| Boron(B) deficiency is frequently observed in citrus orchard. The use of B-efficient rootstocks or scions that are tolerant to low B supply is one of the most common strategies to cope with B deficiency. Our previous studies showed that the navel orange scion ‘Fengjie’ and rootstock Carrizo citrange are more tolerant to B deficiency(B-efficient) than the closely related scion ‘Newhall’ and rootstock trifoliate orange(B-inefficient), respectively. However, all commercial citrus are grafted trees, little information is available on the mechanisms of differences in B efficiency among citrus scion-rootstock combinations. Therefore, in the present study, the methods of sand culture and hydroponics were used to investigate the physiological and molecular mechanisms of B efficiency in citrus scion-rootstock combinations and rootstocks, which might help us to better understand the knowledge of B nutrition in citrus as well as its integrated management. The main results were as follows:1. Physiological differences in four citrus scion-rootstock combinations with contrasting boron efficiency under boron-deficient conditionsTo elucidate the physiological differences in scion-rootstock combinations with different B efficiency, the B-efficient navel orange scion ‘Fengjie-72’ [Fs; Citrus sinensis(L.) Osb. cv. Fengjie-72] and rootstock Carrizo citrange [Cr; Citrus sinensis(L.) Osb. × Poncirus trifoliata(L.) Raf.] as well as the closely related B-inefficient scion ‘Newhall’ [Ns; Citrus sinensis(L.) Osb. cv. Newhall] and rootstock trifoliate orange [Tr; Poncirus trifoliata(L.) Raf.] were selected. Four grafting combinations(Fs/Cr, Fs/Tr, Ns/Cr, and Ns/Tr) were obtained and sand-cultured under adequate(0.25 mg/L) or deficient(0.001 mg/L) B supply for 300 days. 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 with 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 suggested that B efficiency of rootstock Carrizo citrange is associated with the increased ratio of available B in leaves, and B efficiency of scion ‘Fengjie-72’ is related to its less reduced B concentration and distribution in leaves which is achieved by decreasing the ratio of available boron in roots under boron-deficient conditions. In addition, the concentrations of inositol, malic acid, citric acid in leaves as well as the zinc concentration in various parts were significantly lower in Fs/Cr than in the other combinations under B-deficient conditions.2. RNA-Seq analysis of boron deficiency responses in root tips of trifoliate orange with different boron-efficient scionsTo compare the molecular differences of B efficiency in scion-rootstock combinations, the B-efficient scion Fs and B-inefficient scion Ns were grafted onto the B-inefficient rootstock Tr respectively, and treated with 0 mg/L or 0.25 mg/L B nutrient solution. After 0 h, 12 h, 24 h of B deficiency, the concentrations of available and cell wall B in leaves and new roots were determined, and the RNA-Seq in root tip was analysed. Regardless of the scion type, the concentrations of available and total B in roots with B-deficient treatment were significantly reduced at 12 h, and the concentration of cell wall B also decreased at 24 h. By contrast, the available and total B concentrations in leaves were significantly reduced at 12 h and 24 h respectively only in Ns/Tr. RNA-Seq analysis indicated that a total of 1913 genes differentially expressed within 24 h by B deficiency. 20 biological processes, 11 molecular functions and 12 cellular components were significantly influenced under B-deficient conditions. In early B-deficient responses, a total of 9 MIPs gene family, 103 cell-wall-related genes and 51 stress-related transcription factors might be involved. Moreover, the scions with differential B efficiency exerted a significant effect on the differentially expressed gene in the rootstock Tr.3. Physiological differences and critical boron concentration of two citrus rootstocks differing in boron efficiency under low boron conditionsThe B-efficient rootstock Carrizo citrange(CC) and B-inefficient rootstock trifoliate orange(TO) were grown in sand culture receiving seven B concentrations(0, 0.0005, 0.001, 0.0025, 0.005, 0.01 and 0.25 mg L-1) for 155 days. The physiological differences and critical B concentrations of CC and TO were investigated. Compared with the control(0.25 mg L-1), low B treatments reduced plant height, individual leaf area, total root length and surface area, dry weight, and the concentrations of B and B forms. Regardless of the B treatment, no difference in B concentration of roots was observed between CC and TO; but the B concentration in leaves was less reduced in CC than TO. Compared with TO, the available B(free B and semi-bound B) concentration in leaves was less decreased by low B treatmwnts, and accordingly the bound B concentration increased less. Additionally, the proportion of longer roots(0.50-1.00 cm) and semi-bound B in CC was higher as compared with TO. These results suggested that, under low B conditions, CC may have greater abilities to absorb B from medium, to translocate B from roots to shoot, and to utilize B within the plants. The critical B concentration of CC and TO in leaves were 11 and 17 mg kg-1 DW, and that in nutrient solution were 0.0031 and 0.0023 mg L-1, respectively. We proposed that the semi-bound B may pay a key “boron reserve” role in tolerance to B deficiency.4. The effects of inarching boron-efficient rootstock on the boron uptake of navel orange plants under low boron conditionsOne-year-old ‘Newhall’ navel orange plants on trifoliate orange were inarched with Carrizo citrange seedlings. After a 15-month B deficiency, the plants were resupplied with low B for 35 days to investigate the B uptake and the contribution of inarching rootstock to B absorption in different parts by using 10 B labeling and split-root methods. When resupplying low B to B-deficient plants, the B concentration and proportion of newly absorbed B in new leaves, new twigs, and old leaves were higher in inarched than non-inarched plants. By contrast, the B concentration in original rootstock roots of inarched plants was lower than that of non-inarched plants. Compared with adequate B supply, the inarching rootstock contributed more to the absorbed B in the scion, but less to the absorbed B in the rootstock under low B conditions. These results suggest that, under low B conditions, inarching B-efficient Carrizo citrange onto B-deficient navel orange improves the plant’s B uptake, increases the B concentration in young scion parts, and thus enhances the tolerance of the whole plant to B deficiency. |
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