Differences in boron distribution and forms in four citrus scion-rootstock combinations with contrasting boron efficiency under boron-deficient conditions

Key message Boron efficiency of scion 'Fengjie-72' is related to its less reduced boron concentration and distribution in leaves, achieved by decreasing the ratio of available boron in roots under boron-deficient conditions. Boron (B) deficiency is widespread in citrus orchards. Previous studies have demonstrated that the B-efficient navel orange scion 'Fengjie-72' (Fs) and rootstock Carrizo citrange (Cr) are more tolerant to B deficiency than the closely related B-inefficient scion 'Newhall' (Ns) and rootstock trifoliate orange (Tr), respectively. However, the mechanisms underlying such differences remain unclear. Here, we investigated the differences in B distribution and forms among four combinations (Fs/Cr, Fs/Tr, Ns/Cr, and Ns/Tr) under adequate (0.25 mg/L) or deficient (0.001 mg/L) B supply for 300 days in sand culture. The results showed that B concentrations in buds and leaves of Fs-grafted plants were significantly higher than the respective concentrations of Ns-grafted plants under B-deficient conditions. Moreover, B distribution of Fs-grafted plants due to B deficiency was reduced less in leaves, but more in roots as compared to that of Ns-grafted plants. However, Ns/Cr accumulated more B in the scion stem (24 %) than the other combinations (17-19 %) when B was limited. A correlation was established between B efficiency and the ratio of B concentration in the rootstock stem or buds to the scion stem. Under B-deficient conditions, the ratio of available B (free B and semi-bound B) was significantly higher in leaves in Cr-grafted (36 %) than Tr-grafted plants (29 %), but lower in roots of Fs-grafted (22 %) than Ns-grafted plants (28 %). These results suggest that, under B-deficient conditions, differential B efficiency arises probably because Cr transports more B into scion, Fs redistributes B more efficiently within the plant, or both.