Environmental factors, such as limited rainfall and nutritional deficiencies, have been important drawbacks for forest plantations in Brazil. Among the micronutrients, deficiency of boron (B) is the most frequently encountered in the field, requiring constant application. In order to evaluate the transport in soil, absorption and translocation of B by clones of eucalyptus, three experiments were carried out under greenhouse conditions. In the first one, the contributions of mass flow and diffusion in the transport of B to eucalyptus roots were evaluated in response to water potential and doses of B. The experiment consisted of one soil, one clone of eucalyptus, two water potentials (-10 and -40 kPa) and six application rates of B (0, 0.5, 1, 2, 3 and 5 mg dm-3). The water potential was controlled using a tensiometer in each pot. The maximum root dry weight was attained at 0.98 and 2.38 mg dm-3 of B for potential -10 kPa and -40 kPa, respectively. The maximum shoot dry weight was reached at 0.96 and 1.82 mg dm-3 of B for the -10 kPa and -40 kPa potential, respectively. Positive and significative relationships were observed between B application rate, extractable B, B in soil solution and plant B content for both water potentials. Mass flow was the predominant mechanism for B transport in the soil, reaching 100 % when high doses of B were applied. Diffusion was more important under conditions of low B in soil and under water deficit. In the second experiment, eight clones of eucalyptus were grown in nutrient solutions at B concentrations of 0, 10, 20, 50 and 100 µmol L-1 for 70 days, after which, the physiological variables, dry matter production and B content were determined. There was a differential response of the eucalyptus clones to the concentration of B in nutrient solution, with maximum dry matter production occurring between 24 and 69 µmol L-1 of B in nutrient solution. The genotypes differed in B use efficiency. Under B deficiency, the B concentration gradient between younger leaves and older leaves was reversed in comparison to those under normal B conditions, providing evidence of B mobility. In the third experiment, B translocation was evaluated with two clones (68 and 129) of eucalyptus using the 10B stable isotope as a tracer. 10B was applied once to only one mature leaf of seedlings with and without B deficiency. Samples of young tissue, mature leaves and roots were taken at 0, 1, 5, 12 and 17 days after the 10B application. 10B:11B isotopic ratio was determined with a high resolution-inductively coupled plasma mass spectrometer (HR-ICP-MS). Samples of xylem sap and leaves from seedlings were taken for soluble carbohydrate analyses by ion chromatography. In the deficient plants, new branchs were observed after B application. The results showed that B was translocated from mature leaf to young tissues. The clone 129 translocated more B from the treated leaf to young tissue, other mature leaves and roots than the clone 68. The plants grown under sufficient B supply showed no B translocation, providing evidence that B translocation may be an inductive mechanism. Sorbitol was the main polyol found in the leaves of eucalyptus.
