cultivars, and their retention time was 5.41, 6.97, 12.20, 14.36, 15.63 and 7.99 min, respectively. (2) The range and mean content of organic acids were 1.39-3.80 mg·g-1 and 2.32 mg·g-1, respectively. The proportion and content of each organic acid had significantly differences among the cultivars. The median values of organic acid contents from high to low were 1.60 mg·g-1 (malic acid) , 0.48 mg·g-1 (citric acid) , 0.14 mg·g-1 (fumaric acid) , 0.06 mg·g-1 (tartaric acid) and 3.20 μg·g-1 (maleic acid) . Meanwhile, the orders of mean and 5%trimmed mean of the five components were in consistent with their median. (3) The range of the content of ascorbic acid was 0.10-3.07 mg·g-1 and the values of range and interquartile range (IQR) were 2.98 and1.02, respectively. The results showed that the content of ascorbic acid was significantly different among the twelve avocado cultivars. (4) The range of total acid content was 2.71-3.43 mg·g-1, and the values of median, range and IQR were 3.07, 3.29 and 1.91, respectively. (5) In avocado flesh, the content of organic acid was significantly positively correlated with the contents of malic acid (P < 0.01) . Total acid content was significantly positively correlated with that of ascorbic acid and with malic acid. Among the twelve avocado cultivars, the contents of organic acids, malic acid, fumaric acid, and tartaric acid of‘Loretta'were highest. However, the ascorbic aicd content of‘Brooks late'was highest in the twelve cultivars.【Conclusion】The organic acid contents in avocado flesh are significantly different among cultivars.Malic acid is the most abundant organic acid, followed in descending order by fumaric acid, citric acid, maleic acid and tartaric acid in avocado flesh. The content of organic acid is thus mainly determined by the content of malic acid. In addition, the content of ascorbic acid in avocado flesh differs greatly among cultivars.