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A greenhouse experiment was performed to study the effects of boron (B) on growth, nutrient concentration and distribution, nutrient use efficiency, and total nutrient absorption of 2 citrus genotypes. The experimental layout was a 6 × 2 factorial, with 6 B concentrations (0.05, 0.25, 0.50, 1.00, 2.00, and 5.00 mg/L) on 2 genotypes: the sour orange (Citrus aurantium L.) and the Swingle citrumelo (C. paradisi Macf. × Poncirus trifoliata L.). The plants were grown for 3 months in a B-free sand : perlite (1 : 1) medium that was irrigated with 6 half-strength Hoagland's nutrient solutions. Increasing B supply in the nutrient solution increased the B concentration linearly in all parts of the plant in the following order: basal leaves > top leaves > bark > root > stems > wood. There was no consistent effect of B supply on the concentration of other fundamental elements (P, K, Ca, Mg, Mn, Zn, Fe). Furthermore, none of the tested B concentrations significantly affected the total plant content and consequently the absorption of any other element. A concentration of 1.00 mg B/L or higher resulted in less B absorption by the Swingle citrumelo than by the sour orange. Furthermore, the Swingle citrumelo has the ability to retain more B in its stems and roots than the sour orange, thus preventing B transport to leaves. Finally, B and Mn use efficiency in both genotypes correlated significantly and negatively with the B supply.

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