| Xanthoceras sorbifolia, which belongs to Sapindaceae, is one of the most valuable oil trees widely distributed in North China. Its seed has high oil content up to around 60%, in particular, oleic acid and linoleic acid content can reach to more than 30%, respectively. In this study, oil accumulation and its correlation with the carbohydrates and protein were followed during seed development of Xanthoceras sorbifolia. Fatty acid composition was also examined in developing seeds. The results are as follows:Four stages can be distinguished in the developmental period of seed. Stage I, from flowering to around 33 day after anthesis(daa), the embryo progressively acquired the basic plan of the plant and developed into the early cotyledon shape. Stage II, from 34-47daa, the embryo grew rapidly and a green embryo filled the entire interior of the seed. Stageâ…¢, from 48-68daa, the size of embryo ceased to enlarge, the fresh and dry weight of embryo continued to increase rapidly and reached the maximum at 68daa. Stageâ…£, from 69-82daa, the dry weight ceased to increase, but the fresh weight decreased slightly due to desiccation. Oil deposition during seed development exhibited "slow-fast-slow" accumulation pattern, the first oil bodies were observed in the cotyledon cells at around 33daa. From stage I to the first half of stage II, oil formation was slow. Rapid oil accumulation, indeed, commenced at the second half of stage II. Thereafter, oil continued to accumulate rapidly till the end of stageâ…¢. In stageâ…£, oil accumulated slowly with slight fluctuations.During seed development, the accumulation of soluble sugar and starch exhibitied the opposite accumulation pattern in contrast to oil. They accumulated transiently before the rapid oil accumulation and degraded later as the oil accumulated continually. Soluble sugar content ceased to decline with the end of rapid oil accumulation, but starch continually degraded and almost no starch grains were observed in the cotyledon cells of mature seed. Protein accumulation pattern was similar to that of oil, but the rate of accumulation was always slower than that of oil and the total protein content was about 30%in mature seed. Correlation analysis showed oil content was negatively related to soluble sugar and starch, positively related to protein content.During oil accumulation,11 kinds of fatty acids were detected. The content of saturated fatty acids decreased, but the level of unsaturated fatty acids increased as seeds ripened which constituted 95.22% of the oil in the mature seeds. Among the unsaturated fatty acids, the sum of oleic acid(18:1) content and linoleic acid (18:2) content has always been maintained at above 68%. Oleic acid peaked at 61daa while the linoleic acid reached its maximal level at 40daa. Cis-11-eicosenoic acid(20:1), erucic acid(22:1) and nervonic acid(24:1) synthesized later and their content increased gradually as seed matured. Correlation analysis of fatty acid composition showed that fatty acids can be divided into 2 groups. Palmitic acid, linoleic acid and linolenic acid were fallen into group 1, they were mainly accumulated at early stages of seed development and then their content decreased with further development of seeds. Oleic acid, cis-11-Eicosenoic acid, erucic acid and nervonic acid belonged to group 2, and their content increased gradually with seed ripening. The fatty acids of group2 were highly negatively associated with that of group 1.
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