Absorption and Translocation of Boron by Sunflower Plants

Abstract

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 lower levels of the stems in these series. Lateral transport of boron at the lower internodes was demonstrated when boron was supplied to only one-half of the root system in higher quantities (5,000 or 10,000 μg per root half). Little or no lateral transport in leaves or stems was observed with boron application of 50 or 100 μg per root-half. Although mature leaves sprayed with 10-ppm boron solution (as boric acid) showed an increase in dialyzable boron content, this soluble boron was not translocated from the leaves. No differences in growth were evident between plants receiving dialyzable boron extracted from leaf tissues and those receiving boric acid in the nutrient solutions. Non-dialyzable boron in the non-particulate fraction of leaf homogenate, however, was not utilized by plants when it was added to the nutrient solution. Reversing the transpiration stream of a leaf by placing it in a dilute minus-boron nutrient solution and withholding a water supply from other organs of the plant did not result in a movement of boron from the leaf to the apical meristem. Some of the boron in the leaf, however, was readily leached into the nutrient solution. Covering the top surface of a leaf with aluminum foil to limit photosynthesis did not result in a change in its boron content. A nitrogen-deficient nutrient solution adversely affected the capacity of plants to accumulate boron. From the results of the experiments described, it is concluded that one of the prime factors controlling the distribution of boron in sunflower plants is the extent and location of the metabolic requirement for this element. It is further proposed that immobility of boron in the leaves of this species results from some type of translocation block in the vascular tissue and not from a fixation of boron in a soluble but non-utilizable form.