Roots are usually the first plant organs where boron deficiency symptoms appear. The experimental evaluation of the effects of mineral deficiencies on the root system of trees is rather difficult. This experiment aimed to evaluate the effect of boron deficiency on the formation of roots with different diameters and on nutrient accumulation and to characterize visual symptoms of B deficiency in roots of green dwarf coconut trees. The following treatments were applied: complete nutrient solution (+B) and nutrient solution without boron (-B) in a randomized design, with six replicates. The experimental unit consisted of one plant in a plastic pot containing 90 L of purified beach sand. Until the 60th day (3/04/2006), all plants were treated with a complete nutrient solution. On the 61st day after transplanting the above treatments were applied. The roots were collected 513 days (6/01/2007) after the induction of deficiency. After washing, the roots were separated in three diameters: fine (< 1 mm), medium (1 to 5 mm) and thick (> 5 mm). After the separation, the roots were immersed in water several times to remove the sand and finally washed in deionized water. After oven-drying, the dry matter of each root type was determined and thereafter the concentrations of N, P, K, Ca, Mg, S, B, Cu, Fe, Mn, and Zn were determined. Results indicated that the highest N, P, Ca, Mg, S, B, Cu, Fe, Mn, and Zn concentrations were found in the fine roots of the coconut tree in both treatments, but those of K were higher in the thick roots. The boron deficiency increased the N, P and K concentrations in all roots, the Mg, S, Cu and Zn concentrations in the fine roots, but Ca, Fe and Mn were not affected. The boron deficiency reduced the production of total and fine roots by 29.7 and 48.3 %, respectively; it induced root thickening and darkening and short ramifications; the root tips died off, causing root overgrowth.
necrosis; root thickning; nutrient solution; root distribution; boric acid
