Abstract: Changes in contents of boron (B)in different forms in the leaves of Newhall navel orange (Citrus sinensis Osbeck) were investigated during fruit development in the south part of Jiangxi Province. The Newhall orange trees were either grown in low boron soil, where conspicuous symptom of B deficiency was found in mature leaves, or grown soil with adequate boron, serving as the control. The results showed that: (Ⅰ) content of total B decreased significantly either in old leaves from last season or in springflush leaves from current year during fruit enlargement period from 120 to 160 days after full bloom (DAFB), and the content in Newhall (B deficiency, BD) were generally lower than in the control. (Ⅱ) In old leaves, content of free B and semibound B in BD Newhall were very low from 120 to 160 DAFB, in contrast, those in the control were quite high during this period; Compared to free B and semibound B, bound B content in BD Newhall were likewise lower than the control during the same period, whereas its relative contents were higher in the former; (Ⅲ) In springflush leaves, free B and semibound B content were higher than in old leaves, bound B content were lower; Furthermore, compared to the control, content of three B forms in BD Newhall were significantly low from 140 to 160 DAFB. However, the relative contents of bound B were comparatively higher in Newhall (BD). The dynamics of three B forms and physiological mechanism of B deficiency occurrence in Newhall navel orange leaves is also discussed.

Key words: Citrus sinensis Osbeck, Navel orange, Boron nutrition, Boron deficiency, Physiological mechanism