Distribution of boron and its forms in young Newhall navel orange (Citrus sinensis Osb.) plants grafted on two rootstocks in response to deficient and excessive boron

Boron (B) deficiency is widespread and serious in navel orange orchards in southern Jiangxi in China and has been considered an important soil constraint to citrus yield and quality. It has been observed that there was great variation in responses to different B supply levels between oNewhallo navel orange grafted on citrange and trifoliate orange. The aim of this work was to compare the different responses of the two scion-rootstock combinations under different B stresses (low and high) in young citrus trees. oNewhallo navel orange grafted on citrange or trifoliate orange were incubated in nutrient solution containing different levels of B (0, 0.5 and 5.0 mg L-1), of which 0 and 5.0 mg L-1 were considered as deficient and excessive B and 0.5 mg L-1 was considered as an appropriate level of B. The experiment was carried out in a greenhouse for 117 days. The results showed that, under low B or high B conditions, better growth performance was observed in oNewhallo plants grafted on citrange than in those grafted on trifoliate orange, suggesting that citrange-grafted plants were more tolerant to deficient and excessive B than the plants grafted on trifoliate orange. Boron seemed to be preferentially translocated to the new leaves (leaves from current year), and resulted in normal growth of young leaves under limited B supply. Boron distribution in different plant parts indicated that there was a restriction in translocation of B from roots to scion tissues (stems and leaves of scion) under limited B availability. The relatively high B concentration observed in plants grafted on trifoliate orange suggested higher demand for B than those grafted on citrange. There existed a balance within the B forms to maintain normal growth. Thus, R value (R = semi-bound B/free B) was much higher in citrange-grafted plants than in trifoliate orange-grafted plants under the same B supply level. This explained why citrange-grafted plants had less B but produced relatively more dry mass at cellular level. These results suggested that the utilization efficiency of B of citrange-grafted plants was probably greater than trifoliate orange-grafted plants.