Absorption and Translocation of Boron by Sunflower Plants

The absorption and translocation of boron by sunflower plants (Helianthus annuus L.) grown under controlled-environment conditions were investigated. Exudates in a 43-hr period from the cut stem ends of the root systems of detopped plants contained the same amount of boron whether the roots were immersed in nutrient solutions containing 500 μg or 1,000 μg of boron per liter. Plants given a total of 60 μg of boron in the substrate, but in varied amounts and times of application, showed no differences among the treatments in the accumulation of boron by their mature leaves. Two of the four series with multiple application, however, showed delays in the onset of severe symptoms of boron deficiency as compared with the series with an initial single 60-μg application. Significant lateral transport of boron occurred at the stem tip and in leaf blades when boron was supplied to only half of a split root system at 500 or 1,000 μg per root-half. Slight, though not significant, lateral transport occurred across the...