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In order to uncover the mechanism of boron (B) utilization efficiency, B metabolism was studied through B forms and their balance of transformation. According to the reaction between B and some sugars in rape (Brassica napus L.), B was divided into free B, semi-bound B, and bound B using a sequential extraction procedure. One B-efficient rape cultivar (9589), one B-inefficient rape cultivar (95105), and the F1 hybrid (95105 × 9589) were involved in this pot experiment, which was carried out during 1998–1999. Two externally applied B treatments (B1: 0.05 mg kg-1 and B2: 0.25 mg kg-1) were used, and in both B treatments, the results showed that the concentration of the three B forms differed between the different cultivars. The free B and bound B concentration in the B-efficient cultivar (9589) was significantly lower than that in the B-inefficient cultivar (95105), and the F1 hybrid (95105 × 9589) was intermediate, but the semi-bound B concentration showed the opposite trend. Boron could react with some sugars which contained the vicinal group in cis position, which showed closely the relation between B and some sugar in plants. A significant difference of fructose content existed between cultivars. Using thin-layer chromatography and ultraviolet-scan analysis fructose was shown to react easily with B. From the close relationship between B and sugar, a hypothesis concerning the physiological mechanism of B utilization efficiency was formulated; a reaction constant km for the balance of B forms may indicate to some degree whether rape cultivars are B efficient. [ABSTRACT FROM AUTHOR]